2024-18130. Takes of Marine Mammals Incidental to Specified Activities; Taking Marine Mammals Incidental to the Office of Naval Research's Arctic Research Activities in the Beaufort and Chukchi Seas (Year 7)

Table 1—Characteristics of Modeled Acoustic Sources

Platform (total number deployed)	Acoustic source	Purpose/ function	Frequency	Signal strength (dB re 1 μPa at 1 m)	Pulse width/duty cycle
REMUS 600 UUV a (up to 1)	WHOI Micro-modem	Acoustic communications	900-950 Hz	NTE 180 dB by sys design limits	5 pings/hour with 30 sec pulse length.
REMUS 600 UUV a (up to 1)	UUV/WHOI Micro-modem	Acoustic communications	8-14 kHz	NTE 185 dB by sys design limits	10% average duty cycle, with 4 sec pulse length.
IGB (drifting) (2)	WHOI Micro-modem	Acoustic communications	900-950 Hz	NTE 180 dB by sys design limits	Transmit every 4 hours, 30 sec pulse length.
IGB (drifting) (2)	WHOI Micro-modem	Acoustic communications	8-14 kHz	NTE 185 dB by sys design limits	Typically receive only. Transmit is very intermittent.
Mooring (6)	WHOI Micro-modem (4)	Acoustic Navigation	900-950 Hz	NTE 180 dB by sys design limits	Transmit every 4 hours, 30 sec pulse length.
Mooring (6)	VLF (2)	Acoustic Navigation	35 Hz	NTE 190 dB	Up to 4 times per day, 10 minutes each.
Note: dB re 1 μPa at 1 m = decibels referenced to 1 microPascal at 1 meter; Hz = Hertz; IGB = Ice Gateway Buoy; kHz = kilohertz; NTE = not to exceed; VLF = very low frequency; WHOI = Woods Hole Oceanographic Institution.
a  REMUS use is not anticipated during the September 2024 cruise but is included in case of future use during the proposed IHA period.

Activities Not Likely To Result in Take

The following activities have been determined to be unlikely to result in take of marine mammals. These activities are described here but they are not discussed further in this notice.

De minimis Sources—The ONR characterizes de minimis sources as those with the following parameters: low source levels (SLs), narrow beams, downward directed transmission, short pulse lengths, frequencies outside known marine mammal hearing ranges, or some combination of these factors (Navy, 2013). NMFS concurs with the ONR's determination that the sources they have identified here as de minimis are unlikely to result in take of marine mammals. The following are some of the planned de minimis sources which would be used during the proposed action: Woods Hole Oceanographic Institution (WHOI) micromodem, Acoustic Doppler Current Profilers (ADCPs), ice profilers, and additional sources below 160 decibels referenced to 1 microPascal (dB re 1 μPa) used during towing operations. ADCPs may be used on moorings. Ice-profilers measure ice properties and roughness. The ADCPs and ice-profilers would all be above 200 kHz and therefore out of marine mammal hearing ranges, with the exception of the 75 kHz ADCP which has the characteristics and de minimis justification listed in table 2. They may be employed on moorings or UUVs.

A WHOI micromodem will also be employed during the leave behind period. In contrast with the WHOI micromodem usage described in table 1, which covers the use of the micromodem during research cruises, the use of the source during the leave behind period differs in nature. During this period, it is being used for very intermittent communication with vehicles to communicate vehicle status for safety of navigation purposes, and is treated as de minimis while employed in this manner.

Table 2—Parameters for De Minimis Non-Impulsive Acoustic Sources

Source name	Frequency range (kHz)	Sound pressure level (dB re 1 μPa at 1 m)	Pulse length (seconds)	Duty cycle (percent)	De minimis justification
ADCP	>200, 150, or 75	190	<0.001	<0.1	Very low pulse length, narrow beam, moderate source level.
Nortek Signature 500 kHz Doppler Velocity Log	500	214	<0.1	<13	Very high frequency.
CTD Attached Echosounder	5-20	160	0.004	2	Very low source level.
Note: dB re 1 μPa at 1 m = decibels referenced to 1 microPascal at 1 meter; kHz = kilohertz; ADCP = acoustic Doppler current profiler; CTD = conductivity temperature depth.

