BILLING CODE 3510-22-P
DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
[RTID 0648-XE034]
Takes of Marine Mammals Incidental to Specified Activities; Taking Marine
Mammals Incidental to a Marine Geophysical Survey of the Chain Transform Fault
in the Equatorial Atlantic Ocean
AGENCY: National Marine Fisheries Service (NMFS), National Oceanic and
Atmospheric Administration (NOAA), Commerce.
ACTION: Notice; proposed incidental harassment authorization; request for comments
on proposed authorization and possible renewal.
SUMMARY: NMFS received a request from the Lamont-Doherty Earth Observatory of
Columbia University (L-DEO) for authorization to take marine mammals incidental to a
marine geophysical survey at the Chain Transform Fault in the equatorial Atlantic Ocean.
Pursuant to the Marine Mammal Protection Act (MMPA), NMFS is requesting comments
on its proposal to issue an incidental harassment authorization (IHA) to incidentally take
marine mammals during the specified activities. NMFS is also requesting comments on a
possible one-time, 1-year renewal that could be issued under certain circumstances and if
all requirements are met, as described in Request for Public Comments at the end of
this notice. NMFS will consider public comments prior to making any final decision on
the issuance of the requested MMPA authorization and agency responses will be
summarized in the final notice of our decision.
DATES: Comments and information must be received no later than [INSERT DATE 30
DAYS AFTER DATE OF PUBLICATION IN THE FEDERAL REGISTER].
ADDRESSES: Comments should be addressed to Jolie Harrison, Chief, Permits and
Conservation Division, Office of Protected Resources, National Marine Fisheries Service

and should be submitted via email to ITP.harlacher@noaa.gov. Electronic copies of the
application and supporting documents, as well as a list of the references cited in this
document, may be obtained online at: https://www.fisheries.noaa.gov/national/marinemammal-protection/incidental-take-authorizations-research-and-other-activities. In case
of problems accessing these documents, please call the contact listed below.
Instructions: NMFS is not responsible for comments sent by any other method, to
any other address or individual, or received after the end of the comment period.
Comments, including all attachments, must not exceed a 25-megabyte file size. All
comments received are a part of the public record and will generally be posted online at
https://www.fisheries.noaa.gov/national/marine-mammal-protection/incidental-takeauthorizations-research-and-other-activities without change. All personal identifying
information (e.g., name, address) voluntarily submitted by the commenter may be
publicly accessible. Do not submit confidential business information or otherwise
sensitive or protected information.
FOR FURTHER INFORMATION CONTACT: Jenna Harlacher, Office of Protected
Resources, NMFS, (301) 427-8401.
SUPPLEMENTARY INFORMATION:
Background
The MMPA prohibits the “take” of marine mammals, with certain exceptions.
Sections 101(a)(5)(A) and (D) of the MMPA (16 U.S.C. 1361 et seq.) direct the Secretary
of Commerce (as delegated to NMFS) to allow, upon request, the incidental, but not
intentional, taking of small numbers of marine mammals by U.S. citizens who engage in
a specified activity (other than commercial fishing) within a specified geographical
region if certain findings are made and either regulations are proposed or, if the taking is
limited to harassment, a notice of a proposed IHA is provided to the public for review
and comment.

Authorization for incidental takings shall be granted if NMFS finds that the taking
will have a negligible impact on the species or stock(s) and will not have an unmitigable
adverse impact on the availability of the species or stock(s) for taking for subsistence
uses (where relevant). Further, NMFS must prescribe the permissible methods of taking
and other “means of effecting the least practicable adverse impact” on the affected
species or stocks and their habitat, paying particular attention to rookeries, mating
grounds, and areas of similar significance, and on the availability of the species or stocks
for taking for certain subsistence uses (referred to in shorthand as “mitigation”); and
prescribe requirements pertaining to the monitoring and reporting of the takings. The
definitions of all applicable MMPA statutory terms cited above are included in the
relevant sections below.
National Environmental Policy Act
To comply with the National Environmental Policy Act of 1969 (NEPA; 42
U.S.C. 4321 et seq.) and NOAA Administrative Order (NAO) 216-6A, NMFS must
review our proposed action (i.e., the issuance of an IHA) with respect to potential impacts
on the human environment.
This action is consistent with categories of activities identified in Categorical
Exclusion B4 (incidental harassment authorizations with no anticipated serious injury or
mortality) of the Companion Manual for NOAA Administrative Order 216-6A, which do
not individually or cumulatively have the potential for significant impacts on the quality
of the human environment and for which we have not identified any extraordinary
circumstances that would preclude this categorical exclusion. Accordingly, NMFS has
preliminarily determined that the issuance of the proposed IHA qualifies to be
categorically excluded from further NEPA review.
Summary of Request

On April 15, 2024, NMFS received a request from L-DEO for an IHA to take
marine mammals incidental to conducting a marine geophysical survey of the Chain
Transform Fault in the equatorial Atlantic Ocean. Following NMFS review of the
application and additional clarifying information from L-DEO, NMFS deemed the
application adequate and complete on May 22, 2024. L-DEO’s request is for take of 28
marine mammal species by Level B harassment, and for take of a subset of 5 of these
species, by Level A harassment. Neither L-DEO nor NMFS expect serious injury or
mortality to result from this activity and, therefore, an IHA is appropriate.
Description of Proposed Activity
Overview
Researchers from the Woods Hole Oceanographic Institution, University of
Delaware, University of New Hampshire, Boise State University and Boston College,
with funding from the National Science Foundation, propose to conduct a high-energy
seismic survey using airguns as the acoustic source from the research vessel (R/V)
Marcus G. Langseth (Langseth), which is owned and operated by L-DEO. The proposed
survey would occur at the Chain Transform Fault, off the coast of Africa, in the
equatorial Atlantic Ocean during austral summer 2024 in the Southern Hemisphere (i.e.,
between October 2024 and February 2025). The proposed survey would occur within
International Waters more than 600 kilometers (km) in the Gulf of Guinea, Africa. The
survey would occur in water depths ranging from approximately 2,000 to 5,500 meters
(m). To complete this survey, the R/V Langseth would tow a 36-airgun array with a total
discharge volume of approximately (~) 6,600 cubic inches (in3) at a depth of 9 to 12 m.
The airgun array receiving system would consist of a 15 km long solid-state hydrophone
streamer and 20 Ocean Bottom Seismometers (OBS). The airguns would fire at a shot
interval of 37.5 m (~18 seconds (s)) during seismic acquisition. Approximately 2,058 km

of total survey trackline are proposed. Airgun arrays would introduce underwater sounds
that may result in take, by Level A and Level B harassment, of marine mammals.
The purpose of the proposed survey is to understand the rheologic mechanisms
that lead to both seismic and aseismic behavior. Specifically, the aim of the project is to:
(i) understand the tectonic variation along slow-slipping transforms; (ii) identify the
influences of seawater and melt on transform fault rheology; (iii) identify the influences
of seawater and melt on transform fault rheology; (iv) link slip behavior to observed
variations in seismic coupling and microseismicity; and (v) apply the results to
understanding the global spectrum of oceanic transform fault behavior. The goal of this
work is to understand how and why tectonic stresses in some places lead to earthquakes
of varying sizes while in other places the stresses are resolved without resulting in
earthquakes. The seismic survey would image the reflectivity and velocity structure of
seafloor features related to the transform fault within the Chain transform valley,
including the fault itself, ‘flower’ structures surrounding the fault, and the crustal massifs
that comprise the steep walls of the transform valley.
Additional data would be collected using a multibeam echosounder (MBES), a
sub-bottom profiler (SBP), and an Acoustic Doppler Current Profiler (ADCP), which
would be operated from R/V Langseth continuously during the seismic surveys, including
during transit. No take of marine mammals is expected to result from use of this
equipment.
Dates and Duration
The proposed survey is expected to last for approximately 30 days, with 11.5 days
of seismic operations, 3.5 days of OBS deployment, 2.5 days of streamer deployment and
retrieval, 2.5 days of contingency, and 10 days of transit. R/V Langseth would likely
leave from and return to port in Praia, Cape Verde during austral summer 2024 (between
October 2024 and February 2025).

Specific Geographic Region
The proposed survey would occur within approximately 0-2° S, 13-16.5° W,
within international waters more than 600 km off the coast of the Gulf of Guinea, Africa,
in water depths ranging from approximately 2,000 to 5,500 m. The region where the
survey is proposed to occur is depicted in figure 1, and is expected to cover
approximately 2,058 km of survey trackline. Representative survey tracklines are shown;
however, some deviation in actual tracklines, including the order of survey operations,
could be necessary for reasons such as science drivers, poor data quality, inclement
weather, or mechanical issues with the research vessel and/or equipment.

Figure 1. Location of the Proposed Chain Transform Fault Seismic Surveys in the
equatorial Atlantic Ocean
Location of the proposed seismic surveys, OBS deployments, and marine conservation areas in the
Equatorial Atlantic Ocean. Representative survey tracklines are included in the figure; however, the
tracklines could occur anywhere within the survey area. MPA = Marine Protected Area. EBSA =
Ecologically or Biologically Significant Marine Areas. VME = Vulnerable Marine Ecosystem. SEAFO =
South East Atlantic Fisheries Organization.

Detailed Description of the Specified Activity
The procedures to be used for the proposed surveys would be similar to those used
during previous seismic surveys by L-DEO and would use conventional seismic
methodology. The survey would involve one source vessel, R/V Langseth, which is
owned and operated by L-DEO. During the high-energy survey, R/V Langseth would tow
4 strings with 36 airguns, consisting of a mixture of Bolt 1500LL and Bolt 1900LLX.
During the surveys, all 4 strings, totaling 36 active airguns with a total discharge volume
of 6,600 in3, would be used. The four airgun strings would be spaced 8 m apart,
distributed across an area of approximately 24 m x 16 m behind the R/V Langseth, and
would be towed approximately 140 m behind the vessel. The airgun array configurations
are illustrated in figure 2-11 of National Science Foundation (NSF) and the U.S.
Geological Survey's (USGS) Programmatic Environmental Impact Statement (PEIS;
NSF-USGS, 2011). (The PEIS is available online at:
https://www.nsf.gov/geo/oce/envcomp/usgs-nsf-marine-seismic-research/nsf-usgs-finaleis-oeis_3june2011.pdf). The receiving system would consist of a 15-km long solid-state
hydrophone streamer and 20 OBSs. As the airgun arrays are towed along the survey lines,
the hydrophone streamer would transfer the data to the on-board processing system, and
the OBSs would receive and store the returning acoustic signals internally for later
analysis.
Approximately 2,058 km of seismic acquisition are proposed. The survey would
take place in water depths ranging from 2,000 to 5,500 m; all effort would occur in water
more than 2,000 m deep. Twenty OBSs would be deployed by R/V Langseth and left on
the ocean floor for a period of 1 year to record earthquakes. To retrieve the OBSs, the
instrument is released to float to the surface via an acoustic release system from the
anchor, which is not retrieved. In addition to the operations of the airgun array, the ocean
floor would be mapped with the Kongsberg EM 122 MBES and a Knudsen Chirp 3260

SBP. A Teledyne RDI 75 kilohertz (kHz) Ocean Surveyor ADCP would be used to
measure water current velocities, and acoustic pingers would be used to retrieve OBSs.
Take of marine mammals is not expected to occur incidental to the use of the MBES,
SBP, and ADCP, regardless of whether the airguns are operating simultaneously with the
other sources. Given their characteristics (e.g., narrow downward-directed beam), marine
mammals would experience no more than one or two brief ping exposures, if any
exposure were to occur, which would not be expected to provoke a response equating to
take. NMFS does not expect that the use of these sources presents any reasonable
potential to cause take of marine mammals.
Proposed mitigation, monitoring, and reporting measures are described in detail
later in this Notice (please see Proposed Mitigation and Proposed Monitoring and
Reporting).
Description of Marine Mammals in the Area of Specified Activities
Sections 3 and 4 of the application summarize available information regarding
status and trends, distribution and habitat preferences, and behavior and life history of the
potentially affected species. NMFS fully considered all of this information, and we refer
the reader to these descriptions, instead of reprinting the information. Additional
information about these species (e.g., physical and behavioral descriptions) may be found
on NMFS’ website (https://www.fisheries.noaa.gov/find-species). NMFS refers the reader
to the aforementioned source for general information regarding the species listed in table
1.
The populations of marine mammals found in the survey area do not occur within
the U.S. exclusive economic zone (EEZ) and therefore, are not assessed in NMFS' Stock
Assessment Reports (SARs). For most species, there are no stocks defined for
management purposes in the survey area, and NMFS is evaluating impacts at the species
level. As such, information on potential biological removal level (PBR; defined by the

MMPA as the maximum number of animals, not including natural mortalities, that may
be removed from a marine mammal stock while allowing that stock to reach or maintain
its optimum sustainable population) and annual levels of serious injury and mortality
from anthropogenic sources are not available for these marine mammal populations.
Abundance estimates for marine mammals in the survey location are lacking; therefore,
the modeled abundances presented here are based on a variety of proxy sources,
including the U.S Navy Atlantic Fleet Training and Testing Area Marine Mammal
Density (AFTT) model (Roberts et al., 2023) and the International Whaling Commission
(IWC) Population (Abundance) Estimates (IWC 2024). The modeled abundance is
considered the best scientific information available on the abundance of marine mammal
populations in the area.
Table 1 lists all species that occur in the survey area that may be taken as a result
of the proposed survey and summarizes information related to the population, including
regulatory status under the MMPA and Endangered Species Act (ESA).
Table 1 -- Species Likely Impacted by the Specified Activities

Common name

Scientific name

Stock

ESA/MMPA
status;
Strategic
(Y/N)1

Modeled
abundance2

Order Artiodactyla – Cetacea – Mysticeti (baleen whales)
Family Balaenopteridae (rorquals)

Blue Whale

Balaenoptera
musculus

NA

E, D, Y

1912/2,3004

Fin Whale

Balaenoptera
physalus

NA

E, D, Y

11,672

Humpback
Whale

Megaptera
novaeangliae

NA

-, -, N

4,9902/42,0005

Common Minke
Whale

Balaenoptera
acutorostrata

NA

-, -, N

13,784

Antarctic Minke
Whale

Balaenoptera
bonaerensis

NA

-, -, N

515,0003

Sei Whale

Balaenoptera
borealis

NA

E, D, Y

19,530

Bryde’s Whale

Balaenoptera
edeni

NA

-, -, N

64,015

Odontoceti (toothed whales, dolphins, and porpoises)
Family Physeteridae
Sperm Whale

Physeter
macrocephalus

NA

E, D, Y

Kogia breviceps

NA

-, -, N

Family Kogiidae
Pygmy Sperm
Whale

26,0437
Dwarf Sperm
Whale

Kogia sima

NA

-, -, N

Family Ziphiidae (beaked whales)
Blainville's
Beaked Whale

Mesoplodon
densirostris

NA

-, -, N

Cuvier's Beaked
Whale

Ziphius cavirostris

NA

-, -, N

Gervais’ Beaked
Whale

Mesoplodon
europaeus

NA

-, -, N

Killer Whale

Orcinus orca

NA

-, -, N

Short-Finned
Pilot Whale

Globicephala
melas

NA

-, -, N

264,9076

Rough-toothed
Dolphin

Steno bredanensis

NA

-, -, N

32,848

Bottlenose
Dolphin

Tursiops truncatus

NA

-, -, N

418,151

Risso's Dolphin

Grampus griseus

NA

-, -, N

78,205

Common
Dolphin

Delphinus delphis

NA

-, -, N

473,260

Striped Dolphin

Stenella
coeruleoalba

NA

-, -, N

412,729

Pantropical
Spotted Dolphin

Stenella attenuata

NA

-, -, N

321,740

Atlantic Spotted
Dolphin

Stenella frontalis

NA

-, -, N

259,519

65,0698

Family Delphinidae

Spinner Dolphin

Stenella
longirostris

NA

-, -, N

152,511

Clymene
Dolphin

Stenella clymene

NA

-, -, N

181,209

Fraser’s Dolphin

Lagenodelphis
hosei

NA

-, -, N

19,585

Melon-headed
Whale

Peponocephala
electra

NA

-, -, N

64,114

Pygmy Killer
Whale

Feresa attenuata

NA

-, -, N

9,001

False Killer
Whale

Pseudorca
crassidens

NA

-, -, N

12,682

ESA status: Endangered (E), Threatened (T)/MMPA status: Depleted (D). A dash (-) indicates that the
species is not listed under the ESA or designated as depleted under the MMPA. Under the MMPA, a
strategic stock is one for which the level of direct human-caused mortality exceeds PBR or which is
determined to be declining and likely to be listed under the ESA within the foreseeable future. Any species
or stock listed under the ESA is automatically designated under the MMPA as depleted and as a strategic
stock.
2 Modeled abundance value from U.S Navy Atlantic Fleet Training and Testing Area Marine Mammal
Density (AFTT) (Roberts et al., 2023) unless otherwise noted.
3 Abundance of minke whales (species unspecified) for the Southern Hemisphere (IWC 2024)
4 Abundance of blue whales (excluding pygmy blue whales) for Southern Hemisphere (IWC 2024)
5 Abundance of humpback whales on Antarctic feeding grounds (IWC 2024)
6 Pilot whale guild.
7 Estimate includes dwarf and pygmy sperm whales.
8 Beaked whale guild.
All 28 species in table 1 temporally and spatially co-occur with the activity to the
degree that take is reasonably likely to occur. All species that could potentially occur in
the proposed survey area are listed in section 3 of the application. In addition to what is
included in sections 3 and 4 of the application, and NMFS’ website, further detail
informing the baseline for select species of particular or unique vulnerability (i.e.,
information regarding ESA listed species) is provided below.
Blue Whale
The blue whale has a cosmopolitan distribution and tends to be pelagic, only
coming nearshore to feed and possibly to breed (Jefferson et al. 2015). The distribution of
the species, at least during times of the year when feeding is a major activity, occurs in
areas that provide large seasonal concentrations of euphausiids (Yochem and

Leatherwood 1985). Blue whales are most often found in cool, productive waters where
upwelling occurs (Reilly and Thayer 1990). Generally, blue whales are seasonal migrants
between high latitudes in summer, where they feed, and low latitudes in winter, where
they mate and give birth (Lockyer and Brown 1981). Their summer range in the North
Atlantic extends from Davis Strait, Denmark Strait, and the waters north of Svalbard and
the Barents Sea, south to the Gulf of St. Lawrence and the Bay of Biscay (Rice 1998).
Although the winter range is mostly unknown, some occur near Cape Verde at that time
of year (Rice 1998). One individual has been seen in Cape Verde in the month of June
(Reiner et al. 1996). Blue whales have also been sighted elsewhere off northwestern
Africa (Camphuysen 2015; Camphuysen et al. 2012, 2022; Baines and Reichelt 2014;
Djiba et al. 2015; Correia 2020; Samba Bilal et al. 2023).
An extensive data review and analysis by Branch et al. (2007a) showed that blue
whales are essentially absent from the central regions of major ocean basins, including in
the equatorial Atlantic Ocean, where the proposed survey area is located. Similarly,
Jefferson et al. (2015) indicate that the proposed survey area falls within the secondary
range of the blue whale. Blue whales were captured by the thousands off Angola,
Namibia, and South Africa in the 1900s, and a few catches were made near the proposed
survey area (Branch et al. 2007a; Figueiredo and Weir 2014). However, whales were
nearly extirpated in this region, and sightings of Antarctic blue whales in the region are
now rare (Branch et al. 2007a). At least four records of blue whales exist for Angola; all
sightings were made in 2012, with at least one sighting in July, two in August, and one in
October (Figueiredo and Weir 2014).
Sightings were also made off Namibia in 2014 from seismic vessels (Brownell et
al. 2016). Waters off Namibia may serve as a possible wintering and possible breeding
grounds for Antarctic blue whales (Best 1998, 2007; Thomisch 2017). Offshore sightings
in the southern Atlantic Ocean include one sighting at 13.4 º S, 26.8 º W and another at

15.9 º S, 4.6 º W (Branch et al. 2007a). Most blue whales off southeastern Africa are
expected to be Antarctic blue whales; however, ~4 percent may be pygmy blue whales
(Branch et al. 2007b, 2008). In fact, pygmy blue whale vocalizations were detected off
northern Angola in October 2008; these calls were attributed to the Sri Lanka population
(Cerchio et al. 2010). Antarctic blue whale calls were detected on acoustic recorders that
were deployed northwest of Walvis Ridge from November 2011 through May 2013
during all months except during September and October, indicating that not all whales
migrate to higher latitudes during the summer (Thomisch 2017). There are no blue whale
records near the proposed survey area in the Ocean Biodiversity Information System
(OBIS) database (OBIS 2024).
Fin Whale
The fin whale is widely distributed in all the world’s oceans (Gambell 1985),
although it is most abundant in temperate and cold waters (Aguilar and García-Vernet
2018). Nonetheless, its overall range and distribution are not well known (Jefferson et al.
2015). Fin whales most commonly occur offshore but can also be found in coastal areas
(Jefferson et al. 2015). Most populations migrate seasonally between temperate waters
where mating and calving occur in winter, and polar waters where feeding occurs in
summer (Aguilar and García-Vernet 2018).
In the Southern Hemisphere, fin whales are typically distributed south of 50ºS in
the austral summer, migrating northward to breed in the winter (Gambell 1985).
According to Edwards et al. (2015), sightings have been made off northwestern Africa
throughout the year and south of South Africa from December–February. Edwards did
not report any sightings or acoustic detections near the proposed project area, although it
is possible that fin whales could occur there. Fin whales were seen off Mauritania during
April 2004 (Tulp and Leopold 2004), November 2012–January 2013 (Camphuysen et al.
2012; Baines and Reichelt 2014), 2015–2016 (Camphuysen et al. 2017; Correia

2020), and February–March 2022 (Camphuysen et al. 2022). Samba Bilal et al. (2023)
reported several other records for Mauritania.
Several fin whale records exist for Angola (Weir 2011; Weir et al. 2012), South
Africa (Shirshov Institute n.d.), Namibia (NDP unpublished data in Pisces Environmental
Services 2017), and historical whaling data showed several catches off Namibia and
southern Africa (Best 2007), and Tristan da Cunha (Best et al. 2009). Fin whales appear
to be somewhat common in the Tristan da Cunha archipelago from October–December
(Bester and Ryan 2007). Fin whale calls were detected on acoustic recorders that were
deployed northwest of Walvis Ridge from November 2011 through May 2013 during the
months of November, January, and June through August, indicating that the waters off
Namibia serve as wintering grounds (Thomisch 2017). Similarly, Best (2007) also
suggested that waters off Namibia may be wintering grounds. Forty fin whales were seen
during a trans-Atlantic voyage along 20ºS during August 1943 between 5º and 25ºW
(Wheeler 1946 in Best 2007). Although Edwards et al. (2015) reported sightings in Cape
Verde, there were no records of fin whales for the proposed survey area to the south of
Cape Verde. There were no records of fin whales in the OBIS database near the proposed
survey area; the closest record of fin whales in the OBIS database is off the coast of West
Africa north of the proposed survey area (OBIS 2024).
Humpback Whale
For most North Atlantic humpbacks, the summer feeding grounds range from the
northeast coast of the U.S. to the Barents Sea (Katona and Beard 1990; Smith et al.
1999). In the winter, the majority of humpback whales migrate to wintering areas in the
West Indies (Smith et al. 1999); this is known as the West Indies distinct population
segment (DPS) (Bettridge et al. 2015). Some individuals from the North Atlantic migrate
to Cape Verde to breed (Wenzel et al. 2009, 2020); this is known as the Cape
Verde/Northwest Africa DPS which is listed as endangered under the ESA (Wenzel et al.