Drifting Oceanographic Sensors—Observations of ocean-ice interactions require the use of sensors that are moored and embedded in the ice. For the proposed action, it will not be required to break ice to do this, as deployments can be performed in areas of low ice-coverage or free floating ice. Sensors are deployed within a few dozen meters of each other on the same ice floe. Three types of sensors would be used: autonomous ocean flux buoys, Integrated Autonomous Drifters, and ice-tethered profilers. The autonomous ocean flux buoys measure oceanographic properties just below the ocean-ice interface. The autonomous ocean flux buoys would have ADCPs and temperature chains attached, to measure temperature, salinity, and other ocean parameters the top 6 m (20 ft) of the water column. Integrated Autonomous Drifters would have a long temperate string extending down to 200 m (656 ft) depth and would incorporate meteorological sensors, and a temperature spring to estimate ice thickness. The ice-tethered profilers would collect information on ocean temperature, salinity, and velocity down to 250 m (820 ft) depth.

Up to 20 Argo-type autonomous profiling floats may be deployed in the central Beaufort Sea. Argo float drift at 1,500 m (4,921 ft) depth, profiling from 2,000 m (6,562 ft) to the sea surface once every 10 days to collect profiles of ( print page 66074) temperature and salinity. Moored Oceanographic Sensors—Moored sensors would capture a range of ice, ocean, and atmospheric conditions on a year-round basis. These would be bottom anchored, sub-surface moorings measuring velocity, temperature, and salinity in the upper 500 m (1,640 ft) of the water column. The moorings also collect high-resolution acoustic measurements of the ice using the ice profilers described above. Ice velocity and surface waves would be measured by 500 kHz multibeam sonars from Nortek Signatures. The moored oceanographic sensors described above use only de minimis sources and are therefore not anticipated to have the potential for impacts on marine mammals or their habitat. On-ice Measurements—On-ice measurement systems would be used to collect weather data. These would include an Autonomous Weather Station and an Ice Mass Balance Buoy. The Autonomous Weather Station would be deployed on a tripod; the tripod has insulated foot platforms that are frozen into the ice. The system would consist of an anemometer, humidity sensor, and pressure sensor. The Autonomous Weather Station also includes an altimeter that is de minimis due to its very high frequency (200 kHz). The Ice Mass Balance Buoy is a 6 m (20 ft) sensor string, which is deployed through a 5 centimeter (cm; 2 inch (in)) hole drilled into the ice. The string is weighted by a 1 kilogram (kg; 2.2 pound (lb)) lead weight and is supported by a tripod. The buoy contains a de minimis 200 kHz altimeter and snow depth sensor. Autonomous Weather Stations and Ice Mass Balance Buoys will be deployed and will drift with the ice, making measurements until their host ice floes melt, thus destroying the instruments (likely in summer, roughly 1 year after deployment). After the on-ice instruments are destroyed they cannot be recovered and would sink to the seafloor as their host ice floes melted.

Proposed mitigation, monitoring, and reporting measures are described in detail later in this document (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 regarding population trends and threats may be found in NMFS' Stock Assessment Reports (SARs; https://www.fisheries.noaa.gov/​national/​marine-mammal-protection/​marine-mammal-stock-assessments) and more general information about these species ( e.g., physical and behavioral descriptions) may be found on NMFS' website ( https://www.fisheries.noaa.gov/​find-species).

Table 3 lists all species or stocks for which take is expected and proposed to be authorized for this activity and summarizes information related to the population or stock, including regulatory status under the MMPA and Endangered Species Act (ESA) and potential biological removal (PBR), where known. PBR is 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 (as described in NMFS' SARs). While no serious injury or mortality is anticipated or proposed to be authorized here, PBR and annual serious injury and mortality from anthropogenic sources are included here as gross indicators of the status of the species or stocks and other threats.

Marine mammal abundance estimates presented in this document represent the total number of individuals that make up a given stock or the total number estimated within a particular study or survey area. NMFS' stock abundance estimates for most species represent the total estimate of individuals within the geographic area, if known, that comprises that stock. For some species, this geographic area may extend beyond U.S. waters. All managed stocks in this region are assessed in NMFS' U.S. Alaska SARs (Young et al., 2023). All values presented in table 3 are the most recent available at the time of publication and are available online at: https://www.fisheries.noaa.gov/​national/​marine-mammal-protection/​marine-mammal-stock-assessments.