2020). A small proportion of the Atlantic humpback whale population remains at high
latitudes in the eastern North Atlantic during winter (e.g., Christensen et al. 1992). Based
on known migration routes of humpbacks from these breeding areas in the North Atlantic
(see Jann et al. 2003); Bettridge et al. 2015; NMFS 2016b), it is unlikely that individuals
from the aforementioned DPSs would occur in the proposed survey area, south of the
Equator.
In the Southern Hemisphere, humpback whales migrate annually from summer
foraging areas in the Antarctic to breeding grounds in tropical seas (Clapham 2018). It is
uncertain whether humpbacks occur in the proposed offshore survey area; Jefferson et al.
(2015) indicated this region to be within the possible range of this species and deep
offshore waters off West Africa to be the secondary range. The IWC recognizes seven
breeding populations in the Southern Hemisphere that are linked to six foraging areas in
the Antarctic (Clapham 2018). Two of the breeding grounds are in the South Atlantic –
off Brazil and West Africa (Engel and Martin 2009). Bettridge et al. (2015) identified
humpback whales at these breeding locations as the Brazil and Gabon/Southwest Africa
DPSs. Migrations, song similarity, and genetic studies indicate some interchange between
these two DPSs (Darling and Sousa-Lima 2005; Rosenbaum et al. 2009; Kershaw et al.
2017). Based on photo-identification work, one female humpback whale traveled from
Brazil to Madagascar, a distance of > 9800 km (Stevick et al. 2011). Deoxyribonucleic
acid (DNA) sampling showed evidence of a male humpback having traveled from West
Africa to Madagascar (Pomilla and Rosenbaum 2005). Humpback whales likely to be
encountered in the proposed survey area would be from the Gabon/Southwest Africa
DPS.
There may be at least two breeding substocks in Gabon/Southwest Africa,
including individuals in the main breeding area in the Gulf of Guinea and those animals
that feed and migrate off Namibia and South Africa (Rosenbaum et al. 2009, 2014;

Barendse et al. 2010a; Branch 2011; Carvalho et al. 2011). In addition, wintering
humpbacks have also been reported on the continental shelf of northwestern Africa (from
Senegal to Guinea) from July through November, which may represent the northernmost
component of Southern Hemisphere humpback whales that are known to winter in the
Gulf of Guinea (Van Waerebeek et al. 2013). Some humpbacks have also been reported
in the northern Gulf of Guinea during December (Hazevoet et al. 2011). Migration rates
are relatively high between populations within the southeastern Atlantic (Rosenbaum et
al. 2009). However, Barendse et al. (2010a) reported no matches between individuals
sighted in Namibia and South Africa based on a comparison of tail flukes. Feeding areas
for Gabon/Southwest Africa DPS include Bouvet Island (Rosenbaum et al. 2014) and
waters of the Antarctic Peninsula (Barendse et al. 2010b).
Humpbacks have been seen on breeding grounds around São Tomé in the Gulf of
Guinea from August through November (Carvalho et al. 2011). They are regularly seen
in the northern Gulf of Guinea off Togo and Benin during December (Van Waerebeek et
al. 2001; Van Waerebeek 2002). Off Gabon, humpback whales occur from late June–
December (Carvalho et al. 2011). Weir (2011) reported year-round occurrence of
humpback whales off Gabon and Angola, with the highest sighting rates from June
through October. The west coast of South Africa might not be a ‘typical’ migration
corridor, as humpbacks are also known to feed in the area; they are known to occur in the
region during the northward migration (July–August), the southward migration (October–
November), and into February (Barendse et al. 2010b; Carvalho et al. 2011; Seakamela et
al. 2015). The highest sighting rates in the area occurred during mid-spring through
summer (Barendse et al. 2010b).
Humpback whale calls were detected on acoustic recorders that were deployed
northwest of Walvis Ridge from November 2011 through May 2013 during the months of
November, December, January, and May through August, indicating that not all whales

migrate to higher latitudes during the summer (Thomisch 2017). Based on whales that
were satellite-tagged in Gabon in winter 2002, migration routes southward include
offshore waters along Walvis Ridge (Rosenbaum et al. 2014). Humpback whales have
also been sighted off Namibia (Elwen et al. 2014), South Africa (Barendse et al. 2010b),
Tristan da Cunha (Bester and Ryan 2007; Best et al. 2009), St. Helena (MacLeod and
Bennett 2007; Clingham et al. 2013), and they have been detected visually and
acoustically off Angola (Best et al. 1999; Weir 2011; Cerchio et al. 2010, 2014; Weir et
al. 2012). In the OBIS database, there are no records of humpback whales within the
proposed survey area; the closest records of humpback whales are from whaling catches
closer to shore in the Gulf of Guinea and farther north than the proposed survey location
(OBIS 2024).
Minke Whale
In the Northern Hemisphere, minke whales are usually seen in coastal areas but
may also be seen in pelagic waters during their northward migration in spring and
summer and southward migration in fall (Stewart and Leatherwood, 1985). Although
some populations of common minke whale have been well studied on summer feeding
grounds, information on wintering areas and migration routes is lacking (Risch et al.
2014). Minke whales migrate north of 30°N from March–April and migrate south from
Iceland from late September through October (Risch et al. 2014; Víkingsson and HeideJorgensen 2015). Sightings have been made off northwestern Africa (Correia 2020;
Samba Bilal et al. 2023; Shakhovskoy 2023), including off Mauritania during February
2022 (Camphuysen et al. 2022). The Antarctic minke whale occurs south of 60°S during
austral summer and moves northwards to the coasts off western South Africa and
northeast Brazil during austral winter (Perrin et al. 2018).
A smaller form (unnamed subspecies) of the common minke whale, known as the
dwarf minke whale, occurs in the Southern Hemisphere, where its distribution overlaps

with that of the Antarctic minke whale during summer (Perrin et al. 2018). The dwarf
minke whale is generally found in shallow coastal waters and over the outer continental
shelf in regions where it overlaps with the Antarctic minke whale (Perrin et al. 2018).
The range of the dwarf minke whale is thought to extend as far south as 65°S off
Antarctica in the South Atlantic Ocean (Jefferson et al. 2015) and as far north as 2°S in
the Atlantic off South America, where dwarf minke whales can be found nearly yearround (Perrin et al. 2018). Dwarf minke whales are known to occur off South Africa
during autumn and winter (Perrin et al. 2018), but have not been reported for the waters
off Angola or Namibia (Best 2007).
It is unclear which species or form, if any, would occur in the proposed survey
area, as this region is considered to be within the possible range of the common minke
whale and just north of the primary range of the Antarctic minke whale (Jefferson et al.
2015). There are no records of common or Antarctic minke whales near the proposed
survey area in the OBIS database (OBIS 2024).
Sei Whale
Sei whales are found in all ocean basins (Horwood 2018) but appear to prefer
mid-latitude temperate waters (Jefferson et al. 2015). Habitat suitability models indicate
that sei whale distribution is related to cool water with high chlorophyll levels (Palka et
al., 2017; Chavez-Rosales et al. 2019). They occur in deeper waters characteristic of the
continental shelf edge region (Hain et al. 1985) and in other regions of steep bathymetric
relief such as seamounts and canyons (Kenney and Winn 1987; Gregr and Trites 2001).
Sei whales undertake seasonal migrations to feed in subpolar latitudes during
summer and return to lower latitudes during winter to calve (Gambell 1985; Horwood
2018). On summer feeding grounds, sei whales associate with oceanic frontal systems
(Horwood 1987). Sei whales that have been tagged in the Azores have traveled to the
Labrador Sea, where they spend extended periods of time presumably feeding (Olsen et

al. 2009; Prieto et al. 2010, 2014). Sei whales were the most commonly sighted species
during a summer survey along the Mid-Atlantic Ridge from Iceland to north of the
Azores (Waring et al. 2008). One sighting was made on the shelf break off Mauritania
during March 2003 (Burton and Camphuysen 2003), at least seven sightings were made
off Mauritania during November 2012–January 2013 (Baines and Reichelt 2014), and six
sightings were made off Mauritania during February–March 2022 (Camphuysen et al.
2022). Correia (2020) and Samba Bilal et al. (2023) reported additional records for the
waters off northwestern Africa.
In the South Atlantic, waters off northern Namibia may serve as wintering
grounds (Best 2007). Summer concentrations are found between the subtropical and
Antarctic convergences (Horwood 2018). A sighting of a mother and calf were made off
Namibia in March 2012, and one stranding was reported in July 2013 (NDP unpublished
data in Pisces Environmental Services 2017). One sighting was made during seismic
surveys off the coast of northern Angola between 2004 and 2009 (Weir 2011; Weir et al.
2012). A group of two to four sei whales was seen near St. Helena during April 2011
(Clingham et al. 2013). Sei whales were also taken by whaling vessels off southern
Africa during the 1960s (Best and Lockyer 2002). There are no records of sei whales near
the proposed survey in the OBIS database (OBIS 2024). However, one sighting was
made just northeast of the proposed survey area during March 2014 (Jungblut et al.
2017).
Sperm Whale
The sperm whale is widely distributed, occurring from the edge of the polar pack
ice to the Equator in both hemispheres, with the sexes occupying different distributions
(Whitehead 2018). Their distribution and relative abundance can vary in response to prey
availability, most notably squid (Jaquet and Gendron 2002). Females generally inhabit
waters >1,000 m deep at latitudes <40º where sea surface temperatures are <15ºC; adult

males move to higher latitudes as they grow older and larger in size, returning to warmwater breeding grounds (Whitehead 2018).
The primary range of sperm whales includes the waters off West Africa (Jefferson
et al. 2015), including Cape Verde (Reiner et al. 1996; Hazevoet et al. 2010). Sperm
whales have also been reported off Mauritania (Camphuysen 2015; Camphuysen et al.
2017). Sperm whales were the most frequently sighted cetacean during seismic surveys
off the coast of northern Angola between 2004 and 2009; hundreds of sightings were
made off Angola and a few sightings were reported off Gabon (Weir 2011). They occur
there throughout the year, although sighting rates were highest from April through June
(Weir 2011). de Boer (2010) also reported sightings off Gabon in 2009, and Weir et al.
(2012) reported numerous sightings of sperm whales off Angola, the Republic of the
Congo, and the Democratic Republic of the Congo during 2004–2009. Van Waerebeek et
al. (2010) reported sightings off South Africa, and one group was seen at St. Helena
during July 2009 (Clingham et al. 2013). Bester and Ryan (2007) noted that sperm
whales might be common in the Tristan da Cunha archipelago, and catches of sperm
whales were made there in the 19th and 20th centuries (Best et al. 2009). The waters of
northern Angola, Namibia, and South Africa were historical whaling grounds (Best 2007;
Weir 2019). There are thousands of sperm whale records for the South Atlantic in the
OBIS database, but most of these are historical catches (OBIS 2024). Although none of
the records occur within the proposed survey area, there are several records to the north
and southwest of the proposed survey area (OBIS 2024).
Marine Mammal Hearing
Hearing is the most important sensory modality for marine mammals underwater,
and exposure to anthropogenic sound can have deleterious effects. To appropriately
assess the potential effects of exposure to sound, it is necessary to understand the
frequency ranges marine mammals are able to hear. Not all marine mammal species have

equal hearing capabilities (e.g., Richardson et al., 1995; Wartzok and Ketten, 1999; Au
and Hastings, 2008). To reflect this, Southall et al. (2007, 2019) recommended that
marine mammals be divided into hearing groups based on directly measured (behavioral
or auditory evoked potential techniques) or estimated hearing ranges (behavioral response
data, anatomical modeling, etc.). Note that no direct measurements of hearing ability
have been successfully completed for mysticetes (i.e., low-frequency cetaceans).
Subsequently, NMFS (2018) described generalized hearing ranges for these marine
mammal hearing groups. Generalized hearing ranges were chosen based on the
approximately 65 decibel (dB) threshold from the normalized composite audiograms,
with the exception for lower limits for low-frequency cetaceans where the lower bound
was deemed to be biologically implausible and the lower bound from Southall et al.
(2007) retained. Marine mammal hearing groups and their associated hearing ranges are
provided in table 2.
Table 2 -- Marine Mammal Hearing Groups (NMFS, 2018)
Hearing Group
Low-frequency (LF) cetaceans
(baleen whales)

Generalized Hearing
Range*
7 Hz to 35 kHz

Mid-frequency (MF) cetaceans
(dolphins, toothed whales, beaked whales, bottlenose
whales)

150 Hz to 160 kHz

High-frequency (HF) cetaceans
(true porpoises, Kogia, river dolphins, Cephalorhynchid,
Lagenorhynchus cruciger & L. australis)

275 Hz to 160 kHz

Phocid pinnipeds (PW) (underwater)
(true seals)

50 Hz to 86 kHz

Otariid pinnipeds (OW) (underwater)
(sea lions and fur seals)

60 Hz to 39 kHz

* Represents the generalized hearing range for the entire group as a composite (i.e., all species within
the group), where individual species’ hearing ranges are typically not as broad. Generalized hearing
range chosen based on ~65 dB threshold from normalized composite audiogram, with the exception for
lower limits for LF cetaceans (Southall et al. 2007) and PW pinniped (approximation).

For more detail concerning these groups and associated frequency ranges, please
see NMFS (2018) for a review of available information.
Potential Effects of Specified Activities on Marine Mammals and Their Habitat
This section provides a discussion of the ways in which components of the
specified activity may impact marine mammals and their habitat. The Estimated Take of
Marine Mammals section later in this document includes a quantitative analysis of the
number of individuals that are expected to be taken by this activity. The Negligible
Impact Analysis and Determination section considers the content of this section, the
Estimated Take of Marine Mammals section, and the Proposed Mitigation section, to
draw conclusions regarding the likely impacts of these activities on the reproductive
success or survivorship of individuals and whether those impacts are reasonably expected
to, or reasonably likely to, adversely affect the species or stock through effects on annual
rates of recruitment or survival.
Description of Active Acoustic Sound Sources
This section contains a brief technical background on sound, the characteristics of
certain sound types, and on metrics used in this proposal inasmuch as the information is
relevant to the specified activity and to a discussion of the potential effects of the
specified activity on marine mammals found later in this document.
Sound travels in waves, the basic components of which are frequency,
wavelength, velocity, and amplitude. Frequency is the number of pressure waves that
pass by a reference point per unit of time and is measured in hertz (Hz) or cycles per
second. Wavelength is the distance between two peaks or corresponding points of a
sound wave (length of one cycle). Higher frequency sounds have shorter wavelengths
than lower frequency sounds, and typically attenuate (decrease) more rapidly, except in
certain cases in shallower water. Amplitude is the height of the sound pressure wave or
the “loudness” of a sound and is typically described using the relative unit of the dB. A

sound pressure level (SPL) in dB is described as the ratio between a measured pressure
and a reference pressure (for underwater sound, this is 1 micropascal (μPa)) and is a
logarithmic unit that accounts for large variations in amplitude; therefore, a relatively
small change in dB corresponds to large changes in sound pressure. The source level (SL)
represents the SPL referenced at a distance of 1 m from the source (referenced to 1 μPa)
while the received level is the SPL at the listener's position (referenced to 1 μPa).
Root mean square (RMS) is the quadratic mean sound pressure over the duration
of an impulse. Root mean square is calculated by squaring all of the sound amplitudes,
averaging the squares, and then taking the square root of the average (Urick, 1983). Root
mean square accounts for both positive and negative values; squaring the pressures makes
all values positive so that they may be accounted for in the summation of pressure levels
(Hastings and Popper, 2005). This measurement is often used in the context of discussing
behavioral effects, in part because behavioral effects, which often result from auditory
cues, may be better expressed through averaged units than by peak pressures.
Sound exposure level (SEL; represented as dB re 1 μPa2−s) represents the total
energy contained within a pulse and considers both intensity and duration of exposure.
Peak sound pressure (also referred to as zero-to-peak sound pressure or 0-p) is the
maximum instantaneous sound pressure measurable in the water at a specified distance
from the source and is represented in the same units as the RMS sound pressure. Another
common metric is peak-to-peak sound pressure (pk-pk), which is the algebraic difference
between the peak positive and peak negative sound pressures. Peak-to-peak pressure is
typically approximately 6 dB higher than peak pressure (Southall et al., 2007).
When underwater objects vibrate or activity occurs, sound-pressure waves are
created. These waves alternately compress and decompress the water as the sound wave
travels. Underwater sound waves radiate in a manner similar to ripples on the surface of a
pond and may be either directed in a beam or beams or may radiate in all directions

(omnidirectional sources), as is the case for pulses produced by the airgun array
considered here. The compressions and decompressions associated with sound waves are
detected as changes in pressure by aquatic life and man-made sound receptors such as
hydrophones.
Even in the absence of sound from the specified activity, the underwater
environment is typically loud due to ambient sound. Ambient sound is defined as
environmental background sound levels lacking a single source or point (Richardson et
al., 1995), and the sound level of a region is defined by the total acoustical energy being
generated by known and unknown sources. These sources may include physical (e.g.,
wind and waves, earthquakes, ice, atmospheric sound), biological (e.g., sounds produced
by marine mammals, fish, and invertebrates), and anthropogenic (e.g., vessels, dredging,
construction) sound. A number of sources contribute to ambient sound, including the
following (Richardson et al., 1995):
Wind and waves: The complex interactions between wind and water surface,
including processes such as breaking waves and wave-induced bubble oscillations and
cavitation, are a main source of naturally occurring ambient sound for frequencies
between 200 Hz and 50 kHz (Mitson, 1995). In general, ambient sound levels tend to
increase with increasing wind speed and wave height. Surf sound becomes important near
shore, with measurements collected at a distance of 8.5 km from shore showing an
increase of 10 dB in the 100 to 700 Hz band during heavy surf conditions;
Precipitation: Sound from rain and hail impacting the water surface can become
an important component of total sound at frequencies above 500 Hz, and possibly down
to 100 Hz during quiet times;
Biological: Marine mammals can contribute significantly to ambient sound levels,
as can some fish and snapping shrimp. The frequency band for biological contributions is
from approximately 12 Hz to over 100 kHz; and

Anthropogenic: Sources of anthropogenic sound related to human activity include
transportation (surface vessels), dredging and construction, oil and gas drilling and
production, seismic surveys, sonar, explosions, and ocean acoustic studies. Vessel noise
typically dominates the total ambient sound for frequencies between 20 and 300 Hz. In
general, the frequencies of anthropogenic sounds are below 1 kHz and, if higher
frequency sound levels are created, they attenuate rapidly. Sound from identifiable
anthropogenic sources other than the activity of interest (e.g., a passing vessel) is
sometimes termed background sound, as opposed to ambient sound.
The sum of the various natural and anthropogenic sound sources at any given
location and time - which comprise “ambient” or “background” sound - depends not only
on the source levels (as determined by current weather conditions and levels of biological
and human activity) but also on the ability of sound to propagate through the
environment. In turn, sound propagation is dependent on the spatially and temporally
varying properties of the water column and sea floor, and is frequency-dependent. As a
result of this dependence on a large number of varying factors, ambient sound levels can
be expected to vary widely over both coarse and fine spatial and temporal scales. Sound
levels at a given frequency and location can vary by 10-20 dB from day to day
(Richardson et al., 1995). The result is that, depending on the source type and its
intensity, sound from a given activity may be a negligible addition to the local
environment or could form a distinctive signal that may affect marine mammals. Details
of source types are described in the following text.
Sounds are often considered to fall into one of two general types: Pulsed and nonpulsed. The distinction between these two sound types is important because they have
differing potential to cause physical effects, particularly with regard to hearing (e.g.,
NMFS, 2018; Ward, 1997 in Southall et al., 2007). Please see Southall et al. (2007) for
an in-depth discussion of these concepts.

Pulsed sound sources (e.g., airguns, explosions, gunshots, sonic booms, impact
pile driving) produce signals that are brief (typically considered to be less than one
second), broadband, atonal transients (ANSI, 1986, 2005; Harris, 1998; NIOSH, 1998;
ISO, 2003) and occur either as isolated events or repeated in some succession. Pulsed
sounds are all characterized by a relatively rapid rise from ambient pressure to a maximal
pressure value followed by a rapid decay period that may include a period of diminishing,
oscillating maximal and minimal pressures, and generally have an increased capacity to
induce physical injury as compared with sounds that lack these features.
Non-pulsed sounds can be tonal, narrowband, or broadband, brief or prolonged,
and may be either continuous or non-continuous (ANSI, 1995; NIOSH, 1998). Some of
these non-pulsed sounds can be transient signals of short duration but without the
essential properties of pulses (e.g., rapid rise time). Examples of non-pulsed sounds
include those produced by vessels, aircraft, machinery operations such as drilling or
dredging, vibratory pile driving, and active sonar systems (such as those used by the U.S.
Navy). The duration of such sounds, as received at a distance, can be greatly extended in
a highly reverberant environment.
Airgun arrays produce pulsed signals with energy in a frequency range from about
10-2,000 Hz, with most energy radiated at frequencies below 200 Hz. The amplitude of
the acoustic wave emitted from the source is equal in all directions (i.e., omnidirectional),
but airgun arrays do possess some directionality due to different phase delays between
guns in different directions. Airgun arrays are typically tuned to maximize functionality
for data acquisition purposes, meaning that sound transmitted in horizontal directions and
at higher frequencies is minimized to the extent possible.
Acoustic Effects
Here, we discuss the effects of active acoustic sources on marine mammals.

Potential Effects of Underwater Sound1 — Anthropogenic sounds cover a broad
range of frequencies and sound levels and can have a range of highly variable impacts on
marine life, from none or minor to potentially severe responses, depending on received
levels, duration of exposure, behavioral context, and various other factors. The potential
effects of underwater sound from active acoustic sources can potentially result in one or
more of the following: Temporary or permanent hearing impairment; non-auditory
physical or physiological effects; behavioral disturbance; stress; and masking
(Richardson et al., 1995; Gordon et al., 2004; Nowacek et al., 2007; Southall et al., 2007;
Götz et al., 2009). The degree of effect is intrinsically related to the signal characteristics,
received level, distance from the source, and duration of the sound exposure. In general,
sudden, high level sounds can cause hearing loss, as can longer exposures to lower level
sounds. Temporary or permanent loss of hearing, if it occurs at all, will occur almost
exclusively in cases where a noise is within an animal's hearing frequency range. We first
describe specific manifestations of acoustic effects before providing discussion specific
to the use of airgun arrays.
Richardson et al. (1995) described zones of increasing intensity of effect that
might be expected to occur, in relation to distance from a source and assuming that the
signal is within an animal's hearing range. First is the area within which the acoustic
signal would be audible (potentially perceived) to the animal, but not strong enough to
elicit any overt behavioral or physiological response. The next zone corresponds with the
area where the signal is audible to the animal and of sufficient intensity to elicit
behavioral or physiological response. Third is a zone within which, for signals of high
intensity, the received level is sufficient to potentially cause discomfort or tissue damage
to auditory or other systems. Overlaying these zones to a certain extent is the area within

1 Please refer to the information given previously (“Description of Active Acoustic Sound Sources”) regarding

sound, characteristics of sound types, and metrics used in this document.

which masking (i.e., when a sound interferes with or masks the ability of an animal to
detect a signal of interest that is above the absolute hearing threshold) may occur; the
masking zone may be highly variable in size.
We describe the more severe effects of certain non-auditory physical or
physiological effects only briefly as we do not expect that use of airgun arrays are
reasonably likely to result in such effects (see below for further discussion). Potential
effects from impulsive sound sources can range in severity from effects such as
behavioral disturbance or tactile perception to physical discomfort, slight injury of the
internal organs and the auditory system, or mortality (Yelverton et al., 1973). Nonauditory physiological effects or injuries that theoretically might occur in marine
mammals exposed to high level underwater sound or as a secondary effect of extreme
behavioral reactions (e.g., change in dive profile as a result of an avoidance reaction)
caused by exposure to sound include neurological effects, bubble formation, resonance
effects, and other types of organ or tissue damage (Cox et al., 2006; Southall et al., 2007;
Zimmer and Tyack, 2007; Tal et al., 2015). The survey activities considered here do not
involve the use of devices such as explosives or mid-frequency tactical sonar that are
associated with these types of effects.
Threshold Shift — Marine mammals exposed to high-intensity sound, or to lowerintensity sound for prolonged periods, can experience hearing threshold shift (TS), which
is the loss of hearing sensitivity at certain frequency ranges (Finneran, 2015). Threshold
shift can be permanent (PTS), in which case the loss of hearing sensitivity is not fully
recoverable, or temporary (TTS), in which case the animal's hearing threshold would
recover over time (Southall et al., 2007). Repeated sound exposure that leads to TTS
could cause PTS. In severe cases of PTS, there can be total or partial deafness, while in
most cases the animal has an impaired ability to hear sounds in specific frequency ranges
(Kryter, 1985).

When PTS occurs, there is physical damage to the sound receptors in the ear (i.e.,
tissue damage), whereas TTS represents primarily tissue fatigue and is reversible
(Southall et al., 2007). In addition, other investigators have suggested that TTS is within
the normal bounds of physiological variability and tolerance and does not represent
physical injury (e.g., Ward, 1997). Therefore, NMFS does not typically consider TTS to
constitute auditory injury.
Relationships between TTS and PTS thresholds have not been studied in marine
mammals. There is no PTS data for cetaceans, but such relationships are assumed to be
similar to those in humans and other terrestrial mammals. PTS typically occurs at
exposure levels at least several dBs above (a 40-dB threshold shift approximates PTS
onset; e.g., Kryter et al., 1966; Miller, 1974) that inducing mild TTS (a 6-dB threshold
shift approximates TTS onset; e.g., Southall et al. 2007). Based on data from terrestrial
mammals, a precautionary assumption is that the PTS thresholds for impulsive sounds
(such as airgun pulses as received close to the source) are at least 6 dB higher than the
TTS threshold on a peak-pressure basis and PTS cumulative sound exposure level
thresholds are 15 to 20 dB higher than TTS cumulative sound exposure level thresholds
(Southall et al., 2007). Given the higher level of sound or longer exposure duration
necessary to cause PTS as compared with TTS, it is considerably less likely that PTS
could occur.
TTS is the mildest form of hearing impairment that can occur during exposure to
sound (Kryter, 1985). While experiencing TTS, the hearing threshold rises, and a sound
must be at a higher level in order to be heard. In terrestrial and marine mammals, TTS
can last from minutes or hours to days (in cases of strong TTS). In many cases, hearing
sensitivity recovers rapidly after exposure to the sound ends. Few data on sound levels
and durations necessary to elicit mild TTS have been obtained for marine mammals.

Marine mammal hearing plays a critical role in communication with conspecifics,
and interpretation of environmental cues for purposes such as predator avoidance and
prey capture. Depending on the degree (elevation of threshold in dB), duration (i.e.,
recovery time), and frequency range of TTS, and the context in which it is experienced,
TTS can have effects on marine mammals ranging from discountable to serious. For
example, a marine mammal may be able to readily compensate for a brief, relatively
small amount of TTS in a non-critical frequency range that occurs during a time where
ambient noise is lower and there are not as many competing sounds present.
Alternatively, a larger amount and longer duration of TTS sustained during time when
communication is critical for successful mother/calf interactions could have more serious
impacts.
Finneran et al. (2015) measured hearing thresholds in 3 captive bottlenose
dolphins before and after exposure to 10 pulses produced by a seismic airgun in order to
study TTS induced after exposure to multiple pulses. Exposures began at relatively low
levels and gradually increased over a period of several months, with the highest
exposures at peak SPLs from 196 to 210 dB and cumulative (unweighted) SELs from
193-195 dB. No substantial TTS was observed. In addition, behavioral reactions were
observed that indicated that animals can learn behaviors that effectively mitigate noise
exposures (although exposure patterns must be learned, which is less likely in wild
animals than for the captive animals considered in this study). The authors note that the
failure to induce more significant auditory effects was likely due to the intermittent
nature of exposure, the relatively low peak pressure produced by the acoustic source, and
the low-frequency energy in airgun pulses as compared with the frequency range of best
sensitivity for dolphins and other mid-frequency cetaceans.
Currently, TTS data only exist for four species of cetaceans (bottlenose dolphin
(Tursiops truncatus), beluga whale (Delphinapterus leucas), harbor porpoise (Phocoena

phocoena), and Yangtze finless porpoise (Neophocaena asiaeorientalis)) exposed to a
limited number of sound sources (i.e., mostly tones and octave-band noise) in laboratory
settings (Finneran, 2015). In general, harbor porpoises have a lower TTS onset than other
measured cetacean species (Finneran, 2015). Additionally, the existing marine mammal
TTS data come from a limited number of individuals within these species. There is no
direct data available on noise-induced hearing loss for mysticetes.
Critical questions remain regarding the rate of TTS growth and recovery after
exposure to intermittent noise and the effects of single and multiple pulses. Data at
present are also insufficient to construct generalized models for recovery and determine
the time necessary to treat subsequent exposures as independent events. More
information is needed on the relationship between auditory evoked potential and
behavioral measures of TTS for various stimuli. For summaries of data on TTS in marine
mammals or for further discussion of TTS onset thresholds, please see Southall et al.
(2007, 2019), Finneran and Jenkins (2012), Finneran (2015), and NMFS (2018).
Behavioral Effects — Behavioral disturbance may include a variety of effects,
including subtle changes in behavior (e.g., minor or brief avoidance of an area or changes
in vocalizations), more conspicuous changes in similar behavioral activities, and more
sustained and/or potentially severe reactions, such as displacement from or abandonment
of high-quality habitat. Behavioral responses to sound are highly variable and contextspecific, and any reactions depend on numerous intrinsic and extrinsic factors (e.g.,
species, state of maturity, experience, current activity, reproductive state, auditory
sensitivity, time of day), as well as the interplay between factors (e.g., Richardson et al.,
1995; Wartzok et al., 2003; Southall et al., 2007, 2019; Weilgart, 2007; Archer et al.,
2010). Behavioral reactions can vary not only among individuals but also within an
individual, depending on previous experience with a sound source, context, and
numerous other factors (Ellison et al., 2012), and can vary depending on characteristics

associated with the sound source (e.g., whether it is moving or stationary, number of
sources, distance from the source). Please see appendices B-C of Southall et al. (2007)
for a review of studies involving marine mammal behavioral responses to sound.
Habituation can occur when an animal's response to a stimulus wanes with
repeated exposure, usually in the absence of unpleasant associated events (Wartzok et al.,
2003). Animals are most likely to habituate to sounds that are predictable and unvarying.
It is important to note that habituation is appropriately considered as a “progressive
reduction in response to stimuli that are perceived as neither aversive nor beneficial,”
rather than as, more generally, moderation in response to human disturbance (Bejder et
al., 2009). The opposite process is sensitization, when an unpleasant experience leads to
subsequent responses, often in the form of avoidance, at a lower level of exposure. As
noted, behavioral state may affect the type of response. For example, animals that are
resting may show greater behavioral change in response to disturbing sound levels than
animals that are highly motivated to remain in an area for feeding (Richardson et al.,
1995; NRC, 2003; Wartzok et al., 2003). Controlled experiments with captive marine
mammals have shown pronounced behavioral reactions, including avoidance of loud
sound sources (Ridgway et al., 1997). Observed responses of wild marine mammals to
loud pulsed sound sources (typically seismic airguns or acoustic harassment devices)
vary but often consist of avoidance behavior or other behavioral changes suggesting
discomfort (Morton and Symonds, 2002; see also Richardson et al., 1995; Nowacek et
al., 2007). However, many delphinids approach acoustic source vessels with no apparent
discomfort or obvious behavioral change (e.g., Barkaszi et al., 2012).
Available studies show wide variation in response to underwater sound; therefore,
it is difficult to predict specifically how any given sound in a particular instance might
affect marine mammals perceiving the signal. If a marine mammal reacts briefly to
underwater sound by changing its behavior or moving a small distance, the impacts of the

behavioral change are unlikely to be significant to the individual, let alone the stock or
population. However, if a sound source displaces marine mammals from an important
feeding or breeding area for a prolonged period, impacts on individuals and populations
could be significant (e.g., Lusseau and Bejder, 2007; Weilgart, 2007; NRC, 2005). There
are broad categories of potential response, which we describe in greater detail here, that
include changes in dive behavior, disruption of foraging (feeding) behavior, changes in
respiration (breathing), interference with or alteration of vocalization, avoidance, and
flight.
Changes in dive behavior can vary widely, and may consist of increased or
decreased dive times and surface intervals as well as changes in the rates of ascent and
descent during a dive (e.g., Frankel and Clark, 2000; Ng and Leung, 2003; Nowacek et
al., 2004; Goldbogen et al., 2013a, b). Variations in dive behavior may reflect disruptions
in biologically significant activities (e.g., foraging) or they may be of little biological
significance. The impact of an alteration to dive behavior resulting from an acoustic
exposure depends on what the animal is doing at the time of the exposure and the type
and magnitude of the response.
Disruption of foraging (feeding) behavior can be difficult to correlate with
anthropogenic sound exposure, so it is usually inferred by observed displacement from
known foraging areas, the appearance of secondary indicators (e.g., bubble nets or
sediment plumes), or changes in dive behavior. As for other types of behavioral response,
the frequency, duration, and temporal pattern of signal presentation, as well as
differences in species sensitivity, are likely contributing factors to differences in response
in any given circumstance (e.g., Croll et al., 2001; Nowacek et al.; 2004; Madsen et al.,
2006; Yazvenko et al., 2007). A determination of whether foraging disruptions adversely
affect fitness would require information on or estimates of the energetic requirements of

the affected individuals and the relationship between prey availability, foraging effort and
success, and the life history stage of the animal.
Visual tracking, passive acoustic monitoring (PAM), and movement recording
tags were used to quantify sperm whale behavior prior to, during, and following exposure
to airgun arrays at received levels in the range of 140-160 dB and distances of 7-13 km,
following a phase-in of sound intensity and full array exposures at 1-13 km (Madsen et
al., 2006; Miller et al., 2009). Sperm whales did not exhibit horizontal avoidance
behavior at the surface. However, foraging behavior may have been affected. The sperm
whales exhibited 19 percent less vocal, or buzz, rate during full exposure relative to post
exposure, and the whale that was approached most closely had an extended resting period
and did not resume foraging until the airguns ceased firing. The remaining whales
continued to execute foraging dives throughout exposure; however, swimming
movements during foraging dives were 6 percent lower during exposure than they were
during control periods (Miller et al., 2009). These data raise concerns that seismic
surveys may impact foraging behavior in sperm whales, although more data is required to
understand whether the differences were due to exposure or natural variation in sperm
whale behavior (Miller et al., 2009).
Changes in respiration naturally vary with different behaviors and alterations to
breathing rate as a function of acoustic exposure can be expected to co-occur with other
behavioral reactions, such as a flight response or an alteration in diving. However,
respiration rates in and of themselves may be representative of annoyance or an acute
stress response. Various studies have shown that respiration rates may either be
unaffected or could increase, depending on the species and signal characteristics, again
highlighting the importance in understanding species differences in the tolerance of
underwater noise when determining the potential for impacts resulting from

anthropogenic sound exposure (e.g., Kastelein et al., 2001, 2005, 2006; Gailey et al.,
2007, 2016).
Marine mammals vocalize for different purposes and across multiple modes, such
as whistling, echolocation click production, calling, and singing. Changes in vocalization
behavior in response to anthropogenic noise can occur for any of these modes and may
result from a need to compete with an increase in background noise or may reflect
increased vigilance or a startle response. For example, in the presence of potentially
masking signals, humpback whales and killer whales have been observed to increase the
length of their songs or amplitude of calls (Miller et al., 2000; Fristrup et al., 2003; Foote
et al., 2004; Holt et al., 2012), while right whales have been observed to shift the
frequency content of their calls upward while reducing the rate of calling in areas of
increased anthropogenic noise (Parks et al., 2007). In some cases, animals may cease
sound production during production of aversive signals (Bowles et al., 1994).
Cerchio et al. (2014) used PAM to document the presence of singing humpback
whales off the coast of northern Angola and to opportunistically test for the effect of
seismic survey activity on the number of singing whales. Two recording units were
deployed between March and December 2008 in the offshore environment; numbers of
singers were counted every hour. Generalized Additive Mixed Models were used to
assess the effect of survey day (seasonality), hour (diel variation), moon phase, and
received levels of noise (measured from a single pulse during each 10 minutes sampled
period) on singer number. The number of singers significantly decreased with increasing
received level of noise, suggesting that humpback whale communication was disrupted to
some extent by the survey activity.
Castellote et al. (2012) reported acoustic and behavioral changes by fin whales in
response to shipping and airgun noise. Acoustic features of fin whale song notes recorded
in the Mediterranean Sea and northeast Atlantic Ocean were compared for areas with

different shipping noise levels and traffic intensities and during a seismic airgun survey.
During the first 72 hours of the survey, a steady decrease in song received levels and
bearings to singers indicated that whales moved away from the acoustic source and out of
the study area. This displacement persisted for a time period well beyond the 10-day
duration of seismic airgun activity, providing evidence that fin whales may avoid an area
for an extended period in the presence of increased noise. The authors hypothesize that
fin whale acoustic communication is modified to compensate for increased background
noise and that a sensitization process may play a role in the observed temporary
displacement.
Seismic pulses at average received levels of 131 dB re 1 μPa2-s caused blue
whales to increase call production (Di Iorio and Clark, 2010). In contrast, McDonald et
al. (1995) tracked a blue whale with seafloor seismometers and reported that it stopped
vocalizing and changed its travel direction at a range of 10 km from the acoustic source
vessel (estimated received level 143 dB pk-pk). Blackwell et al. (2013) found that
bowhead whale call rates dropped significantly at onset of airgun use at sites with a
median distance of 41-45 km from the survey. Blackwell et al. (2015) expanded this
analysis to show that whales actually increased calling rates as soon as airgun signals
were detectable before ultimately decreasing calling rates at higher received levels (i.e.,
10-minute cumulative sound exposure level (SELcum) of ~127 dB). Overall, these results
suggest that bowhead whales may adjust their vocal output in an effort to compensate for
noise before ceasing vocalization effort and ultimately deflecting from the acoustic
source (Blackwell et al., 2013, 2015). These studies demonstrate that even low levels of
noise received far from the source can induce changes in vocalization and/or behavior for
mysticetes.
Avoidance is the displacement of an individual from an area or migration path as
a result of the presence of sound or other stressors, and is one of the most obvious

manifestations of disturbance in marine mammals (Richardson et al., 1995). For example,
gray whales are known to change direction - deflecting from customary migratory paths in order to avoid noise from seismic surveys (Malme et al., 1984). Humpback whales
show avoidance behavior in the presence of an active seismic array during observational
studies and controlled exposure experiments in western Australia (McCauley et al.,
2000). Avoidance may be short-term, with animals returning to the area once the noise
has ceased (e.g., Bowles et al., 1994; Goold, 1996; Stone et al., 2000; Morton and
Symonds, 2002; Gailey et al., 2007). Longer-term displacement is possible, however,
which may lead to changes in abundance or distribution patterns of the affected species in
the affected region if habituation to the presence of the sound does not occur (e.g., Bejder
et al., 2006; Teilmann et al., 2006).
Forney et al. (2017) detail the potential effects of noise on marine mammal
populations with high site fidelity, including displacement and auditory masking, noting
that a lack of observed response does not imply absence of fitness costs and that apparent
tolerance of disturbance may have population-level impacts that are less obvious and
difficult to document. Avoidance of spatiotemporal overlap between disturbing noise and
areas and/or times of particular importance for sensitive species may be critical to
avoiding population-level impacts because (particularly for animals with high site
fidelity) there may be a strong motivation to remain in the area despite negative impacts.
Forney et al. (2017) state that, for these animals, remaining in a disturbed area may
reflect a lack of alternatives rather than a lack of effects.
Forney et al. (2017) specifically discuss beaked whales, stating that until recently
most knowledge of beaked whales was derived from strandings, as they have been
involved in atypical mass stranding events associated with mid-frequency active sonar
(MFAS) training operations. Given these observations and recent research, beaked
whales appear to be particularly sensitive and vulnerable to certain types of acoustic

disturbance relative to most other marine mammal species. Individual beaked whales
reacted strongly to experiments using simulated MFAS at low received levels, by moving
away from the sound source and stopping foraging for extended periods. These
responses, if on a frequent basis, could result in significant fitness costs to individuals
(Forney et al., 2017). Additionally, difficulty in detection of beaked whales due to their
cryptic surfacing behavior and silence when near the surface pose problems for
mitigation measures employed to protect beaked whales. Forney et al. (2017) specifically
states that failure to consider both displacement of beaked whales from their habitat and
noise exposure could lead to more severe biological consequences.
A flight response is a dramatic change in normal movement to a directed and
rapid movement away from the perceived location of a sound source. The flight response
differs from other avoidance responses in the intensity of the response (e.g., directed
movement, rate of travel). Relatively little information on flight responses of marine
mammals to anthropogenic signals exist, although observations of flight responses to the
presence of predators have occurred (Connor and Heithaus, 1996). The result of a flight
response could range from brief, temporary exertion and displacement from the area
where the signal provokes flight to, in extreme cases, marine mammal strandings (Evans
and England, 2001). However, it should be noted that response to a perceived predator
does not necessarily invoke flight (Ford and Reeves, 2008), and whether individuals are
alone or in groups may influence the response.
Behavioral disturbance can also impact marine mammals in more subtle ways.
Increased vigilance may result in costs related to diversion of focus and attention (i.e.,
when a response consists of increased vigilance, it may come at the cost of decreased
attention to other critical behaviors such as foraging or resting). These effects have
generally not been demonstrated for marine mammals, but studies involving fish and
terrestrial animals have shown that increased vigilance may substantially reduce feeding