Table 3—Species Likely Impacted by the Specified Activities ¹

Common name	Scientific name	Stock	ESA/MMPA status; strategic (Y/N) 2	Stock abundance (CV, Nmin, most recent abundance survey) 3	PBR	Annual M/SI 4
Beluga Whale	Delphinapterus leucas	Beaufort Sea	-, -, N	39,258 (0.229, N/A, 1992)	UND	104
Beluga Whale	Delphinapterus leucas	Eastern Chukchi	-, -, N	13,305 (0.51, 8,875, 2017)	178	56
Ringed Seal	Pusa hispida	Arctic	T, D, Y	UND 5 (UND, UND, 2013)	UND	6,459
1  Information on the classification of marine mammal species can be found on the web page for The Society for Marine Mammalogy's Committee on Taxonomy ( https://marinemammalscience.org/​science-and-publications/​list-marine-mammal-species-subspecies/​).
2  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.
3  NMFS marine mammal stock assessment reports online at: https://www.fisheries.noaa.gov/​national/​marine-mammal-protection/​marine-mammal-stock-assessment-reports-region. CV is coefficient of variation; Nmin is the minimum estimate of stock abundance.
4  These values, found in NMFS's SARs, represent annual levels of human-caused mortality plus serious injury from all sources combined ( e.g., commercial fisheries, vessel strike). Annual M/SI often cannot be determined precisely and is in some cases presented as a minimum value or range. A CV associated with estimated mortality due to commercial fisheries is presented in some cases.
5  A reliable population estimate for the entire stock is not available. Using a sub-sample of data collected from the U.S. portion of the Bering Sea, an abundance estimate of 171,418 ringed seals has been calculated, but this estimate does not account for availability bias due to seals in the water or in the shore-fast ice zone at the time of the survey. The actual number of ringed seals in the U.S. portion of the Bering Sea is likely much higher. Using the Nmin based upon this negatively biased population estimate, the PBR is calculated to be 4,755 seals, although this is also a negatively biased estimate.

As indicated above, both species (with three managed stocks) in table 3 temporally and spatially co-occur with the activity to the degree that take is reasonably likely to occur. While bowhead whales ( Balaena mysticetus), gray whales ( Eschrichtius robustus), bearded seals ( Erignathus barbatus), spotted seals ( Phoca largha), and ribbon seals ( Histriophoca fasciata) have been documented in the area, the temporal and/or spatial occurrence of these ( print page 66075) species is such that take is not expected to occur, and they are not discussed further beyond the explanation provided below.

Due to the location of the study area ( i.e., northern offshore, deep water), there were no calculated exposures for the bowhead whale, gray whale, bearded seal, spotted seal, and ribbon seal from quantitative modeling of acoustic sources. Bowhead and gray whales are closely associated with the shallow waters of the continental shelf in the Beaufort Sea and are unlikely to be exposed to acoustic harassment from this activity (Young et al., 2023). Gray whales feed primarily in the Beaufort Sea, Chukchi Sea, and Northwestern Bering Sea during the summer and fall, but migrate south to winter in Baja California lagoons (Young et al., 2023). Gray whales are primarily bottom feeders (Swartz et al., 2006) in water depths of less than 60 m (196.9 ft) (Pike, 1962). Therefore, on the rare occasion that a gray whale does overwinter in the Beaufort Sea (Stafford et al., 2007), we would expect an overwintering individual to remain in shallow water over the continental shelf where it could feed. Spotted seals tend to prefer pack ice areas with water depths less than 200 m (656.2 ft) during the spring and move to coastal habitats in the summer and fall, found as far north as 69-72 degrees N (Muto et al., 2021). Although the study area includes some waters south of 72 degrees N, the acoustic sources with the potential to result in take of marine mammals are not found below that latitude and spotted seals are not expected to be exposed. Ribbon seals are found year-round in the Bering Sea but may seasonally range into the Chukchi Sea (Muto et al., 2021). The proposed action occurs primarily in the Beaufort Sea, outside of the core range of ribbon seals, thus ribbon seals are not expected to be behaviorally harassed. Narwhals ( Monodon monoceros) are considered extralimital in the project area and are not expected to be encountered. As no harassment is expected of the bowhead whale, gray whale, spotted seal, bearded seal, ribbon seal, and narwhal, these species will not be discussed further in this proposed notice.