rates (e.g., Beauchamp and Livoreil, 1997; Fritz et al., 2002; Purser and Radford, 2011).
In addition, chronic disturbance can cause population declines through reduction of
fitness (e.g., decline in body condition) and subsequent reduction in reproductive success,
survival, or both (e.g., Harrington and Veitch, 1992; Daan et al., 1996; Bradshaw et al.,
1998). However, Ridgway et al. (2006) reported that increased vigilance in bottlenose
dolphins exposed to sound over a 5-day period did not cause any sleep deprivation or
stress effects.
Many animals perform vital functions, such as feeding, resting, traveling, and
socializing, on a diel cycle (24-hour cycle). Disruption of such functions resulting from
reactions to stressors, such as sound exposure, are more likely to be significant if they last
more than one diel cycle or recur on subsequent days (Southall et al., 2007).
Consequently, a behavioral response lasting less than 1 day and not recurring on
subsequent days is not considered particularly severe unless it could directly affect
reproduction or survival (Southall et al., 2007). Note that there is a difference between
multi-day substantive behavioral reactions and multi-day anthropogenic activities. For
example, just because an activity lasts for multiple days does not necessarily mean that
individual animals are either exposed to activity-related stressors for multiple days or,
further, exposed in a manner resulting in sustained multi-day substantive behavioral
responses.
Stone (2015) reported data from at-sea observations during 1,196 seismic surveys
from 1994 to 2010. When large arrays of airguns (considered to be 500 in3 or more in that
study) were firing, lateral displacement, more localized avoidance, or other changes in
behavior were evident for most odontocetes. However, significant responses to large
arrays were found only for the minke whale and fin whale. Behavioral responses
observed included changes in swimming or surfacing behavior, with indications that
cetaceans remained near the water surface at these times. Cetaceans were recorded as

feeding less often when large arrays were active. Behavioral observations of gray whales
during a seismic survey monitored whale movements and respirations pre-, during, and
post-seismic survey (Gailey et al., 2016). Behavioral state and water depth were the best
“natural” predictors of whale movements and respiration and, after considering natural
variation, none of the response variables were significantly associated with seismic
survey or vessel sounds.
Stress Responses — An animal's perception of a threat may be sufficient to trigger
stress responses consisting of some combination of behavioral responses, autonomic
nervous system responses, neuroendocrine responses, or immune responses (e.g., Seyle,
1950; Moberg, 2000). In many cases, an animal's first and sometimes most economical
(in terms of energetic costs) response is behavioral avoidance of the potential stressor.
Autonomic nervous system responses to stress typically involve changes in heart rate,
blood pressure, and gastrointestinal activity. These responses have a relatively short
duration and may or may not have a significant long-term effect on an animal's fitness.
Neuroendocrine stress responses often involve the hypothalamus-pituitary-adrenal
system. Virtually all neuroendocrine functions that are affected by stress - including
immune competence, reproduction, metabolism, and behavior - are regulated by pituitary
hormones. Stress-induced changes in the secretion of pituitary hormones have been
implicated in failed reproduction, altered metabolism, reduced immune competence, and
behavioral disturbance (e.g., Moberg, 1987; Blecha, 2000). Increases in the circulation of
glucocorticoids are also equated with stress (Romano et al., 2004).
The primary distinction between stress (which is adaptive and does not normally
place an animal at risk) and distress is the cost of the response. During a stress response,
an animal uses glycogen stores that can be quickly replenished once the stress is
alleviated. In such circumstances, the cost of the stress response would not pose serious
fitness consequences. However, when an animal does not have sufficient energy reserves

to satisfy the energetic costs of a stress response, energy resources must be diverted from
other functions. This state of distress will last until the animal replenishes its energetic
reserves sufficiently to restore normal function.
Relationships between these physiological mechanisms, animal behavior, and the
costs of stress responses are well-studied through controlled experiments and for both
laboratory and free-ranging animals (e.g., Holberton et al., 1996; Hood et al., 1998;
Jessop et al., 2003; Krausman et al., 2004; Lankford et al., 2005). Stress responses due to
exposure to anthropogenic sounds or other stressors and their effects on marine mammals
have also been reviewed (Fair and Becker, 2000; Romano et al., 2002b) and, more rarely,
studied in wild populations (e.g., Romano et al., 2002a). For example, Rolland et al.
(2012) found that noise reduction from reduced ship traffic in the Bay of Fundy was
associated with decreased stress in North Atlantic right whales. These and other studies
lead to a reasonable expectation that some marine mammals will experience
physiological stress responses upon exposure to acoustic stressors and that it is possible
that some of these would be classified as “distress.” In addition, any animal experiencing
TTS would likely also experience stress responses (NRC, 2003).
Auditory Masking — Sound can disrupt behavior through masking, or interfering
with, an animal's ability to detect, recognize, or discriminate between acoustic signals of
interest (e.g., those used for intraspecific communication and social interactions, prey
detection, predator avoidance, navigation) (Richardson et al., 1995; Erbe et al., 2016).
Masking occurs when the receipt of a sound is interfered with by another coincident
sound at similar frequencies and at similar or higher intensity, and may occur whether the
sound is natural (e.g., snapping shrimp, wind, waves, precipitation) or anthropogenic
(e.g., shipping, sonar, seismic exploration) in origin. The ability of a noise source to mask
biologically important sounds depends on the characteristics of both the noise source and
the signal of interest (e.g., signal-to-noise ratio, temporal variability, direction), in

relation to each other and to an animal's hearing abilities (e.g., sensitivity, frequency
range, critical ratios, frequency discrimination, directional discrimination, age or TTS
hearing loss), and existing ambient noise and propagation conditions.
Under certain circumstances, significant masking could disrupt behavioral
patterns, which in turn could affect fitness for survival and reproduction. It is important to
distinguish TTS and PTS, which persist after the sound exposure, from masking, which
occurs during the sound exposure. Because masking (without resulting in TS) is not
associated with abnormal physiological function, it is not considered a physiological
effect, but rather a potential behavioral effect.
The frequency range of the potentially masking sound is important in predicting
any potential behavioral impacts. For example, low-frequency signals may have less
effect on high-frequency echolocation sounds produced by odontocetes but are more
likely to affect detection of mysticete communication calls and other potentially
important natural sounds such as those produced by surf and some prey species. The
masking of communication signals by anthropogenic noise may be considered as a
reduction in the communication space of animals (e.g., Clark et al., 2009) and may result
in energetic or other costs as animals change their vocalization behavior (e.g., Miller et
al., 2000; Foote et al., 2004; Parks et al., 2007; Di Iorio and Clark, 2009; Holt et al.,
2009). Masking may be less in situations where the signal and noise come from different
directions (Richardson et al., 1995), through amplitude modulation of the signal, or
through other compensatory behaviors (Houser and Moore, 2014). Masking can be tested
directly in captive species (e.g., Erbe, 2008), but in wild populations it must be either
modeled or inferred from evidence of masking compensation. There are few studies
addressing real-world masking sounds likely to be experienced by marine mammals in
the wild (e.g., Branstetter et al., 2013).

Masking affects both senders and receivers of acoustic signals and can potentially
have long-term chronic effects on marine mammals at the population level as well as at
the individual level. Low-frequency ambient sound levels have increased by as much as
20 dB (more than three times in terms of SPL) in the world's ocean from pre-industrial
periods, with most of the increase from distant commercial shipping (Hildebrand, 2009).
All anthropogenic sound sources, but especially chronic and lower-frequency signals
(e.g., from vessel traffic), contribute to elevated ambient sound levels, thus intensifying
masking.
Masking effects of pulsed sounds (even from large arrays of airguns) on marine
mammal calls and other natural sounds are expected to be limited, although there is little
specific data on this. Because of the intermittent nature and low duty cycle of seismic
pulses, animals can emit and receive sounds in the relatively quiet intervals between
pulses. However, in exceptional situations, reverberation occurs for much or all of the
interval between pulses (e.g., Simard et al. 2005; Clark and Gagnon 2006), which could
mask calls. Situations with prolonged strong reverberation are infrequent. However, it is
common for reverberation to cause some lesser degree of elevation of the background
level between airgun pulses (e.g., Gedamke 2011; Guerra et al. 2011, 2016; Klinck et al.
2012; Guan et al. 2015), and this weaker reverberation presumably reduces the detection
range of calls and other natural sounds to some degree. Guerra et al. (2016) reported that
ambient noise levels between seismic pulses were elevated as a result of reverberation at
ranges of 50 km from the seismic source. Based on measurements in deep water of the
Southern Ocean, Gedamke (2011) estimated that the slight elevation of background noise
levels during intervals between seismic pulses reduced blue and fin whale
communication space by as much as 36-51 percent when a seismic survey was operating
450-2,800 km away. Based on preliminary modeling, Wittekind et al. (2016) reported
that airgun sounds could reduce the communication range of blue and fin whales 2,000

km from the seismic source. Nieukirk et al. (2012) and Blackwell et al. (2013) noted the
potential for masking effects from seismic surveys on large whales.
Some baleen and toothed whales are known to continue calling in the presence of
seismic pulses, and their calls usually can be heard between the pulses (e.g., Nieukirk et
al. 2012; Thode et al. 2012; Bröker et al. 2013; Sciacca et al. 2016). Cerchio et al. (2014)
suggested that the breeding display of humpback whales off Angola could be disrupted
by seismic sounds, as singing activity declined with increasing received levels. In
addition, some cetaceans are known to change their calling rates, shift their peak
frequencies, or otherwise modify their vocal behavior in response to airgun sounds (e.g.,
Di Iorio and Clark 2010; Castellote et al. 2012; Blackwell et al. 2013, 2015). The hearing
systems of baleen whales are more sensitive to low-frequency sounds than are the ears of
the small odontocetes that have been studied directly (e.g., MacGillivray et al., 2014).
The sounds important to small odontocetes are predominantly at much higher frequencies
than are the dominant components of airgun sounds, thus limiting the potential for
masking. In general, masking effects of seismic pulses are expected to be minor, given
the normally intermittent nature of seismic pulses.
Vessel Noise
Vessel noise from the R/V Langseth could affect marine animals in the proposed
survey areas. Houghton et al. (2015) proposed that vessel speed is the most important
predictor of received noise levels, and Putland et al. (2017) also reported reduced sound
levels with decreased vessel speed. However, some energy is also produced at higher
frequencies (Hermannsen et al., 2014); low levels of high-frequency sound from vessels
has been shown to elicit responses in harbor porpoise (Dyndo et al., 2015).
Vessel noise, through masking, can reduce the effective communication distance
of a marine mammal if the frequency of the sound source is close to that used by the
animal, and if the sound is present for a significant fraction of time (e.g., Richardson et

al. 1995; Clark et al., 2009; Jensen et al., 2009; Gervaise et al., 2012; Hatch et al., 2012;
Rice et al., 2014; Dunlop 2015; Erbe et al., 2015; Jones et al., 2017; Putland et al., 2017).
In addition to the frequency and duration of the masking sound, the strength, temporal
pattern, and location of the introduced sound also play a role in the extent of the masking
(Branstetter et al., 2013, 2016; Finneran and Branstetter 2013; Sills et al., 2017).
Branstetter et al. (2013) reported that time-domain metrics are also important in
describing and predicting masking.
Baleen whales are thought to be more sensitive to sound at these low frequencies
than are toothed whales (e.g., MacGillivray et al. 2014), possibly causing localized
avoidance of the proposed survey area during seismic operations. Many odontocetes
show considerable tolerance of vessel traffic, although they sometimes react at long
distances if confined by ice or shallow water, if previously harassed by vessels, or have
had little or no recent exposure to vessels (Richardson et al. 1995). Pirotta et al. (2015)
noted that the physical presence of vessels, not just ship noise, disturbed the foraging
activity of bottlenose dolphins. There is little data on the behavioral reactions of beaked
whales to vessel noise, though they seem to avoid approaching vessels (e.g., Würsig et
al., 1998) or dive for an extended period when approached by a vessel (e.g., Kasuya
1986).
In summary, project vessel sounds would not be at levels expected to cause
anything more than possible localized and temporary behavioral changes in marine
mammals, and would not be expected to result in significant negative effects on
individuals or at the population level. In addition, in all oceans of the world, large vessel
traffic is currently so prevalent that it is commonly considered a usual source of ambient
sound (NSF-USGS 2011).
Vessel Strike

Vessel collisions with marine mammals, or vessel strikes, can result in death or
serious injury of the animal. Wounds resulting from vessel strike may include massive
trauma, hemorrhaging, broken bones, or propeller lacerations (Knowlton and Kraus,
2001). An animal at the surface may be struck directly by a vessel, a surfacing animal
may hit the bottom of a vessel, or an animal just below the surface may be cut by a
vessel’s propeller. Superficial strikes may not kill or result in the death of the animal.
These interactions are typically associated with large whales (e.g., fin whales), which are
occasionally found draped across the bulbous bow of large commercial vessels upon
arrival in port. Although smaller cetaceans are more maneuverable in relation to large
vessels than are large whales, they may also be susceptible to vessel strikes. The severity
of injuries typically depends on the size and speed of the vessel, with the probability of
death or serious injury increasing as vessel speed increases (Knowlton and Kraus, 2001;
Laist et al., 2001; Vanderlaan and Taggart, 2007; Conn and Silber, 2013). Impact forces
increase with speed, as does the probability of a strike at a given distance (Silber et al.,
2010; Gende et al., 2011).
Pace and Silber (2005) also found that the probability of death or serious injury
increased rapidly with increasing vessel speed. Specifically, the predicted probability of
serious injury or death increased from 45 to 75 percent as vessel speed increased from 10
to 14 knots (kn (26 kilometer per hour (kph)), and exceeded 90 percent at 17 kn (31 kph).
Higher speeds during collisions result in greater force of impact, but higher speeds also
appear to increase the chance of severe injuries or death through increased likelihood of
collision by pulling whales toward the vessel (Clyne, 1999; Knowlton et al., 1995). In a
separate study, Vanderlaan and Taggart (2007) analyzed the probability of lethal
mortality of large whales at a given speed, showing that the greatest rate of change in the
probability of a lethal injury to a large whale as a function of vessel speed occurs between
8.6 and 15 kn (28 kph). The chances of a lethal injury decline from approximately 80

percent at 15 kn (28 kph) to approximately 20 percent at 8.6 kn (16 kph). At speeds
below 11.8 kn (22 kph), the chances of lethal injury drop below 50 percent, while the
probability asymptotically increases toward 100 percent above 15 kn (28 kph).
The R/V Langseth will travel at a speed of 5 kn (9 kph) while towing seismic
survey gear. At this speed, both the possibility of striking a marine mammal and the
possibility of a strike resulting in serious injury or mortality are discountable. At average
transit speed, the probability of serious injury or mortality resulting from a strike is less
than 50 percent. However, the likelihood of a strike actually happening is again
discountable. Vessel strikes, as analyzed in the studies cited above, generally involve
commercial shipping, which is much more common in both space and time than is
geophysical survey activity. Jensen and Silber (2004) summarized vessel strikes of large
whales worldwide from 1975-2003 and found that most collisions occurred in the open
ocean and involved large vessels (e.g., commercial shipping). No such incidents were
reported for geophysical survey vessels during that time period.
It is possible for vessel strikes to occur while traveling at slow speeds. For
example, a hydrographic survey vessel traveling at low speed (5.5 kn (10 kph)) while
conducting mapping surveys off the central California coast struck and killed a blue
whale in 2009. The State of California determined that the whale had suddenly and
unexpectedly surfaced beneath the hull, with the result that the propeller severed the
whale’s vertebrae, and that this was an unavoidable event. This strike represents the only
such incident in approximately 540,000 hours of similar coastal mapping activity (p = 1.9
x 10-6; 95 percent confidence interval = 0-5.5 x 10-6; NMFS, 2013). In addition, a
research vessel reported a fatal strike in 2011 of a dolphin in the Atlantic, demonstrating
that it is possible for strikes involving smaller cetaceans to occur. In that case, the
incident report indicated that an animal apparently was struck by the vessel’s propeller as
the animal was intentionally swimming near the vessel. While indicative of the type of

unusual events that cannot be ruled out, neither of these instances represents a
circumstance that would be considered reasonably foreseeable or that would be
considered preventable.
Although the likelihood of the vessel striking a marine mammal is low, we
propose a robust vessel strike avoidance protocol (see Proposed Mitigation), which we
believe eliminates any foreseeable risk of vessel strike during transit. We anticipate that
vessel collisions involving a seismic data acquisition vessel towing gear, while not
impossible, represent unlikely, unpredictable events for which there are no preventive
measures. Given the proposed mitigation measures, the relatively slow speed of the
vessel towing gear, the presence of bridge crew watching for obstacles at all times
(including marine mammals), and the presence of marine mammal observers, the
possibility of vessel strike is discountable and, further, were a strike of a large whale to
occur, it would be unlikely to result in serious injury or mortality. No incidental take
resulting from vessel strike is anticipated, and this potential effect of the specified activity
will not be discussed further in the following analysis.
Stranding – When a living or dead marine mammal swims or floats onto shore
and becomes “beached” or incapable of returning to sea, the event is a “stranding”
(Geraci et al., 1999; Perrin and Geraci, 2002; Geraci and Lounsbury, 2005; NMFS,
2007). The legal definition for a stranding under the MMPA is that a marine mammal is
dead and is on a beach or shore of the United States; or in waters under the jurisdiction of
the United States (including any navigable waters); or a marine mammal is alive and is
on a beach or shore of the United States and is unable to return to the water; on a beach or
shore of the United States and, although able to return to the water, is in need of apparent
medical attention; or in the waters under the jurisdiction of the United States (including
any navigable waters), but is unable to return to its natural habitat under its own power or
without assistance.

Marine mammals strand for a variety of reasons, such as infectious agents,
biotoxicosis, starvation, fishery interaction, vessel strike, unusual oceanographic or
weather events, sound exposure, or combinations of these stressors sustained
concurrently or in series. However, the cause or causes of most strandings are unknown
(Geraci et al., 1976; Eaton, 1979; Odell et al., 1980; Best, 1982). Numerous studies
suggest that the physiology, behavior, habitat relationships, age, or condition of cetaceans
may cause them to strand or might predispose them to strand when exposed to another
phenomenon. These suggestions are consistent with the conclusions of numerous other
studies that have demonstrated that combinations of dissimilar stressors commonly
combine to kill an animal or dramatically reduce its fitness, even though one exposure
without the other does not produce the same result (Chroussos, 2000; Creel, 2005;
DeVries et al., 2003; Fair and Becker, 2000; Foley et al., 2001; Moberg, 2000; Relyea,
2005a; 2005b, Romero, 2004; Sih et al., 2004).
There is no conclusive evidence that exposure to airgun noise results in
behaviorally-mediated forms of injury. Behaviorally-mediated injury (i.e., mass stranding
events) has been primarily associated with beaked whales exposed to mid-frequency
active (MFA) naval sonar. MFA sonar and the alerting stimulus used in Nowacek et al.
(2004) are very different from the noise produced by airguns. As explained below,
military MFA sonar is very different from airguns, and one should not assume that
airguns will cause the same effects as MFA sonar (including strandings).
To understand why military MFA sonar affects beaked whales differently than
airguns do, it is important to note the distinction between behavioral sensitivity and
susceptibility to auditory injury. To understand the potential for auditory injury in a
particular marine mammal species in relation to a given acoustic signal, the frequency
range the species is able to hear is critical, as well as the species’ auditory sensitivity to
frequencies within that range. Current data indicate that not all marine mammal species

have equal hearing capabilities across all frequencies and, therefore, species are grouped
into hearing groups with generalized hearing ranges assigned on the basis of available
data (Southall et al., 2007, 2019). Hearing ranges as well as auditory
sensitivity/susceptibility to frequencies within those ranges vary across the different
groups. For example, in terms of hearing range, the high-frequency cetaceans (e.g., Kogia
spp.) have a generalized hearing range of frequencies between 275 Hz and 160 kHz,
while mid-frequency cetaceans—such as dolphins and beaked whales—have a
generalized hearing range between 150 Hz to 160 kHz. Regarding auditory susceptibility
within the hearing range, while mid-frequency cetaceans and high-frequency cetaceans
have roughly similar hearing ranges, the high-frequency group is much more susceptible
to noise-induced hearing loss during sound exposure, i.e., these species have lower
thresholds for these effects than other hearing groups (NMFS, 2018). Referring to a
species as behaviorally sensitive to noise simply means that an animal of that species is
more likely to respond to lower received levels of sound than an animal of another
species that is considered less behaviorally sensitive. So, while dolphin species and
beaked whale species—both in the mid-frequency cetacean hearing group—are assumed
to generally hear the same sounds equally well and be equally susceptible to noiseinduced hearing loss (auditory injury), the best available information indicates that a
beaked whale is more likely to behaviorally respond to that sound at a lower received
level compared to an animal from other mid-frequency cetacean species that are less
behaviorally sensitive. This distinction is important because, while beaked whales are
more likely to respond behaviorally to sounds than are many other species (even at lower
levels), they cannot hear the predominant, lower frequency sounds from seismic airguns
as well as sounds that have more energy at frequencies that beaked whales can hear better
(such as military MFA sonar).

Military MFA sonar affects beaked whales differently than airguns do because it
produces energy at different frequencies than airguns. Mid-frequency cetacean hearing is
generically thought to be best between 8.8 to 110 kHz, i.e., these cutoff values define the
range above and below which a species in the group is assumed to have declining
auditory sensitivity, until reaching frequencies that cannot be heard (NMFS, 2018).
However, beaked whale hearing is likely best within a higher, narrower range (20-80
kHz, with best sensitivity around 40 kHz), based on a few measurements of hearing in
stranded beaked whales (Cook et al., 2006; Finneran et al., 2009; Pacini et al., 2011) and
several studies of acoustic signals produced by beaked whales (e.g., Frantzis et al., 2002;
Johnson et al., 2004, 2006; Zimmer et al., 2005). While precaution requires that the full
range of audibility be considered when assessing risks associated with noise exposure
(Southall et al., 2007, 2019), animals typically produce sound at frequencies where they
hear best. More recently, Southall et al. (2019) suggested that certain species in the
historical mid-frequency hearing group (beaked whales, sperm whales, and killer whales)
are likely more sensitive to lower frequencies within the group’s generalized hearing
range than are other species within the group, and state that the data for beaked whales
suggest sensitivity to approximately 5 kHz. However, this information is consistent with
the general conclusion that beaked whales (and other mid-frequency cetaceans) are
relatively insensitive to the frequencies where most energy of an airgun signal is found.
Military MFA sonar is typically considered to operate in the frequency range of
approximately 3-14 kHz (D’Amico et al., 2009), i.e., outside the range of likely best
hearing for beaked whales but within or close to the lower bounds, whereas most energy
in an airgun signal is radiated at much lower frequencies, below 500 Hz (Dragoset,
1990).
It is important to distinguish between energy (loudness, measured in dB) and
frequency (pitch, measured in Hz). In considering the potential impacts of mid-frequency

components of airgun noise (1-10 kHz, where beaked whales can be expected to hear) on
marine mammal hearing, one needs to account for the energy associated with these higher
frequencies and determine what energy is truly “significant.” Although there is midfrequency energy associated with airgun noise (as expected from a broadband source),
airgun sound is predominantly below 1 kHz (Breitzke et al., 2008; Tashmukhambetov et
al., 2008; Tolstoy et al., 2009). As stated by Richardson et al. (1995), “[…] most emitted
[seismic airgun] energy is at 10-120 Hz, but the pulses contain some energy up to 5001,000 Hz.” Tolstoy et al. (2009) conducted empirical measurements, demonstrating that
sound energy levels associated with airguns were at least 20 dB lower at 1 kHz
(considered “mid-frequency”) compared to higher energy levels associated with lower
frequencies (below 300 Hz) (“all but a small fraction of the total energy being
concentrated in the 10-300 Hz range” [Tolstoy et al., 2009]), and at higher frequencies
(e.g., 2.6-4 kHz), power might be less than 10 percent of the peak power at 10 Hz (Yoder,
2002). Energy levels measured by Tolstoy et al. (2009) were even lower at frequencies
above 1 kHz. In addition, as sound propagates away from the source, it tends to lose
higher-frequency components faster than low-frequency components (i.e., low-frequency
sounds typically propagate longer distances than high-frequency sounds) (Diebold et al.,
2010). Although higher-frequency components of airgun signals have been recorded, it is
typically in surface-ducting conditions (e.g., DeRuiter et al., 2006; Madsen et al., 2006)
or in shallow water, where there are advantageous propagation conditions for the higher
frequency (but low-energy) components of the airgun signal (Hermannsen et al., 2015).
This should not be of concern because the likely behavioral reactions of beaked whales
that can result in acute physical injury would result from noise exposure at depth
(because of the potentially greater consequences of severe behavioral reactions). In
summary, the frequency content of airgun signals is such that beaked whales will not be

able to hear the signals well (compared to MFA sonar), especially at depth where we
expect the consequences of noise exposure could be more severe.
Aside from frequency content, there are other significant differences between
MFA sonar signals and the sounds produced by airguns that minimize the risk of severe
behavioral reactions that could lead to strandings or deaths at sea, e.g., significantly
longer signal duration, horizontal sound direction, typical fast and unpredictable source
movement. All of these characteristics of MFA sonar tend towards greater potential to
cause severe behavioral or physiological reactions in exposed beaked whales that may
contribute to stranding. Although both sources are powerful, MFA sonar contains
significantly greater energy in the mid-frequency range, where beaked whales hear better.
Short-duration, high energy pulses—such as those produced by airguns—have greater
potential to cause damage to auditory structures (though this is unlikely for midfrequency cetaceans, as explained later in this document), but it is longer duration signals
that have been implicated in the vast majority of beaked whale strandings. Faster, less
predictable movements in combination with multiple source vessels are more likely to
elicit a severe, potentially anti-predator response. Of additional interest in assessing the
divergent characteristics of MFA sonar and airgun signals and their relative potential to
cause stranding events or deaths at sea is the similarity between the MFA sonar signals
and stereotyped calls of beaked whales’ primary predator: the killer whale (Zimmer and
Tyack, 2007). Although generic disturbance stimuli—as airgun noise may be considered
in this case for beaked whales—may also trigger antipredator responses, stronger
responses should generally be expected when perceived risk is greater, as when the
stimulus is confused for a known predator (Frid and Dill, 2002). In addition, because the
source of the perceived predator (i.e., MFA sonar) will likely be closer to the whales
(because attenuation limits the range of detection of mid-frequencies) and moving faster
(because it will be on faster-moving vessels), any antipredator response would be more

likely to be severe (with greater perceived predation risk, an animal is more likely to
disregard the cost of the response; Frid and Dill, 2002). Indeed, when analyzing
movements of a beaked whale exposed to playback of killer whale predation calls, Allen
et al. (2014) found that the whale engaged in a prolonged, directed avoidance response,
suggesting a behavioral reaction that could pose a risk factor for stranding. Overall, these
significant differences between sound from MFA sonar and the mid-frequency sound
component from airguns and the likelihood that MFA sonar signals will be interpreted in
error as a predator are critical to understanding the likely risk of behaviorally-mediated
injury due to seismic surveys.
The available scientific literature also provides a useful contrast between airgun
noise and MFA sonar regarding the likely risk of behaviorally-mediated injury. There is
strong evidence for the association of beaked whale stranding events with MFA sonar
use, and particularly detailed accounting of several events is available (e.g., a 2000
Bahamas stranding event for which investigators concluded that MFA sonar use was
responsible; Evans and England, 2001). D’Amico et al., (2009) reviewed 126 beaked
whale mass stranding events over the period from 1950 (i.e., from the development of
modern MFA sonar systems) through 2004. Of these, there were two events where
detailed information was available on both the timing and location of the stranding and
the concurrent nearby naval activity, including verification of active MFA sonar usage,
with no evidence for an alternative cause of stranding. An additional 10 events were at
minimum spatially and temporally coincident with naval activity likely to have included
MFA sonar use and, despite incomplete knowledge of timing and location of the
stranding or the naval activity in some cases, there was no evidence for an alternative
cause of stranding. The U.S. Navy has publicly stated agreement that five such events
since 1996 were associated in time and space with MFA sonar use, either by the U.S.
Navy alone or in joint training exercises with the North Atlantic Treaty Organization. The

U.S. Navy additionally noted that, as of 2017, a 2014 beaked whale stranding event in
Crete coincident with naval exercises was under review and had not yet been determined
to be linked to sonar activities (U.S. Navy, 2017). Separately, the International Council
for the Exploration of the Sea reported in 2005 that, worldwide, there have been about 50
known strandings, consisting mostly of beaked whales, with a potential causal link to
MFA sonar (ICES, 2005). In contrast, very few such associations have been made to
seismic surveys, despite widespread use of airguns as a geophysical sound source in
numerous locations around the world.
A review of possible stranding associations with seismic surveys (Castellote and
Llorens, 2016) states that, “[s]peculation concerning possible links between seismic
survey noise and cetacean strandings is available for a dozen events but without
convincing causal evidence.” The authors’ search of available information found 10
events worth further investigation via a ranking system representing a rough metric of the
relative level of confidence offered by the data for inferences about the possible role of
the seismic survey in a given stranding event. Only three of these events involved beaked
whales. Whereas D’Amico et al., (2009) used a 1-5 ranking system, in which “1”
represented the most robust evidence connecting the event to MFA sonar use, Castellote
and Llorens (2016) used a 1-6 ranking system, in which “6” represented the most robust
evidence connecting the event to the seismic survey. As described above, D’Amico et al.
(2009) found that two events were ranked “1” and 10 events were ranked “2” (i.e., 12
beaked whale stranding events were found to be associated with MFA sonar use). In
contrast, Castellote and Llorens (2016) found that none of the three beaked whale
stranding events achieved their highest ranks of 5 or 6. Of the 10 total events, none
achieved the highest rank of 6. Two events were ranked as 5: one stranding in Peru
involving dolphins and porpoises and a 2008 stranding in Madagascar. This latter ranking
can only be broadly associated with the survey itself, as opposed to use of seismic

airguns. An investigation of this stranding event, which did not involve beaked whales,
concluded that use of a high-frequency mapping system (12-kHz multibeam
echosounder) was the most plausible and likely initial behavioral trigger of the event,
which was likely exacerbated by several site- and situation-specific secondary factors.
The review panel found that seismic airguns were used after the initial strandings and
animals entering a lagoon system, that airgun use clearly had no role as an initial trigger,
and that there was no evidence that airgun use dissuaded animals from leaving (Southall
et al., 2013).
However, one of these stranding events, involving two Cuvier’s beaked whales,
was contemporaneous with and reasonably associated spatially with a 2002 seismic
survey in the Gulf of California conducted by L-DEO, as was the case for the 2007 Gulf
of Cadiz seismic survey discussed by Castellote and Llorens (also involving two Cuvier’s
beaked whales). Neither event was considered a “true atypical mass stranding”
(according to Frantzis (1998)) as used in the analysis of Castellote and Llorens (2016).
While we agree with the authors that this lack of evidence should not be considered
conclusive, it is clear that there is very little evidence that seismic surveys should be
considered as posing a significant risk of acute harm to beaked whales or other midfrequency cetaceans. We have considered the potential for the proposed surveys to result
in marine mammal stranding and, based on the best available information, do not expect a
stranding to occur.
Entanglement – Entanglements occur when marine mammals become wrapped
around cables, lines, nets, or other objects suspended in the water column. During seismic
operations, numerous cables, lines, and other objects primarily associated with the airgun
array and hydrophone streamers will be towed behind the R/V Langseth near the water’s
surface. However, we are not aware of any cases of entanglement of marine mammals in
seismic survey equipment. No incidents of entanglement of marine mammals with

seismic survey gear have been documented in over 54,000 nautical miles (100,000 km) of
previous NSF-funded seismic surveys when observers were aboard (e.g., Smultea and
Holst 2003; Haley and Koski 2004; Holst 2004; Smultea et al., 2004; Holst et al., 2005a;
Haley and Ireland 2006; SIO and NSF 2006b; Hauser et al., 2008; Holst and Smultea
2008). Although entanglement with the streamer is theoretically possible, it has not been
documented during tens of thousands of miles of NSF-sponsored seismic cruises or, to
our knowledge, during hundreds of thousands of miles of industrial seismic cruises.
There are relatively few deployed devices, and no interaction between marine mammals
and any such device has been recorded during prior NSF surveys using the devices. There
are no meaningful entanglement risks posed by the proposed survey, and entanglement
risks are not discussed further in this document.
Anticipated Effects on Marine Mammal Habitat
Physical Disturbance – Sources of seafloor disturbance related to geophysical
surveys that may impact marine mammal habitat include placement of anchors, nodes,
cables, sensors, or other equipment on or in the seafloor for various activities. Equipment
deployed on the seafloor has the potential to cause direct physical damage and could
affect bottom-associated fish resources. During this survey, OBSs would be deployed on
the seafloor, secured with anchors that would eventually disintegrate on the seafloor.
Placement of equipment could damage areas of hard bottom where direct contact
with the seafloor occurs and could crush epifauna (organisms that live on the seafloor or
surface of other organisms). Damage to unknown or unseen hard bottom could occur, but
because of the small area covered by most bottom-founded equipment and the patchy
distribution of hard bottom habitat, contact with unknown hard bottom is expected to be
rare and impacts minor. Seafloor disturbance in areas of soft bottom can cause loss of
small patches of epifauna and infauna due to burial or crushing, and bottom-feeding

fishes could be temporarily displaced from feeding areas. Overall, any effects of physical
damage to habitat are expected to be minor and temporary.
Effects to Prey – Marine mammal prey varies by species, season, and location
and, for some, is not well documented. Fish react to sounds which are especially strong
and/or intermittent low-frequency sounds, and behavioral responses such as flight or
avoidance are the most likely effects. However, the reaction of fish to airguns depends on
the physiological state of the fish, past exposures, motivation (e.g., feeding, spawning,
migration), and other environmental factors. Several studies have demonstrated that
airgun sounds might affect the distribution and behavior of some fishes, potentially
impacting marine mammal foraging opportunities or increasing energetic costs (e.g.,
Fewtrell and McCauley, 2012; Pearson et al., 1992; Skalski et al., 1992; Santulli et al.,
1999; Paxton et al., 2017), though the bulk of studies indicate no or slight reaction to
noise (e.g., Miller and Cripps, 2013; Dalen and Knutsen, 1987; Pena et al., 2013;
Chapman and Hawkins, 1969; Wardle et al., 2001; Sara et al., 2007; Jorgenson and
Gyselman, 2009; Blaxter et al., 1981; Cott et al., 2012; Boeger et al., 2006), and that,
most commonly, while there are likely to be impacts to fish as a result of noise from
nearby airguns, such effects will be temporary. For example, investigators reported
significant, short-term declines in commercial fishing catch rate of gadid fishes during
and for up to 5 days after seismic survey operations, but the catch rate subsequently
returned to normal (Engas et al., 1996; Engas and Lokkeborg, 2002). Other studies have
reported similar findings (Hassel et al., 2004).
Skalski et al., (1992) also found a reduction in catch rates—for rockfish (Sebastes
spp.) in response to controlled airgun exposure—but suggested that the mechanism
underlying the decline was not dispersal but rather decreased responsiveness to baited
hooks associated with an alarm behavioral response. A companion study showed that
alarm and startle responses were not sustained following the removal of the sound source

(Pearson et al., 1992). Therefore, Skalski et al. (1992) suggested that the effects on fish
abundance may be transitory, primarily occurring during the sound exposure itself. In
some cases, effects on catch rates are variable within a study, which may be more broadly
representative of temporary displacement of fish in response to airgun noise (i.e., catch
rates may increase in some locations and decrease in others) than any long-term damage
to the fish themselves (Streever et al., 2016).
Sound pressure levels of sufficient strength have been known to cause injury to
fish and fish mortality and, in some studies, fish auditory systems have been damaged by
airgun noise (McCauley et al., 2003; Popper et al., 2005; Song et al., 2008). However, in
most fish species, hair cells in the ear continuously regenerate and loss of auditory
function likely is restored when damaged cells are replaced with new cells. Halvorsen et
al. (2012) showed that a TTS of 4-6 dB was recoverable within 24 hours for one species.
Impacts would be most severe when the individual fish is close to the source and when
the duration of exposure is long; both of which are conditions unlikely to occur for this
survey that is necessarily transient in any given location and likely result in brief,
infrequent noise exposure to prey species in any given area. For this survey, the sound
source is constantly moving, and most fish would likely avoid the sound source prior to
receiving sound of sufficient intensity to cause physiological or anatomical damage. In
addition, ramp-up may allow certain fish species the opportunity to move further away
from the sound source.
A comprehensive review (Carroll et al., 2017) found that results are mixed as to
the effects of airgun noise on the prey of marine mammals. While some studies suggest a
change in prey distribution and/or a reduction in prey abundance following the use of
seismic airguns, others suggest no effects or even positive effects in prey abundance. As
one specific example, Paxton et al. (2017), which describes findings related to the effects
of a 2014 seismic survey on a reef off of North Carolina, showed a 78 percent decrease in

observed nighttime abundance for certain species. It is important to note that the evening
hours during which the decline in fish habitat use was recorded (via video recording)
occurred on the same day that the seismic survey passed, and no subsequent data is
presented to support an inference that the response was long-lasting. Additionally, given
that the finding is based on video images, the lack of recorded fish presence does not
support a conclusion that the fish actually moved away from the site or suffered any
serious impairment. In summary, this particular study corroborates prior studies
indicating that a startle response or short-term displacement should be expected.
Available data suggest that cephalopods are capable of sensing the particle motion
of sounds and detect low frequencies up to 1-1.5 kHz, depending on the species, and so
are likely to detect airgun noise (Kaifu et al., 2008; Hu et al., 2009; Mooney et al., 2010;
Samson et al., 2014). Auditory injuries (lesions occurring on the statocyst sensory hair
cells) have been reported upon controlled exposure to low-frequency sounds, suggesting
that cephalopods are particularly sensitive to low-frequency sound (Andre et al., 2011;
Sole et al., 2013). Behavioral responses, such as inking and jetting, have also been
reported upon exposure to low-frequency sound (McCauley et al., 2000b; Samson et al.,
2014). Similar to fish, however, the transient nature of the survey leads to an expectation
that effects will be largely limited to behavioral reactions and would occur as a result of
brief, infrequent exposures.
With regard to potential impacts on zooplankton, McCauley et al. (2017) found
that exposure to airgun noise resulted in significant depletion for more than half the taxa
present and that there were two to three times more dead zooplankton after airgun
exposure compared with controls for all taxa, within 1 km of the airguns. However, the
authors also stated that in order to have significant impacts on r-selected species (i.e.,
those with high growth rates and that produce many offspring) such as plankton, the
spatial or temporal scale of impact must be large in comparison with the ecosystem

concerned, and it is possible that the findings reflect avoidance by zooplankton rather
than mortality (McCauley et al., 2017). In addition, the results of this study are
inconsistent with a large body of research that generally finds limited spatial and
temporal impacts to zooplankton as a result of exposure to airgun noise (e.g., Dalen and
Knutsen, 1987; Payne, 2004; Stanley et al., 2011). Most prior research on this topic,
which has focused on relatively small spatial scales, has showed minimal effects (e.g.,
Kostyuchenko, 1973; Booman et al., 1996; Sætre and Ona, 1996; Pearson et al., 1994;
Bolle et al., 2012).
A modeling exercise was conducted as a follow-up to the McCauley et al. (2017)
study (as recommended by McCauley et al.), in order to assess the potential for impacts
on ocean ecosystem dynamics and zooplankton population dynamics (Richardson et al.,
2017). Richardson et al. (2017) found that for copepods with a short life cycle in a highenergy environment, a full-scale airgun survey would impact copepod abundance up to 3
days following the end of the survey, suggesting that effects such as those found by
McCauley et al. (2017) would not be expected to be detectable downstream of the survey
areas, either spatially or temporally.
Notably, a more recently described study produced results inconsistent with those
of McCauley et al. (2017). Researchers conducted a field and laboratory study to assess if
exposure to airgun noise affects mortality, predator escape response, or gene expression
of the copepod Calanus finmarchicus (Fields et al., 2019). Immediate mortality of
copepods was significantly higher, relative to controls, at distances of 5 m or less from
the airguns. Mortality 1 week after the airgun blast was significantly higher in the
copepods placed 10 m from the airgun but was not significantly different from the
controls at a distance of 20 m from the airgun. The increase in mortality, relative to
controls, did not exceed 30 percent at any distance from the airgun. Moreover, the
authors caution that even this higher mortality in the immediate vicinity of the airguns

may be more pronounced than what would be observed in free-swimming animals due to
increased flow speed of fluid inside bags containing the experimental animals. There
were no sublethal effects on the escape performance or the sensory threshold needed to
initiate an escape response at any of the distances from the airgun that were tested.
Whereas McCauley et al. (2017) reported an SEL of 156 dB at a range of 509-658 m,
with zooplankton mortality observed at that range, Fields et al. (2019) reported an SEL of
186 dB at a range of 25 m, with no reported mortality at that distance. Regardless, if we
assume a worst-case likelihood of severe impacts to zooplankton within approximately 1
km of the acoustic source, the brief time to regeneration of the potentially affected
zooplankton populations does not lead us to expect any meaningful follow-on effects to
the prey base for marine mammals.
A review article concluded that, while laboratory results provide scientific
evidence for high-intensity and low-frequency sound-induced physical trauma and other
negative effects on some fish and invertebrates, the sound exposure scenarios in some
cases are not realistic to those encountered by marine organisms during routine seismic
operations (Carroll et al., 2017). The review finds that there has been no evidence of
reduced catch or abundance following seismic activities for invertebrates, and that there
is conflicting evidence for fish with catch observed to increase, decrease, or remain the
same. Further, where there is evidence for decreased catch rates in response to airgun
noise, these findings provide no information about the underlying biological cause of
catch rate reduction (Carroll et al., 2017).
In summary, impacts of the specified activity on marine mammal prey species
will likely be limited to behavioral responses, the majority of prey species will be capable
of moving out of the area during the survey, a rapid return to normal recruitment,
distribution, and behavior for prey species is anticipated, and, overall, impacts to prey
species will be minor and temporary. Prey species exposed to sound might move away

from the sound source, experience TTS, experience masking of biologically relevant
sounds, or show no obvious direct effects. Mortality from decompression injuries is
possible in close proximity to a sound, but only limited data on mortality in response to
airgun noise exposure are available (Hawkins et al., 2014). The most likely impacts for
most prey species in the survey area would be temporary avoidance of the area. The
proposed survey would move through an area relatively quickly, limiting exposure to
multiple impulsive sounds. In all cases, sound levels would return to ambient once the
survey moves out of the area or ends and the noise source is shut down and, when
exposure to sound ends, behavioral and/or physiological responses are expected to end
relatively quickly (McCauley et al., 2000b). The duration of fish avoidance of a given
area after survey effort stops is unknown, but a rapid return to normal recruitment,
distribution, and behavior is anticipated. While the potential for disruption of spawning
aggregations or schools of important prey species can be meaningful on a local scale, the
mobile and temporary nature of this survey and the likelihood of temporary avoidance
behavior suggest that impacts would be minor.
Acoustic Habitat – Acoustic habitat is the soundscape—which encompasses all of
the sound present in a particular location and time, as a whole—when considered from
the perspective of the animals experiencing it. Animals produce sound for, or listen for
sounds produced by, conspecifics (communication during feeding, mating, and other
social activities), other animals (finding prey or avoiding predators), and the physical
environment (finding suitable habitats, navigating). Together, sounds made by animals
and the geophysical environment (e.g., produced by earthquakes, lightning, wind, rain,
waves) make up the natural contributions to the total acoustics of a place. These acoustic
conditions, termed acoustic habitat, are one attribute of an animal’s total habitat.
Soundscapes are also defined by, and acoustic habitat influenced by, the total
contribution of anthropogenic sound. This may include incidental emissions from sources

such as vessel traffic, or may be intentionally introduced to the marine environment for
data acquisition purposes (as in the use of airgun arrays). Anthropogenic noise varies
widely in its frequency content, duration, and loudness and these characteristics greatly
influence the potential habitat-mediated effects to marine mammals (please see also the
previous discussion on masking under Acoustic Effects), which may range from local
effects for brief periods of time to chronic effects over large areas and for long durations.
Depending on the extent of effects to habitat, animals may alter their communications
signals (thereby potentially expending additional energy) or miss acoustic cues (either
conspecific or adventitious). For more detail on these concepts see, e.g., Barber et al.,
2010; Pijanowski et al., 2011; Francis and Barber, 2013; Lillis et al., 2014.
Problems arising from a failure to detect cues are more likely to occur when noise
stimuli are chronic and overlap with biologically relevant cues used for communication,
orientation, and predator/prey detection (Francis and Barber, 2013). Although the signals
emitted by seismic airgun arrays are generally low frequency, they would also likely be
of short duration and transient in any given area due to the nature of these surveys. As
described previously, exploratory surveys such as these cover a large area but would be
transient rather than focused in a given location over time and therefore would not be
considered chronic in any given location.
Based on the information discussed herein, we conclude that impacts of the
specified activity are not likely to have more than short-term adverse effects on any prey
habitat or populations of prey species. Further, any impacts to marine mammal habitat are
not expected to result in significant or long-term consequences for individual marine
mammals, or to contribute to adverse impacts on their populations.
Estimated Take of Marine Mammals