The ONR utilized Conn et al. (2014) in their IHA application as an abundance estimate for ringed seals, which is based upon aerial abundance and distribution surveys conducted in the U.S. portion Bering Sea in 2012 (171,418 ringed seals) (Muto et al., 2021). This value is likely an underestimate due to the lack of accounting for availability bias for seals that were in the water at the time of the surveys as well as not including seals located within the shore-fast ice zone (Muto et al., 2021). Muto et al. (2021) notes that an accurate population estimate is likely larger by a factor of two or more. However, no accepted population estimate is present for Arctic ringed seals. Therefore, NMFS will also adopt the Conn et al. (2014) abundance estimate (171,418) for further analyses and discussions on this proposed action by ONR.

In addition, the polar bear ( Ursus maritimus) and Pacific walrus ( Odobenus rosmarus) may be found both on sea ice and/or in the water within the Beaufort Sea and Chukchi Sea. These species are managed by the U.S. Fish and Wildlife Service rather than NMFS and, therefore, they are not considered further in this document.

Beluga Whale

Beluga whales are distributed throughout seasonally ice-covered arctic and subarctic waters of the Northern Hemisphere (Gurevich, 1980), and are closely associated with open leads and polynyas in ice-covered regions (Hazard, 1988). Belugas may be either migratory or residential (non-migratory), depending on the population. Seasonal distribution is affected by ice cover, tidal conditions, access to prey, temperature, and human interaction (Frost et al., 1985; Hauser et al., 2014).

There are five beluga whale stocks recognized within U.S. waters: Cook Inlet, Bristol Bay, eastern Bering Sea, eastern Chukchi Sea, and Beaufort Sea. Two stocks, the Beaufort Sea and eastern Chukchi Sea stocks, have the potential to occur in the location of this proposed action.

Migratory Biologically Important Areas (BIAs) for belugas in the eastern Chukchi and Alaskan Beaufort Sea overlap the southern and western portion of the Study Area (Clarke et al., 2023). A migration corridor for both stocks of beluga whale includes the eastern Chukchi Sea through the Beaufort Sea, with the Beaufort Sea stock utilizing the migratory BIA in April-May and the Eastern Chukchi Sea stock utilizing portions of the area in November. There are also feeding BIAs for both stocks throughout the Arctic region (Clarke et al., 2023). During the winter, they can be found foraging in offshore waters associated with pack ice. When the sea ice melts in summer, they move to warmer river estuaries and coastal areas for molting and calving (Muto et al., 2021). Annual migrations can span over thousands of kilometers. The residential Beaufort Sea populations participate in short distance movements within their range throughout the year. Based on satellite tags (Suydam et al., 2001; Hauser et al., 2014), there is some overlap in distribution with the eastern Chukchi Sea beluga whale stock.

During the winter, eastern Chukchi Sea belugas occur in offshore waters associated with pack ice. In the spring, they migrate to warmer coastal estuaries, bays, and rivers where they may molt (Finley, 1982; Suydam, 2009), give birth to, and care for their calves (Sergeant and Brodie, 1969). Eastern Chukchi Sea belugas move into coastal areas, including Kasegaluk Lagoon (outside of the proposed project site), in late June and animals are sighted in the area until about mid-July (Frost and Lowry, 1990; Frost et al., 1993). Satellite tags attached to eastern Chukchi Sea belugas captured in Kasegaluk Lagoon during the summer showed these whales traveled 1,100 km (593 nm) north of the Alaska coastline, into the Canadian Beaufort Sea within three months (Suydam et al., 2001). Satellite telemetry data from 23 whales tagged during 1998-2007 suggest variation in movement patterns for different age and/or sex classes during July-September (Suydam et al., 2005). Adult males used deeper waters and remained there for the duration of the summer; all belugas that moved into the Arctic Ocean (north of 75 degrees N) were males, and males traveled through 90 percent pack ice cover to reach deeper waters in the Beaufort Sea and Arctic Ocean (79-80 degrees N) by late July/early August. Adult and immature female belugas remained at or near the shelf break in the south through the eastern Bering Strait into the northern Bering Sea, remaining north of Saint Lawrence Island over the winter.