This section provides an estimate of the number of incidental takes proposed for
authorization through the IHA, which will inform both NMFS’ consideration of “small
numbers,” and the negligible impact determinations.
Harassment is the only type of take expected to result from these activities.
Except with respect to certain activities not pertinent here, section 3(18) of the MMPA
defines “harassment” as any act of pursuit, torment, or annoyance, which (i) has the
potential to injure a marine mammal or marine mammal stock in the wild (Level A
harassment); or (ii) has the potential to disturb a marine mammal or marine mammal
stock in the wild by causing disruption of behavioral patterns, including, but not limited
to, migration, breathing, nursing, breeding, feeding, or sheltering (Level B harassment).
Anticipated takes would primarily be by Level B harassment, the noise from use
of the airgun array has the potential to result in disruption of behavioral patterns for
individual marine mammals. There is also some potential for auditory injury (Level A
harassment) to result for species of certain hearing groups (LF and HF) due to the size of
the predicted auditory injury zones for those groups. Auditory injury is less likely to
occur for mid-frequency species due to their relative lack of sensitivity to the frequencies
at which the primary energy of an airgun signal is found as well as such species’ general
lower sensitivity to auditory injury as compared to high-frequency cetaceans. As
discussed in further detail below, we do not expect auditory injury for mid-frequency
cetaceans. No mortality or serious injury is anticipated as a result of these activities.
Below we describe how the proposed take numbers are estimated.
For acoustic impacts, generally speaking, we estimate take by considering: (1)
acoustic thresholds above which NMFS believes the best available science indicates
marine mammals will be behaviorally harassed or incur some degree of permanent
hearing impairment; (2) the area or volume of water that will be ensonified above these
levels in a day; (3) the density or occurrence of marine mammals within these ensonified

areas; and, (4) the number of days of activities. We note that while these factors can
contribute to a basic calculation to provide an initial prediction of potential takes,
additional information that can qualitatively inform take estimates is also sometimes
available (e.g., previous monitoring results or average group size). Below, we describe
the factors considered here in more detail and present the proposed take estimates.
Acoustic Thresholds
NMFS recommends the use of acoustic thresholds that identify the received level
of underwater sound above which exposed marine mammals would be reasonably
expected to be behaviorally harassed (equated to Level B harassment) or to incur PTS of
some degree (equated to Level A harassment).
Level B Harassment – Though significantly driven by received level, the onset of
behavioral disturbance from anthropogenic noise exposure is also informed to varying
degrees by other factors related to the source or exposure context (e.g., frequency,
predictability, duty cycle, duration of the exposure, signal-to-noise ratio, distance to the
source), the environment (e.g., bathymetry, other noises in the area, predators in the area),
and the receiving animals (hearing, motivation, experience, demography, life stage,
depth) and can be difficult to predict (e.g., Southall et al., 2007, 2021, Ellison et al.,
2012). Based on what the available science indicates and the practical need to use a
threshold based on a metric that is both predictable and measurable for most activities,
NMFS typically uses a generalized acoustic threshold based on received level to estimate
the onset of behavioral harassment. NMFS generally predicts that marine mammals are
likely to be behaviorally harassed in a manner considered to be Level B harassment when
exposed to underwater anthropogenic noise above root-mean-squared pressure received
levels (RMS SPL) of 120 dB (re 1 μPa) for continuous (e.g., vibratory pile driving,
drilling) and above RMS SPL 160 dB (re 1 μPa) for non-explosive impulsive (e.g.,
seismic airguns) or intermittent (e.g., scientific sonar) sources. Generally speaking, Level

B harassment take estimates based on these behavioral harassment thresholds are
expected to include any likely takes by TTS as, in most cases, the likelihood of TTS
occurs at distances from the source less than those at which behavioral harassment is
likely. TTS of a sufficient degree can manifest as behavioral harassment, as reduced
hearing sensitivity and the potential reduced opportunities to detect important signals
(conspecific communication, predators, prey) may result in changes in behavior patterns
that would not otherwise occur.
L-DEO’s proposed survey includes the use of impulsive seismic sources (i.e.,
airguns), and therefore the 160 dB re 1 μPa is applicable for analysis of Level B
harassment.
Level A harassment – NMFS’ Technical Guidance for Assessing the Effects of
Anthropogenic Sound on Marine Mammal Hearing (Version 2.0) (Technical Guidance,
2018) identifies dual criteria to assess auditory injury (Level A harassment) to five
different marine mammal groups (based on hearing sensitivity) as a result of exposure to
noise from two different types of sources (impulsive or non-impulsive). L-DEO’s
proposed survey includes the use of impulsive seismic sources (i.e., airguns).
These thresholds are provided in the table below. The references, analysis, and
methodology used in the development of the thresholds are described in NMFS’ 2018
Technical Guidance, which may be accessed at:
https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammalacoustic-technical-guidance.
Table 3 -- Thresholds Identifying the Onset of Permanent Threshold Shift.
PTS Onset Acoustic Thresholds*
(Received Level)
Hearing Group
Low-Frequency (LF)
Cetaceans

Impulsive

Non-impulsive

Cell 1
Lpk,flat: 219 dB

Cell 2
LE,LF,24h: 199 dB

LE,LF,24h: 183 dB

Mid-Frequency (MF)
Cetaceans

Cell 3
Lpk,flat: 230 dB
LE,MF,24h: 185 dB

Cell 4
LE,MF,24h: 198 dB

High-Frequency (HF)
Cetaceans

Cell 5
Lpk,flat: 202 dB
LE,HF,24h: 155 dB

Cell 6
LE,HF,24h: 173 dB

Phocid Pinnipeds (PW)
(Underwater)

Cell 7
Lpk,flat: 218 dB
LE,PW,24h: 185 dB

Cell 8
LE,PW,24h: 201 dB

Otariid Pinnipeds (OW)
(Underwater)

Cell 9
Lpk,flat: 232 dB
LE,OW,24h: 203 dB

Cell 10
LE,OW,24h: 219 dB

* Dual metric acoustic thresholds for impulsive sounds: Use whichever results in the
largest isopleth for calculating PTS onset. If a non-impulsive sound has the potential of
exceeding the peak sound pressure level thresholds associated with impulsive sounds,
these thresholds should also be considered.
Note: Peak sound pressure (Lpk) has a reference value of 1 µPa, and cumulative sound
exposure level (LE) has a reference value of 1µPa2s. In this table, thresholds are
abbreviated to reflect American National Standards Institute standards (ANSI 2013).
However, peak sound pressure is defined by ANSI as incorporating frequency
weighting, which is not the intent for this Technical Guidance. Hence, the subscript
“flat” is being included to indicate peak sound pressure should be flat weighted or
unweighted within the generalized hearing range. The subscript associated with
cumulative sound exposure level thresholds indicates the designated marine mammal
auditory weighting function (LF, MF, and HF cetaceans, and PW and OW pinnipeds)
and that the recommended accumulation period is 24 hours. The cumulative sound
exposure level thresholds could be exceeded in a multitude of ways (i.e., varying
exposure levels and durations, duty cycle). When possible, it is valuable for action
proponents to indicate the conditions under which these acoustic thresholds will be
exceeded.
Ensonified Area
Here, we describe operational and environmental parameters of the activity that
are used in estimating the area ensonified above the acoustic thresholds, including source
levels and transmission loss coefficient.
When the Technical Guidance was published (NMFS, 2016), in recognition of the
fact that ensonified area/volume could be more technically challenging to predict because

of the duration component in the new thresholds, we developed a user spreadsheet that
includes tools to help predict a simple isopleth that can be used in conjunction with
marine mammal density or occurrence to help predict takes. We note that because of
some of the assumptions included in the methods used for these tools, we anticipate that
isopleths produced are typically going to be overestimates of some degree, which may
result in some degree of overestimate of Level A harassment take. However, these tools
offer the best way to predict appropriate isopleths when more sophisticated 3D modeling
methods are not available, and NMFS continues to develop ways to quantitatively refine
these tools and will qualitatively address the output where appropriate.
The proposed survey would entail the use of a 36-airgun array with a total
discharge volume of 6,600 in3 at a tow depth of 9 m to 12 m. L-DEO’s model results are
used to determine the 160 dBrms radius for the airgun source down to a maximum depth
of 2,000 m. Received sound levels have been predicted by L-DEO’s model (Diebold et
al. 2010) as a function of distance from the 36-airgun array. This modeling approach uses
ray tracing for the direct wave traveling from the array to the receiver and its associated
source ghost (reflection at the air-water interface in the vicinity of the array), in a
constant-velocity half-space (infinite homogeneous ocean layer, unbounded by a
seafloor). In addition, propagation measurements of pulses from the 36-airgun array at a
tow depth of 6 m have been reported in deep water (~1,600 m), intermediate water depth
on the slope (~600–1,100 m), and shallow water (~50 m) in the Gulf of Mexico (Tolstoy
et al. 2009; Diebold et al. 2010).
For deep and intermediate water cases, the field measurements cannot be used
readily to derive the harassment isopleths, as at those sites the calibration hydrophone
was located at a roughly constant depth of 350-550 m, which may not intersect all the
SPL isopleths at their widest point from the sea surface down to the assumed maximum
relevant water depth (~2000 m) for marine mammals. At short ranges, where the direct

arrivals dominate and the effects of seafloor interactions are minimal, the data at the deep
sites are suitable for comparison with modeled levels at the depth of the calibration
hydrophone. At longer ranges, the comparison with the model—constructed from the
maximum SPL through the entire water column at varying distances from the airgun
array—is the most relevant.
In deep and intermediate water depths at short ranges, sound levels for direct
arrivals recorded by the calibration hydrophone and L-DEO model results for the same
array tow depth are in good alignment (see figures 12 and 14 in Diebold et al. 2010).
Consequently, isopleths falling within this domain can be predicted reliably by the LDEO model, although they may be imperfectly sampled by measurements recorded at a
single depth. At greater distances, the calibration data show that seafloor-reflected and
sub-seafloor-refracted arrivals dominate, whereas the direct arrivals become weak and/or
incoherent (see figures 11, 12, and 16 in Diebold et al. 2010). Aside from local
topography effects, the region around the critical distance is where the observed levels
rise closest to the model curve. However, the observed sound levels are found to fall
almost entirely below the model curve. Thus, analysis of the Gulf of Mexico calibration
measurements demonstrates that although simple, the L-DEO model is a robust tool for
conservatively estimating isopleths.
The proposed high-energy survey would acquire data with the 36-airgun array at a
tow depth of 9 to 12 m. For this survey, which occurs only in deep water (>1,000 m), we
use the deep-water radii obtained from L-DEO model results down to a maximum water
depth of 2,000 m for the 36-airgun array.
L-DEO's modeling methodology is described in greater detail in L-DEO’s
application. The estimated distances to the Level B harassment isopleth for the proposed
airgun configuration are shown in table 4.
Table 4 -- Predicted Radial Distances from the R/V Langseth Seismic Source to
Isopleth Corresponding to Level B Harassment Threshold

Airgun
Configuration

Tow Depth (m)1

Water Depth (m)

Predicted
distances (in m) to
the Level B
harassment
threshold

4 strings, 36
airguns, 6,600 in3

>1,000

6,7332

1Maximum
2Distance

tow depth was used for conservative distances.
is based on L-DEO model results.

Table 5 -- Modeled Radial Distance to Isopleths Corresponding to Level A
Harassment Thresholds
Low
Frequency
Cetaceans

Mid
Frequency
Cetaceans

High
Frequency
Cetaceans

PTS SELcum

426.9

1.3

PTS Peak

38.9

13.6

268.3

The largest distance (in bold) of the dual criteria (SEL cum or Peak) was used to estimate threshold
distances and potential takes by Level A harassment.

Table 5 presents the modeled PTS isopleths for each cetacean hearing group
based on L-DEO modeling incorporated in the companion user spreadsheet, for the highenergy surveys with the shortest shot interval (i.e. greatest potential to cause PTS based
on accumulated sound energy) (NMFS 2018).
Predicted distances to Level A harassment isopleths, which vary based on marine
mammal hearing groups, were calculated based on modeling performed by L-DEO using
the Nucleus software program and the NMFS user spreadsheet, described below. The
acoustic thresholds for impulsive sounds contained in the NMFS Technical Guidance
were presented as dual metric acoustic thresholds using both SELcum and peak sound
pressure metrics (NMFS 2016). As dual metrics, NMFS considers onset of PTS (Level A
harassment) to have occurred when either one of the two metrics is exceeded (i.e., metric
resulting in the largest isopleth). The SELcum metric considers both level and duration of
exposure, as well as auditory weighting functions by marine mammal hearing group.

The SELcum for the 36-airgun array is derived from calculating the modified
farfield signature. The farfield signature is often used as a theoretical representation of
the source level. To compute the farfield signature, the source level is estimated at a large
distance (right) below the array (e.g., 9 km), and this level is back projected
mathematically to a notional distance of 1 m from the array's geometrical center.
However, it has been recognized that the source level from the theoretical farfield
signature is never physically achieved at the source when the source is an array of
multiple airguns separated in space (Tolstoy et al., 2009). Near the source (at short
ranges, distances <1 km), the pulses of sound pressure from each individual airgun in the
source array do not stack constructively as they do for the theoretical farfield signature.
The pulses from the different airguns spread out in time such that the source levels
observed or modeled are the result of the summation of pulses from a few airguns, not the
full array (Tolstoy et al., 2009). At larger distances, away from the source array center,
sound pressure of all the airguns in the array stack coherently, but not within one time
sample, resulting in smaller source levels (a few dB) than the source level derived from
the far-field signature. Because the far-field signature does not take into account the large
array effect near the source and is calculated as a point source, the far-field signature is
not an appropriate measure of the sound source level for large arrays. See L-DEO’s
application for further detail on acoustic modeling.
Auditory injury is unlikely to occur for mid-frequency cetaceans, given the very
small modeled zones of injury for those species (all estimated zones are less than 15 m
for mid-frequency cetaceans), in the context of distributed source dynamics.
In consideration of the received sound levels in the near-field as described above,
we expect the potential for Level A harassment of mid-frequency cetaceans to be de
minimis, even before the likely moderating effects of aversion and/or other compensatory
behaviors (e.g., Nachtigall et al., 2018) are considered. We do not anticipate that Level A

harassment is a likely outcome for any mid-frequency cetacean and do not propose to
authorize any take by Level A harassment for these species.
The Level A and Level B harassment estimates are based on a consideration of
the number of marine mammals that could be within the area around the operating airgun
array where received levels of sound ≥160 dB re 1 µPa rms are predicted to occur. The
estimated numbers are based on the densities (numbers per unit area) of marine mammals
expected to occur in the area in the absence of seismic surveys. To the extent that marine
mammals tend to move away from seismic sources before the sound level reaches the
criterion level and tend not to approach an operating airgun array, these estimates likely
overestimate the numbers actually exposed to the specified level of sound.
Marine Mammal Occurrence
In this section, we provide information about the occurrence of marine mammals,
including density or other relevant information, which will inform the take calculations.
Habitat-based stratified marine mammal densities for the North Atlantic are taken
from the US Navy Atlantic Fleet Training and Testing Area Marine Mammal Density
(Roberts et al., 2023; Mannocci et al., 2017), which represent the best available
information regarding marine mammal densities in the region. This density information
incorporates visual line-transect surveys of marine mammals for over 35 years, resulting
in various studies that estimated the abundance, density, and distributions of marine
mammal populations. The habitat-based density models consisted of 5 km x 5 km grid
cells. As the AFTT model does not overlap the proposed survey area, the average
densities in the grid cells for the AFTT area that encompassed a similar-sized area as the
proposed survey area in the southeastern-most part of the AFTT area were used (between
~21.1°N–22.5°N and ~45.1°W–49.5°W). Even though these densities are for the western
Atlantic Ocean, they are for an area of the Mid-Atlantic Ridge, which would be most
representative of densities occurring at the Mid-Atlantic Ridge in the proposed survey

area. More information is available online at
https://seamap.env.duke.edu/models/Duke/AFTT/.
Since there was no density data available for the actual proposed survey area, LDEO used OBIS sightings, available literature, and regional distribution maps of the
actual survey area (or greater region) to determine which species would be expected to be
encountered in the proposed survey area. From the AFTT models, L-DEO excluded the
following species, as they were not expected to occur in the survey area: seals, northern
bottlenose whales, North Atlantic right whale (these had densities of zero) and harbor
porpoise, white-beaked dolphin, and Atlantic white-sided dolphin (these species had nonzero densities). There were no additional species that might occur in the survey area that
were not available in the AFTT model.
For most species, only annual densities were available. For some baleen whale
species (fin, sei and humpback whale), monthly densities were available. For these
species, the highest monthly densities were used. Densities for fin whales were near zero
and the calculations did not result in any estimated takes. However, because this species
could be encountered in the proposed survey area, we propose to authorize take of one
individual.
Take Estimate
Here, we describe how the information provided above is synthesized to produce
a quantitative estimate of the take that is reasonably likely to occur and proposed for
authorization. In order to estimate the number of marine mammals predicted to be
exposed to sound levels that would result in Level A or Level B harassment, radial
distances from the airgun array to the predicted isopleth corresponding to the Level A
harassment and Level B harassment thresholds are calculated, as described above. Those
radial distances were then used to calculate the area(s) around the airgun array predicted
to be ensonified to sound levels that exceed the harassment thresholds. The distance for

the 160-dB Level B harassment threshold and PTS (Level A harassment) thresholds
(based on L-DEO model results) was used to draw a buffer around the area expected to
be ensonified (i.e., the survey area). The ensonified areas were then increased by 25
percent to account for potential delays, which is equivalent to adding 25 percent to the
proposed line km to be surveyed. The density for each species was then multiplied by the
daily ensonified areas (increased as described above) and then multiplied by the number
of survey days (11.5) to estimate potential takes (see appendix B of L-DEO’s application
for more information).
L-DEO assumed that their estimates of marine mammal exposures above
harassment thresholds equate to take and requested authorization of those takes. Those
estimates in turn form the basis for our proposed take authorization numbers. For the
species for which NMFS does not expect there to be a reasonable potential for take by
Level A harassment to occur (i.e., mid-frequency cetaceans), we have added L-DEO’s
estimated exposures above Level A harassment thresholds to their estimated exposures
above the Level B harassment threshold to produce a total number of incidents of take by
Level B harassment that is proposed for authorization. Estimated exposures and proposed
take numbers for authorization are shown in table 6.
Table 6 -- Estimated Take Proposed for Authorization
Estimated Take
Species

Proposed Authorized Take
Modeled
Abundance1

Percent of
Abundance

Level
B

Level A

Level B

Level A

Humpback whale

2

2

4,990

0.82

Bryde’s whale

0

0

0.75

Minke whale3

1

1

13,784

0.17

Fin whale

0

0

11,672

0.01

Sei whale

1

1

19,530

0.06

Blue whale

0

0

0.52

Sperm whale

0

0

64,015

0.17

Beaked whales4

0

0

65,069

0.16

Risso’s dolphin

0

0

78,205

0.11

Rough-toothed dolphin

0

0

32,848

0.51

Bottlenose dolphin

2

0

418,151

0.30

Pantropical spotted
dolphin

0

0

321,740

0.02

Atlantic spotted
dolphin

1

0

259,519

0.17

Spinner dolphin

2

0

152,511

0.59

Striped dolphin

0

0

412,729

0.02

Clymene dolphin

2

0

181,209

0.57

Fraser’s dolphin

0

0

19,585

0.56

Common dolphin

0

0

473,206

0.02

Short-finned pilot
whale5

2

0

264,907

0.49

Melon-headed whale

1

0

64,114

0.78

False killer whale

0

0

12,682

0.78

Pygmy killer whale

0

0

9,001

0.79

Killer whale

0

0

0.51

Kogia spp6

5

5

26,043

0.49

1 Modeled

abundance (Roberts et al. 2023) or North Atlantic abundance (NAMMCO 2023), where
applicable.
2 Requested take authorization is expressed as percent of population for the AFTT Area only (Roberts et al.
2023).
3Takes assigned equally between Common minke whales (11 Level B takes and 1 Level A take) and
Antarctic minke whales (12 Level B takes).
4 Beaked whale guild. Includes Cuvier’s beaked whale, Blaineville’s beaked whale, and Gervais’ beaked
whale.
5 Takes based on density for Globicephala sp. All takes are assumed to be for short-finned pilot whales
6 Kogia spp. Includes Pygmy sperm whale and Dwarf sperm whale.
7Takes rounded to a mean group size (Weir 2011)

Proposed Mitigation
In order to issue an IHA under section 101(a)(5)(D) of the MMPA, NMFS must
set forth the permissible methods of taking pursuant to the activity and other means of
effecting the least practicable impact on the species or stock and its habitat, paying

particular attention to rookeries, mating grounds, and areas of similar significance, and on
the availability of the species or stock for taking for certain subsistence uses (latter not
applicable for this action). NMFS regulations require applicants for incidental take
authorizations to include information about the availability and feasibility (economic and
technological) of equipment, methods, and manner of conducting the activity or other
means of effecting the least practicable adverse impact upon the affected species or
stocks, and their habitat (50 CFR 216.104(a)(11)).
In evaluating how mitigation may or may not be appropriate to ensure the least
practicable adverse impact on species or stocks and their habitat, as well as subsistence
uses where applicable, NMFS considers two primary factors:
(1) The manner in which, and the degree to which, the successful implementation
of the measure(s) is expected to reduce impacts to marine mammals, marine mammal
species SZor stocks, and their habitat. This considers the nature of the potential adverse
impact being mitigated (likelihood, scope, range). It further considers the likelihood that
the measure will be effective if implemented (probability of accomplishing the mitigating
result if implemented as planned), the likelihood of effective implementation (probability
implemented as planned), and;
(2) The practicability of the measures for applicant implementation, which may
consider such things as cost and impact on operations.
Vessel-Based Visual Mitigation Monitoring
Visual monitoring requires the use of trained observers (herein referred to as
visual protected species observers (PSOs)) to scan the ocean surface for the presence of
marine mammals. The area to be scanned visually includes primarily the shutdown zone
(SZ), within which observation of certain marine mammals requires shutdown of the
acoustic source, a buffer zone, and to the extent possible depending on conditions, the
surrounding waters. The buffer zone means an area beyond the SZ to be monitored for

the presence of marine mammals that may enter the SZ. During pre-start clearance
monitoring (i.e., before ramp-up begins), the buffer zone also acts as an extension of the
SZ in that observations of marine mammals within the buffer zone would also prevent
airgun operations from beginning (i.e., ramp-up). The buffer zone encompasses the area
at and below the sea surface from the edge of the 0–500 m SZ, out to a radius of 1,000 m
from the edges of the airgun array (500–1,000 m). This 1,000-m zone (SZ plus buffer)
represents the pre-start clearance zone. Visual monitoring of the SZ and adjacent waters
(buffer plus surrounding waters) is intended to establish and, when visual conditions
allow, maintain zones around the sound source that are clear of marine mammals, thereby
reducing or eliminating the potential for injury and minimizing the potential for more
severe behavioral reactions for animals occurring closer to the vessel. Visual monitoring
of the buffer zone is intended to (1) provide additional protection to marine mammals that
may be in the vicinity of the vessel during pre-start clearance, and (2) during airgun use,
aid in establishing and maintaining the SZ by alerting the visual observer and crew of
marine mammals that are outside of, but may approach and enter, the SZ.
During survey operations (e.g., any day on which use of the airgun array is
planned to occur and whenever the airgun array is in the water, whether activated or not),
a minimum of two visual PSOs must be on duty and conducting visual observations at all
times during daylight hours (i.e., from 30 minutes prior to sunrise through 30 minutes
following sunset). Visual monitoring of the pre-start clearance zone must begin no less
than 30 minutes prior to ramp-up and monitoring must continue until 1 hour after use of
the airgun array ceases or until 30 minutes past sunset. Visual PSOs shall coordinate to
ensure 360° visual coverage around the vessel from the most appropriate observation
posts and shall conduct visual observations using binoculars and the naked eye while free
from distractions and in a consistent, systematic, and diligent manner.

PSOs shall establish and monitor the SZ and buffer zone. These zones shall be
based upon the radial distance from the edges of the airgun array (rather than being based
on the center of the array or around the vessel itself). During use of the airgun array (i.e.,
anytime airguns are active, including ramp-up), detections of marine mammals within the
buffer zone (but outside the SZ) shall be communicated to the operator to prepare for the
potential shutdown of the airgun array. Visual PSOs will immediately communicate all
observations to the on duty acoustic PSO(s), including any determination by the visual
PSO regarding species identification, distance, and bearing and the degree of confidence
in the determination. Any observations of marine mammals by crew members shall be
relayed to the PSO team. During good conditions (e.g., daylight hours; Beaufort sea state
(BSS) 3 or less), visual PSOs shall conduct observations when the airgun array is not
operating for comparison of sighting rates and behavior with and without use of the
airgun array and between acquisition periods, to the maximum extent practicable.
Visual PSOs may be on watch for a maximum of 4 consecutive hours followed by
a break of at least 1 hour between watches and may conduct a maximum of 12 hours of
observation per 24-hour period. Combined observational duties (visual and acoustic but
not at same time) may not exceed 12 hours per 24-hour period for any individual PSO.
Passive Acoustic Monitoring
Passive acoustic monitoring means the use of trained personnel (sometimes
referred to as PAM operators, herein referred to as acoustic PSOs) to operate PAM
equipment to acoustically detect the presence of marine mammals. Acoustic monitoring
involves acoustically detecting marine mammals regardless of distance from the source,
as localization of animals may not always be possible. Acoustic monitoring is intended to
further support visual monitoring (during daylight hours) in maintaining a SZ around the
sound source that is clear of marine mammals. In cases where visual monitoring is not

effective (e.g., due to weather, nighttime), acoustic monitoring may be used to allow
certain activities to occur, as further detailed below.
PAM would take place in addition to the visual monitoring program. Visual
monitoring typically is not effective during periods of poor visibility or at night and even
with good visibility, is unable to detect marine mammals when they are below the surface
or beyond visual range. Acoustic monitoring can be used in addition to visual
observations to improve detection, identification, and localization of cetaceans. The
acoustic monitoring would serve to alert visual PSOs (if on duty) when vocalizing
cetaceans are detected. It is only useful when marine mammals vocalize, but it can be
effective either by day or by night and does not depend on good visibility. It would be
monitored in real time so that the visual observers can be advised when cetaceans are
detected.
The R/V Langseth will use a towed PAM system, which must be monitored by at
a minimum one on duty acoustic PSO beginning at least 30 minutes prior to ramp-up and
at all times during use of the airgun array. Acoustic PSOs may be on watch for a
maximum of 4 consecutive hours followed by a break of at least 1 hour between watches
and may conduct a maximum of 12 hours of observation per 24-hour period. Combined
observational duties (acoustic and visual but not at same time) may not exceed 12 hours
per 24-hour period for any individual PSO.
Survey activity may continue for 30 minutes when the PAM system malfunctions
or is damaged, while the PAM operator diagnoses the issue. If the diagnosis indicates that
the PAM system must be repaired to solve the problem, operations may continue for an
additional 10 hours without acoustic monitoring during daylight hours only under the
following conditions:
●

Sea state is less than or equal to BSS 4;

●

No marine mammals (excluding delphinids) detected solely by PAM in

the SZ in the previous 2 hours;
●

NMFS is notified via email as soon as practicable with the time and

location in which operations began occurring without an active PAM system; and
●

Operations with an active airgun array, but without an operating PAM

system, do not exceed a cumulative total of 10 hours in any 24-hour period.
Establishment of Shutdown and Pre-Start Clearance Zones
A SZ is a defined area within which occurrence of a marine mammal triggers
mitigation action intended to reduce the potential for certain outcomes (e.g., auditory
injury, disruption of critical behaviors). The PSOs would establish a minimum SZ with a
500-m radius. The 500-m SZ would be based on radial distance from the edge of the
airgun array (rather than being based on the center of the array or around the vessel
itself). With certain exceptions (described below), if a marine mammal appears within or
enters this zone, the airgun array would be shut down.
The pre-start clearance zone is defined as the area that must be clear of marine
mammals prior to beginning ramp-up of the airgun array and includes the SZ plus the
buffer zone. Detections of marine mammals within the pre-start clearance zone would
prevent airgun operations from beginning (i.e., ramp-up).
The 500-m SZ is intended to be precautionary in the sense that it would be
expected to contain sound exceeding the injury criteria for all cetacean hearing groups,
(based on the dual criteria of SELcum and peak SPL), while also providing a consistent,
reasonably observable zone within which PSOs would typically be able to conduct
effective observational effort. Additionally, a 500-m SZ is expected to minimize the
likelihood that marine mammals will be exposed to levels likely to result in more severe
behavioral responses. Although significantly greater distances may be observed from an
elevated platform under good conditions, we expect that 500 m is likely regularly

attainable for PSOs using the naked eye during typical conditions. The pre-start clearance
zone simply represents the addition of a buffer to the SZ, doubling the SZ size during
pre-clearance.
An extended SZ of 1,500 m must be enforced for all beaked whales, Kogia spp, a
large whale with a calf, and groups of six or more large whales. No buffer of this
extended SZ is required, as NMFS concludes that this extended SZ is sufficiently
protective to mitigate harassment to these groups.
Pre-start Clearance and Ramp-up
Ramp-up (sometimes referred to as "soft start") means the gradual and systematic
increase of emitted sound levels from an airgun array. Ramp-up begins by first activating
a single airgun of the smallest volume, followed by doubling the number of active
elements in stages until the full complement of an array's airguns are active. Each stage
should be approximately the same duration, and the total duration should not be less than
approximately 20 minutes. The intent of pre-start clearance observation (30 minutes) is to
ensure no marine mammals are observed within the pre-start clearance zone (or extended
SZ, for beaked whales, Kogia spp, a large whale with a calf, and groups of six or more
large whales) prior to the beginning of ramp-up. During the pre-start clearance period is
the only time observations of marine mammals in the buffer zone would prevent
operations (i.e., the beginning of ramp-up). The intent of the ramp-up is to warn marine
mammals of pending seismic survey operations and to allow sufficient time for those
animals to leave the immediate vicinity prior to the sound source reaching full intensity.
A ramp-up procedure, involving a stepwise increase in the number of airguns firing and
total array volume until all operational airguns are activated and the full volume is
achieved, is required at all times as part of the activation of the airgun array. All operators
must adhere to the following pre-start clearance and ramp-up requirements:

●

The operator must notify a designated PSO of the planned start of ramp-up

as agreed upon with the lead PSO; the notification time should not be less than 60
minutes prior to the planned ramp-up in order to allow the PSOs time to monitor the prestart clearance zone (and extended SZ) for 30 minutes prior to the initiation of ramp-up
(pre-start clearance);
●

Ramp-ups shall be scheduled so as to minimize the time spent with the

source activated prior to reaching the designated run-in;
●

One of the PSOs conducting pre-start clearance observations must be

notified again immediately prior to initiating ramp-up procedures and the operator must
receive confirmation from the PSO to proceed;
●

Ramp-up may not be initiated if any marine mammal is within the

applicable shutdown or buffer zone. If a marine mammal is observed within the pre-start
clearance zone (or extended SZ, for beaked whales, a large whale with a calf, and groups
of six or more large whales) during the 30 minute pre-start clearance period, ramp-up
may not begin until the animal(s) has been observed exiting the zones or until an
additional time period has elapsed with no further sightings (15 minutes for small
odontocetes, and 30 minutes for all mysticetes and all other odontocetes, including sperm
whales, beaked whales, and large delphinids, such as pilot whales);
●

Ramp-up shall begin by activating a single airgun of the smallest volume

in the array and shall continue in stages by doubling the number of active elements at the
commencement of each stage, with each stage of approximately the same duration.
Duration shall not be less than 20 minutes. The operator must provide information to the
PSO documenting that appropriate procedures were followed;
●

PSOs must monitor the pre-start clearance zone and extended SZ during

ramp-up, and ramp-up must cease and the source must be shut down upon detection of a
marine mammal within the applicable zone. Once ramp-up has begun, detections of

marine mammals within the buffer zone do not require shutdown, but such observation
shall be communicated to the operator to prepare for the potential shutdown;
●

Ramp-up may occur at times of poor visibility, including nighttime, if

appropriate acoustic monitoring has occurred with no detections in the 30 minutes prior
to beginning ramp-up. Airgun array activation may only occur at times of poor visibility
where operational planning cannot reasonably avoid such circumstances;
●

If the airgun array is shut down for brief periods (i.e., less than 30

minutes) for reasons other than implementation of prescribed mitigation (e.g., mechanical
difficulty), it may be activated again without ramp-up if PSOs have maintained constant
visual and/or acoustic observation and no visual or acoustic detections of marine
mammals have occurred within the pre-start clearance zone (or extended SZ, where
applicable). For any longer shutdown, pre-start clearance observation and ramp-up are
required; and
●

Testing of the airgun array involving all elements requires ramp-up.

Testing limited to individual source elements or strings does not require ramp-up but
does require pre-start clearance watch of 30 minutes.
Shutdown
The shutdown of an airgun array requires the immediate de-activation of all
individual airgun elements of the array. Any PSO on duty will have the authority to call
for shutdown of the airgun array if a marine mammal is detected within the applicable
SZ. The operator must also establish and maintain clear lines of communication directly
between PSOs on duty and crew controlling the airgun array to ensure that shutdown
commands are conveyed swiftly while allowing PSOs to maintain watch. When both
visual and acoustic PSOs are on duty, all detections will be immediately communicated
to the remainder of the on-duty PSO team for potential verification of visual observations
by the acoustic PSO or of acoustic detections by visual PSOs. When the airgun array is

active (i.e., anytime one or more airguns is active, including during ramp-up) and (1) a
marine mammal appears within or enters the applicable SZ and/or (2) a marine mammal
(other than delphinids, see below) is detected acoustically and localized within the
applicable SZ, the airgun array will be shut down. When shutdown is called for by a
PSO, the airgun array will be immediately deactivated and any dispute resolved only
following deactivation. Additionally, shutdown will occur whenever PAM alone (without
visual sighting), confirms the presence of marine mammal(s) in the SZ. If the acoustic
PSO cannot confirm presence within the SZ, visual PSOs will be notified but shutdown is
not required.
Following a shutdown, airgun activity would not resume until the marine mammal
has cleared the SZ. The animal would be considered to have cleared the SZ if it is
visually observed to have departed the SZ (i.e., animal is not required to fully exit the
buffer zone where applicable), or it has not been seen within the SZ for 15 minutes for
small odontocetes or 30 minutes for all mysticetes and all other odontocetes, including
sperm whales, beaked whales, and large delphinids, such as pilot whales.
The shutdown requirement is waived for specific genera of small dolphins if an
individual is detected within the SZ. The small dolphin group is intended to encompass
those members of the Family Delphinidae most likely to voluntarily approach the source
vessel for purposes of interacting with the vessel and/or airgun array (e.g., bow riding).
This exception to the shutdown requirement applies solely to the specific genera of small
dolphins (Delphinus, Lagenodelphis, Stenella, Steno and Tursiops).
We include this small dolphin exception because shutdown requirements for these
species under all circumstances represent practicability concerns without likely
commensurate benefits for the animals in question. Small dolphins are generally the most
commonly observed marine mammals in the specific geographic region and would
typically be the only marine mammals likely to intentionally approach the vessel. As

described above, auditory injury is extremely unlikely to occur for mid-frequency
cetaceans (e.g., delphinids), as this group is relatively insensitive to sound produced at
the predominant frequencies in an airgun pulse while also having a relatively high
threshold for the onset of auditory injury (i.e., permanent threshold shift).
A large body of anecdotal evidence indicates that small dolphins commonly
approach vessels and/or towed arrays during active sound production for purposes of bow
riding with no apparent effect observed (e.g., Barkaszi et al., 2012, Barkaszi and Kelly,
2018). The potential for increased shutdowns resulting from such a measure would
require the R/V Langseth to revisit the missed track line to reacquire data, resulting in an
overall increase in the total sound energy input to the marine environment and an increase
in the total duration over which the survey is active in a given area. Although other midfrequency hearing specialists (e.g., large delphinids) are no more likely to incur auditory
injury than are small dolphins, they are much less likely to approach vessels. Therefore,
retaining a shutdown requirement for large delphinids would not have similar impacts in
terms of either practicability for the applicant or corollary increase in sound energy
output and time on the water. We do anticipate some benefit for a shutdown requirement
for large delphinids in that it simplifies somewhat the total range of decision-making for
PSOs and may preclude any potential for physiological effects other than to the auditory
system as well as some more severe behavioral reactions for any such animals in close
proximity to the R/V Langseth.
Visual PSOs shall use best professional judgment in making the decision to call
for a shutdown if there is uncertainty regarding identification (i.e., whether the observed
marine mammal(s) belongs to one of the delphinid genera for which shutdown is waived
or one of the species with a larger SZ).
L-DEO must implement shutdown if a marine mammal species for which take
was not authorized or a species for which authorization was granted but the authorized

takes have been met approaches the Level A or Level B harassment zones. L-DEO must
also implement an extended shutdown of 1,500 m if any large whale (defined as a sperm
whale or any mysticete species) with a calf (defined as an animal less than two-thirds the
body size of an adult observed to be in close association with an adult) and/or an
aggregation of six or more large whales.
Vessel Strike Avoidance Mitigation Measures
Vessel personnel should use an appropriate reference guide that includes
identifying information on all marine mammals that may be encountered. Vessel
operators must comply with the below measures except under extraordinary
circumstances when the safety of the vessel or crew is in doubt or the safety of life at sea
is in question. These requirements do not apply in any case where compliance would
create an imminent and serious threat to a person or vessel or to the extent that a vessel is
restricted in its ability to maneuver and, because of the restriction, cannot comply.
Vessel operators and crews must maintain a vigilant watch for all marine
mammals and slow down, stop their vessel, or alter course, as appropriate and regardless
of vessel size, to avoid striking any marine mammal. A single marine mammal at the
surface may indicate the presence of submerged animals in the vicinity of the vessel;
therefore, precautionary measures should always be exercised. A visual observer aboard
the vessel must monitor a vessel strike avoidance zone around the vessel (separation
distances stated below). Visual observers monitoring the vessel strike avoidance zone
may be third-party observers (i.e., PSOs) or crew members, but crew members
responsible for these duties must be provided sufficient training to 1) distinguish marine
mammals from other phenomena and 2) broadly to identify a marine mammal as a large
whale (defined in this context as sperm whales or baleen whales), or other marine
mammals.

Vessel speeds must be reduced to 10 kn (18.5 kph) or less when mother/calf pairs,
pods, or large assemblages of cetaceans are observed near a vessel. All vessels must
maintain a minimum separation distance of 100 m from sperm whales and all other
baleen whales. All vessels must, to the maximum extent practicable, attempt to maintain
a minimum separation distance of 50 m from all other marine mammals, with an
understanding that at times this may not be possible (e.g., for animals that approach the
vessel).
When marine mammals are sighted while a vessel is underway, the vessel shall
take action as necessary to avoid violating the relevant separation distance (e.g., attempt
to remain parallel to the animal’s course, avoid excessive speed or abrupt changes in
direction until the animal has left the area). If marine mammals are sighted within the
relevant separation distance, the vessel must reduce speed and shift the engine to neutral,
not engaging the engines until animals are clear of the area. This does not apply to any
vessel towing gear or any vessel that is navigationally constrained.
Based on our evaluation of the applicant’s proposed measures, as well as other
measures considered by NMFS, NMFS has preliminarily determined that the proposed
mitigation measures provide the means of effecting the least practicable impact on the
affected species or stocks and their habitat, paying particular attention to rookeries,
mating grounds, and areas of similar significance.
Proposed Monitoring and Reporting
In order to issue an IHA for an activity, section 101(a)(5)(D) of the MMPA states
that NMFS must set forth requirements pertaining to the monitoring and reporting of such
taking. The MMPA implementing regulations at 50 CFR 216.104(a)(13) indicate that
requests for authorizations must include the suggested means of accomplishing the
necessary monitoring and reporting that will result in increased knowledge of the species
and of the level of taking or impacts on populations of marine mammals that are expected

to be present while conducting the activities. Effective reporting is critical both to
compliance as well as ensuring that the most value is obtained from the required
monitoring.
L-DEO must use dedicated, trained, and NMFS-approved PSOs. The PSOs must
have no tasks other than to conduct observational effort, record observational data, and
communicate with and instruct relevant vessel crew with regard to the presence of marine
mammals and mitigation requirements. PSO resumes shall be provided to NMFS for
advance approval (prior to embarking on the vessel).
At least one of the visual and two of the acoustic PSOs (discussed below) aboard
the vessel must have a minimum of 90 days at-sea experience working in those roles,
respectively, with no more than 18 months elapsed since the conclusion of the at-sea
experience. One visual PSO with such experience shall be designated as the lead for the
entire protected species observation team. The lead PSO shall serve as primary point of
contact for the vessel operator and ensure all PSO requirements per the IHA are met. To
the maximum extent practicable, the experienced PSOs should be scheduled to be on duty
with those PSOs with appropriate training but who have not yet gained relevant
experience.
Monitoring and reporting requirements prescribed by NMFS should contribute to
improved understanding of one or more of the following:
●

Occurrence of marine mammal species or stocks in the area in which take

is anticipated (e.g., presence, abundance, distribution, density);
●

Nature, scope, or context of likely marine mammal exposure to potential

stressors/impacts (individual or cumulative, acute or chronic), through better
understanding of: (1) action or environment (e.g., source characterization, propagation,
ambient noise); (2) affected species (e.g., life history, dive patterns); (3) co-occurrence of

marine mammal species with the activity; or (4) biological or behavioral context of
exposure (e.g., age, calving or feeding areas);
●

Individual marine mammal responses (behavioral or physiological) to

acoustic stressors (acute, chronic, or cumulative), other stressors, or cumulative impacts
from multiple stressors;
●

How anticipated responses to stressors impact either: (1) long-term fitness

and survival of individual marine mammals; or (2) populations, species, or stocks;
●

Effects on marine mammal habitat (e.g., marine mammal prey species,

acoustic habitat, or other important physical components of marine mammal habitat);
and,
●

Mitigation and monitoring effectiveness.

Vessel-Based Visual Monitoring
As described above, PSO observations would take place during daytime airgun
operations. During seismic survey operations, at least five visual PSOs would be based
aboard the R/V Langseth. Two visual PSOs would be on duty at all times during daytime
hours. Monitoring shall be conducted in accordance with the following requirements:
●

The operator shall provide PSOs with bigeye reticle binoculars (e.g., 25 x

150; 2.7 view angle; individual ocular focus; height control) of appropriate quality solely
for PSO use. These binoculars shall be pedestal-mounted on the deck at the most
appropriate vantage point that provides for optimal sea surface observation, PSO safety,
and safe operation of the vessel; and
●

The operator will work with the selected third-party observer provider to

ensure PSOs have all equipment (including backup equipment) needed to adequately
perform necessary tasks, including accurate determination of distance and bearing to
observed marine mammals.
PSOs must have the following requirements and qualifications:

●

PSOs shall be independent, dedicated, trained visual and acoustic PSOs

and must be employed by a third-party observer provider;
●

PSOs shall have no tasks other than to conduct observational effort (visual

or acoustic), collect data, and communicate with and instruct relevant vessel crew with
regard to the presence of protected species and mitigation requirements (including brief
alerts regarding maritime hazards);
●

PSOs shall have successfully completed an approved PSO training course

appropriate for their designated task (visual or acoustic). Acoustic PSOs are required to
complete specialized training for operating PAM systems and are encouraged to have
familiarity with the vessel with which they will be working;
●

PSOs can act as acoustic or visual observers (but not at the same time) as

long as they demonstrate that their training and experience are sufficient to perform the
task at hand;
●

NMFS must review and approve PSO resumes accompanied by a relevant

training course information packet that includes the name and qualifications (i.e.,
experience, training completed, or educational background) of the instructor(s), the
course outline or syllabus, and course reference material as well as a document stating
successful completion of the course;
●

PSOs must successfully complete relevant training, including completion

of all required coursework and passing (80 percent or greater) a written and/or oral
examination developed for the training program;
●

PSOs must have successfully attained a bachelor's degree from an

accredited college or university with a major in one of the natural sciences, a minimum of
30 semester hours or equivalent in the biological sciences, and at least one undergraduate
course in math or statistics; and

●

The educational requirements may be waived if the PSO has acquired the

relevant skills through alternate experience. Requests for such a waiver shall be
submitted to NMFS and must include written justification. Requests shall be granted or
denied (with justification) by NMFS within 1 week of receipt of submitted information.
Alternate experience that may be considered includes, but is not limited to (1) secondary
education and/or experience comparable to PSO duties; (2) previous work experience
conducting academic, commercial, or government-sponsored protected species surveys;
or (3) previous work experience as a PSO; the PSO should demonstrate good standing
and consistently good performance of PSO duties.
●

For data collection purposes, PSOs shall use standardized electronic data

collection forms. PSOs shall record detailed information about any implementation of
mitigation requirements, including the distance of animals to the airgun array and
description of specific actions that ensued, the behavior of the animal(s), any observed
changes in behavior before and after implementation of mitigation, and if shutdown was
implemented, the length of time before any subsequent ramp-up of the airgun array. If
required mitigation was not implemented, PSOs should record a description of the
circumstances. At a minimum, the following information must be recorded:
â—‹ Vessel name, vessel size and type, maximum speed capability of vessel;
â—‹ Dates (MM/DD/YYYY) of departures and returns to port with port name;
â—‹ PSO names and affiliations, PSO ID (initials or other identifier);
â—‹ Date (MM/DD/YYYY) and participants of PSO briefings;
â—‹ Visual monitoring equipment used (description);
â—‹ PSO location on vessel and height (meters) of observation location above
water surface;
â—‹ Watch status (description);

â—‹ Dates (MM/DD/YYYY) and times (Greenwich Mean Time/UTC) of
survey on/off effort and times (GMC/UTC) corresponding with PSO
on/off effort;
â—‹ Vessel location (decimal degrees) when survey effort began and ended and
vessel location at beginning and end of visual PSO duty shifts;
â—‹ Vessel location (decimal degrees) at 30-second intervals if obtainable
from data collection software, otherwise at practical regular interval;
â—‹ Vessel heading (compass heading) and speed (knots) at beginning and end
of visual PSO duty shifts and upon any change;
â—‹ Water depth (meters) (if obtainable from data collection software);
â—‹ Environmental conditions while on visual survey (at beginning and end of
PSO shift and whenever conditions changed significantly), including BSS
and any other relevant weather conditions including cloud cover, fog, sun
glare, and overall visibility to the horizon;
â—‹ Factors that may have contributed to impaired observations during each
PSO shift change or as needed as environmental conditions changed
(description) (e.g., vessel traffic, equipment malfunctions); and
â—‹ Vessel/Survey activity information (and changes thereof) (description),
such as airgun power output while in operation, number and volume of
airguns operating in the array, tow depth of the array, and any other notes
of significance (i.e., pre-start clearance, ramp-up, shutdown, testing,
shooting, ramp-up completion, end of operations, streamers, etc.).
●

Upon visual observation of any marine mammals, the following
information must be recorded:
â—‹ Sighting ID (numeric);

â—‹ Watch status (sighting made by PSO on/off effort, opportunistic, crew,
alternate vessel/platform);
â—‹ Location of PSO/observer (description);
â—‹ Vessel activity at the time of the sighting (e.g., deploying, recovering,
testing, shooting, data acquisition, other);
â—‹ PSO who sighted the animal/ID;
â—‹ Time/date of sighting (GMT/UTC, MM/DD/YYYY);
â—‹ Initial detection method (description);
â—‹ Sighting cue (description);
â—‹ Vessel location at time of sighting (decimal degrees);
â—‹ Water depth (meters);
○ Direction of vessel’s travel (compass direction);
â—‹ Speed (knots) of the vessel from which the observation was made;
○ Direction of animal’s travel relative to the vessel (description, compass
heading);
â—‹ Bearing to sighting (degrees);
â—‹ Identification of the animal (e.g., genus/species, lowest possible
taxonomic level, or unidentified) and the composition of the group if there
is a mix of species;
â—‹ Species reliability (an indicator of confidence in identification) (1 =
unsure/possible, 2 = probable, 3 = definite/sure, 9 = unknown/not
recorded);
â—‹ Estimated distance to the animal (meters) and method of estimating
distance;
â—‹ Estimated number of animals (high/low/best) (numeric);

â—‹ Estimated number of animals by cohort (adults, yearlings, juveniles,
calves, group composition, etc.);
â—‹ Description (as many distinguishing features as possible of each individual
seen, including length, shape, color, pattern, scars or markings, shape and
size of dorsal fin, shape of head, and blow characteristics);
â—‹ Detailed behavior observations (e.g., number of blows/breaths, number of
surfaces, breaching, spyhopping, diving, feeding, traveling; as explicit and
detailed as possible; note any observed changes in behavior);
○ Animal’s closest point of approach (meters) and/or closest distance from
any element of the airgun array;
â—‹ Description of any actions implemented in response to the sighting (e.g.,
delays, shutdown, ramp-up) and time and location of the action.
â—‹ Photos (Yes/No);
â—‹ Photo Frame Numbers (List of numbers); and
â—‹ Conditions at time of sighting (Visibility; Beaufort Sea State).
If a marine mammal is detected while using the PAM system, the following
information should be recorded:
●

An acoustic encounter identification number, and whether the detection

was linked with a visual sighting;
●

Date and time when first and last heard;

●

Types and nature of sounds heard (e.g., clicks, whistles, creaks, burst

pulses, continuous, sporadic, strength of signal); and
●

Any additional information recorded such as water depth of the

hydrophone array, bearing of the animal to the vessel (if determinable), species or
taxonomic group (if determinable), spectrogram screenshot, and any other notable
information.

Reporting
L-DEO shall submit a draft comprehensive report on all activities and monitoring
results within 90 days of the completion of the survey or expiration of the IHA,
whichever comes sooner. The report must describe all activities conducted and sightings
of marine mammals, must provide full documentation of methods, results, and
interpretation pertaining to all monitoring, and must summarize the dates and locations of
survey operations and all marine mammal sightings (dates, times, locations, activities,
associated survey activities). The draft report shall also include geo-referenced timestamped vessel tracklines for all time periods during which airgun arrays were operating.
Tracklines should include points recording any change in airgun array status (e.g., when
the sources began operating, when they were turned off, or when they changed
operational status such as from full array to single gun or vice versa). Geographic
Information System files shall be provided in Environmental Systems Research Institute
shapefile format and include the UTC date and time, latitude in decimal degrees, and
longitude in decimal degrees. All coordinates shall be referenced to the WGS84
geographic coordinate system. In addition to the report, all raw observational data shall
be made available. The report must summarize data collected as described above. A final
report must be submitted within 30 days following resolution of any comments on the
draft report.
The report must include a validation document concerning the use of PAM, which
should include necessary noise validation diagrams and demonstrate whether background
noise levels on the PAM deployment limited achievement of the planned detection goals.
Copies of any vessel self-noise assessment reports must be included with the report.
Reporting Injured or Dead Marine Mammals
Discovery of injured or dead marine mammals—In the event that personnel
involved in the survey activities discover an injured or dead marine mammal, the L-DEO

shall report the incident to the Office of Protected Resources (OPR) as soon as feasible.
The report must include the following information:
●

Time, date, and location (latitude/longitude) of the first discovery (and

updated location information if known and applicable);
●

Species identification (if known) or description of the animal(s) involved;

●

Condition of the animal(s) (including carcass condition if the animal is

●

Observed behaviors of the animal(s), if alive;

●

If available, photographs or video footage of the animal(s); and

●

General circumstances under which the animal was discovered.

dead);

Vessel strike—In the event of a strike of a marine mammal by any vessel involved
in the activities covered by the authorization, L-DEO shall report the incident to OPR as
soon as feasible. The report must include the following information:
●

Time, date, and location (latitude/longitude) of the incident;

●

Vessel's speed during and leading up to the incident;

●

Vessel's course/heading and what operations were being conducted (if

applicable);
●

Status of all sound sources in use;

●

Description of avoidance measures/requirements that were in place at the

time of the strike and what additional measure were taken, if any, to avoid strike;
●

Environmental conditions (e.g., wind speed and direction, BSS, cloud

cover, visibility) immediately preceding the strike;
●

Species identification (if known) or description of the animal(s) involved;

●

Estimated size and length of the animal that was struck;

●

Description of the behavior of the marine mammal immediately preceding

and following the strike;

●

If available, description of the presence and behavior of any other marine

mammals present immediately preceding the strike;
●

Estimated fate of the animal (e.g., dead, injured but alive, injured and

moving, blood or tissue observed in the water, status unknown, disappeared); and
●

To the extent practicable, photographs or video footage of the animal(s).

Negligible Impact Analysis and Determination
NMFS has defined negligible impact as an impact resulting from the specified
activity that cannot be reasonably expected to, and is not reasonably likely to, adversely
affect the species or stock through effects on annual rates of recruitment or survival (50
CFR 216.103). A negligible impact finding is based on the lack of likely adverse effects
on annual rates of recruitment or survival (i.e., population-level effects). An estimate of
the number of takes alone is not enough information on which to base an impact
determination. In addition to considering estimates of the number of marine mammals
that might be “taken” through harassment, NMFS considers other factors, such as the
likely nature of any impacts or responses (e.g., intensity, duration), the context of any
impacts or responses (e.g., critical reproductive time or location, foraging impacts
affecting energetics), as well as effects on habitat, and the likely effectiveness of the
mitigation. We also assess the number, intensity, and context of estimated takes by
evaluating this information relative to population status. Consistent with the 1989
preamble for NMFS’ implementing regulations (54 FR 40338, September 29, 1989), the
impacts from other past and ongoing anthropogenic activities are incorporated into this
analysis via their impacts on the baseline (e.g., as reflected in the regulatory status of the
species, population size and growth rate where known, ongoing sources of human-caused
mortality, or ambient noise levels).
To avoid repetition, the discussion of our analysis applies to all the species listed
in table 1, given that the anticipated effects of this activity on these different marine

mammal stocks are expected to be similar. Where there are meaningful differences
between species or stocks they are included as separate subsections below. NMFS does
not anticipate that serious injury or mortality would occur as a result of L-DEO’s planned
survey, even in the absence of mitigation, and no serious injury or mortality is proposed
to be authorized. As discussed in the Potential Effects of Specified Activities on
Marine Mammals and Their Habitat section above, non-auditory physical effects and
vessel strike are not expected to occur. NMFS expects that the majority of potential takes
would be in the form of short-term Level B harassment, resulting from temporary
avoidance of the area or decreased foraging (if such activity was occurring), reactions
that are considered to be of low severity and with no lasting biological consequences
(e.g., Southall et al., 2007).
We are proposing to authorize a limited number of Level A harassment events of
five species in the form of PTS (humpback whale, minke whale, sei whale, and Kogia spp
(i.e., pygmy and dwarf sperm whales)) and Level B harassment of all 28 marine mammal
species (table 6). If any PTS is incurred in marine mammals as a result of the specified
activity, we expect only a small degree of PTS that would not result in severe hearing
impairment because of the constant movement of both the R/V Langseth and of the
marine mammals in the project areas, as well as the fact that the vessel is not expected to
remain in any one area in which individual marine mammals would be expected to
concentrate for an extended period of time. Additionally, L-DEO would shut down the
airgun array if marine mammals approach within 500 m (with the exception of specific
genera of dolphins, see Proposed Mitigation), further reducing the expected duration
and intensity of sound and therefore, the likelihood of marine mammals incurring PTS.
Since the duration of exposure to loud sounds will be relatively short, it would be
unlikely to affect the fitness of any individuals. Also, as described above, we expect that
marine mammals would likely move away from a sound source that represents an

aversive stimulus, especially at levels that would be expected to result in PTS, given
sufficient notice of the R/V Langseth's approach due to the vessel's relatively low speed
when conducting seismic surveys.
In addition, the maximum expected Level B harassment zone around the survey
vessel is 6,733 m. Therefore, the ensonified area surrounding the vessel is relatively small
compared to the overall distribution of animals in the area and their use of the habitat.
Feeding behavior is not likely to be significantly impacted as prey species are mobile and
are broadly distributed throughout the survey area; therefore, marine mammals that may
be temporarily displaced during survey activities are expected to be able to resume
foraging once they have moved away from areas with disturbing levels of underwater
noise. Because of the short duration (11.5 days) and temporary nature of the disturbance
and the availability of similar habitat and resources in the surrounding area, the impacts
to marine mammals and marine mammal prey species are not expected to cause
significant or long-term fitness consequences for individual marine mammals or their
populations.
Additionally, the acoustic “footprint” of the proposed survey would be very small
relative to the ranges of all marine mammals that would potentially be affected. Sound
levels would increase in the marine environment in a relatively small area surrounding
the vessel compared to the range of the marine mammals within the proposed survey
area. The seismic array would be active 24 hours per day throughout the duration of the
proposed survey. However, the very brief overall duration of the proposed survey (30
survey days) would further limit potential impacts that may occur as a result of the
proposed activity.
Of the marine mammal species that are likely to occur in the project area, the
following species are listed as endangered under the ESA: blue whales, fin whales, sei
whales, and sperm whales. The take numbers proposed for authorization for these species

(table 6) are minimal relative to their modeled population sizes; therefore, we do not
expect population-level impacts to any of these species. Moreover, the actual range of the
populations extends past the area covered by the model, so modeled population sizes are
likely smaller than their actual population size. The other marine mammal species that
may be taken by harassment during L-DEO's seismic survey are not listed as threatened
or endangered under the ESA. There is no designated critical habitat for any ESA-listed
marine mammals within the project area.
There are no rookeries, mating, or calving grounds known to be biologically
important to marine mammals within the survey area, and there are no feeding areas
known to be biologically important to marine mammals within the survey area.
The proposed mitigation measures are expected to reduce, to the extent
practicable, the intensity and/or duration of takes for all species listed in table 1. In
particular, they would provide animals the opportunity to move away from the sound
source throughout the survey area before seismic survey equipment reaches full energy,
thus, preventing them from being exposed to sound levels that have the potential to cause
injury (Level A harassment) or more severe Level B harassment.
In summary and as described above, the following factors primarily support our
preliminary determination that the impacts resulting from this activity are not expected to
adversely affect any of the species or populations through effects on annual rates of
recruitment or survival:
●

No serious injury or mortality is anticipated or proposed to be authorized;

●

We are proposing to authorize a limited number of Level A harassment

events of five species in the form of PTS; if any PTS is incurred as a result of the
specified activity, we expect only a small degree of PTS that would not result in severe
hearing impairment because of the constant movement of both the vessel and of the
marine mammals in the project areas, as well as the fact that the vessel is not expected to

remain in any one area in which individual marine mammals would be expected to
concentrate for an extended period of time.
●

The proposed activity is temporary and of relatively short duration (11.5

days of planned survey activity);
●

The vast majority of anticipated impacts of the proposed activity on

marine mammals would be temporary behavioral changes due to avoidance of the
ensonified area, which is relatively small (see table 4);
●

The availability of alternative areas of similar habitat value for marine

mammals to temporarily vacate the survey area during the proposed survey to avoid
exposure to sounds from the activity is readily abundant;
●

The potential adverse effects on fish or invertebrate species that serve as

prey species for marine mammals from the proposed survey would be temporary and
spatially limited and impacts to marine mammal foraging would be minimal;
●

The proposed mitigation measures are expected to reduce the number and

severity of takes, to the extent practicable, by visually and/or acoustically detecting
marine mammals within the established zones and implementing corresponding
mitigation measures (e.g., delay; shutdown).
Based on the analysis contained herein of the likely effects of the specified
activity on marine mammals and their habitat and taking into consideration the
implementation of the proposed monitoring and mitigation measures, NMFS
preliminarily finds that the marine mammal take from the proposed activity will have a
negligible impact on all affected marine mammal species or populations.
Small Numbers
As noted previously, only take of small numbers of marine mammals may be
authorized under sections 101(a)(5)(A) and (D) of the MMPA for specified activities
other than military readiness activities. The MMPA does not define small numbers and

so, in practice, where estimated numbers are available, NMFS compares the number of
individuals taken to the most appropriate estimation of abundance of the relevant species
or population in our determination of whether an authorization is limited to small
numbers of marine mammals. When the predicted number of individuals to be taken is
fewer than one-third of the species or population abundance, the take is considered to be
of small numbers. Additionally, other qualitative factors may be considered in the
analysis, such as the temporal or spatial scale of the activities.
The number of takes NMFS proposes to authorize is below one-third of the most
appropriate abundance estimate for all relevant populations (specifically, take of
individuals is less than 1 percent of the modeled abundance of each affected population,
see table 6). This is conservative because the modeled abundance represents a population
of the species and we assume all takes are of different individual animals, which is likely
not the case. Some individuals may be encountered multiple times in a day, but PSOs
would count them as separate individuals if they cannot be identified.
Based on the analysis contained herein of the proposed activity, including the
proposed mitigation and monitoring measures, and the proposed authorized take of
marine mammals, NMFS preliminarily finds that small numbers of marine mammals
would be taken relative to the size of the affected species or populations.
Unmitigable Adverse Impact Analysis and Determination
There are no relevant subsistence uses of the affected marine mammal stocks or
species implicated by this action. Therefore, NMFS has determined that the total taking
of affected species or stocks would not have an unmitigable adverse impact on the
availability of such species or stocks for taking for subsistence purposes.
Endangered Species Act
Section 7(a)(2) of the Endangered Species Act of 1973 (ESA; 16 U.S.C. 1531 et
seq.) requires that each Federal agency insure that any action it authorizes, funds, or

carries out is not likely to jeopardize the continued existence of any endangered or
threatened species or result in the destruction or adverse modification of designated
critical habitat. To ensure ESA compliance for the issuance of IHAs, NMFS consults
internally whenever we propose to authorize take for endangered or threatened species.
NMFS is proposing to authorize take of blue whales, fin whales, sei whales, and
sperm whales, which are listed under the ESA. The NMFS OPR Permits and
Conservation Division has requested initiation of section 7 consultation with the OPR
ESA Interagency Cooperation Division for the issuance of this IHA. NMFS will conclude
the ESA consultation prior to reaching a determination regarding the proposed issuance
of the authorization.
Proposed Authorization
As a result of these preliminary determinations, NMFS proposes to issue an IHA
to L-DEO for conducting a marine geophysical survey at the Chain Transform Fault in
the equatorial Atlantic Ocean during austral summer 2024, provided the previously
mentioned mitigation, monitoring, and reporting requirements are incorporated. A draft
of the proposed IHA can be found at: https://www.fisheries.noaa.gov/national/marinemammal-protection/incidental-take-authorizations-research-and-other-activities.
Request for Public Comments
We request comment on our analyses, the proposed authorization, and any other
aspect of this notice of proposed IHA for the proposed marine geophysical survey. We
also request comment on the potential renewal of this proposed IHA as described in the
paragraph below. Please include with your comments any supporting data or literature
citations to help inform decisions on the request for this IHA or a subsequent renewal
IHA.
On a case-by-case basis, NMFS may issue a one-time, 1 year renewal IHA
following notice to the public providing an additional 15 days for public comments when

(1) up to another year of identical or nearly identical activities as described in the
Description of Proposed Activity section of this notice is planned or (2) the activities as
described in the Description of Proposed Activity section of this notice would not be
completed by the time the IHA expires and a renewal would allow for completion of the
activities beyond that described in the Dates and Duration section of this notice, provided
all of the following conditions are met:
●

A request for renewal is received no later than 60 days prior to the needed

renewal IHA effective date (recognizing that the renewal IHA expiration date cannot
extend beyond 1 year from expiration of the initial IHA).
●

The request for renewal must include the following:

(1) An explanation that the activities to be conducted under the requested renewal
IHA are identical to the activities analyzed under the initial IHA, are a subset of the
activities, or include changes so minor (e.g., reduction in pile size) that the changes do
not affect the previous analyses, mitigation and monitoring requirements, or take
estimates (with the exception of reducing the type or amount of take).
(2) A preliminary monitoring report showing the results of the required
monitoring to date and an explanation showing that the monitoring results do not indicate
impacts of a scale or nature not previously analyzed or authorized.
Upon review of the request for renewal, the status of the affected species or
stocks, and any other pertinent information, NMFS determines that there are no more
than minor changes in the activities, the mitigation and monitoring measures will remain
the same and appropriate, and the findings in the initial IHA remain valid.
Dated: July 1, 2024.
Kimberly Damon-Randall,
Director, Office of Protected Resources,
National Marine Fisheries Service.

[FR Doc. 2024-14737 Filed: 7/5/2024 8:45 am; Publication Date: 7/8/2024]