Ringed Seal

Ringed seals are the most common pinniped in the Study Area and have wide distribution in seasonally and permanently ice-covered waters of the Northern Hemisphere (North Atlantic Marine Mammal Commission, 2004). Throughout their range, ringed seals have an affinity for ice-covered waters and are well adapted to occupying both shore-fast and pack ice (Kelly, 1988). Ringed seals can be found further offshore than other pinnipeds since they can maintain breathing holes in ice thickness greater than 2 m (6.6 ft) (Smith and Stirling, 1975). The breathing holes are maintained by ringed seals using their sharp teeth and claws found on their fore flippers. They remain in contact with ice most of the year and use it as a platform for molting in late spring to early summer, for pupping and nursing in late winter to ( print page 66076) early spring, and for resting at other times of the year (Muto et al., 2018).

Ringed seals have at least two distinct types of subnivean lairs: Haulout lairs and birthing lairs (Smith and Stirling, 1975). Haul-out lairs are typically single-chambered and offer protection from predators and cold weather. Birthing lairs are larger, multi-chambered areas that are used for pupping in addition to protection from predators. Ringed seals pup on both shore-fast ice as well as stable pack ice. Lentfer (1972) found that ringed seals north of Utqiaġvik, Alaska, build their subnivean lairs on the pack ice near pressure ridges. Since subnivean lairs were found north of Utqiaġvik, Alaska, in pack ice, they are also assumed to be found within the sea ice in the proposed project site. Ringed seals excavate subnivean lairs in drifts over their breathing holes in the ice, in which they rest, give birth, and nurse their pups for 5-9 weeks during late winter and spring (Chapskii, 1940; McLaren, 1958; Smith and Stirling, 1975). Ringed seals are born beginning in March but the majority of births occur in early April. About a month after parturition, mating begins in late April and early May.

In Alaskan waters, during winter and early spring when sea ice is at its maximum extent, ringed seals are abundant in the northern Bering Sea, Norton and Kotzebue Sounds, and throughout the Chukchi and Beaufort seas (Frost, 1985; Kelly, 1988). Passive acoustic monitoring of ringed seals from a high frequency recording package deployed at a depth of 240 m (787 ft) in the Chukchi Sea 120 km (65 nm) north-northwest of Utqiaġvik, Alaska detected ringed seals in the area between mid-December and late May over the 4 year study (Jones et al., 2014). In addition, ringed seals have been observed near and beyond the outer boundary of the U.S. EEZ (Beland and Ireland, 2010). During the spring and early summer, ringed seals may migrate north as the ice edge recedes and spend their summers in the open water period of the northern Beaufort and Chukchi Seas (Frost, 1985). Foraging-type movements have been recorded over the continental shelf and north of the continental shelf waters (Von Duyke et al., 2020). During this time, sub-adult ringed seals may also occur in the Arctic Ocean Basin (Hamilton et al., 2015; Hamilton et al., 2017).

With the onset of fall freeze, ringed seal movements become increasingly restricted and seals will either move west and south with the advancing ice pack with many seals dispersing throughout the Chukchi and Bering Seas, or remaining in the Beaufort Sea (Crawford et al., 2012; Frost and Lowry, 1984; Harwood et al., 2012). Kelly et al. (2010a) tracked home ranges for ringed seals in the subnivean period (using shore-fast ice); the size of the home ranges varied from less than 1 up to 279 km² (median = 0.62 km² for adult males, 0.65 km² for adult females). Most (94 percent) of the home ranges were less than 3 km² during the subnivean period (Kelly et al., 2010a). Near large polynyas, ringed seals maintain ranges, up to 7,000 km² during winter and 2,100 km² during spring (Born et al., 2004). Some adult ringed seals return to the same small home ranges they occupied during the previous winter (Kelly et al., 2010a). The size of winter home ranges can vary by up to a factor of 10 depending on the amount of fast ice; seal movements were more restricted during winters with extensive fast ice, and were much less restricted where fast ice did not form at high levels (Harwood et al., 2015).

Of the five recognized subspecies of ringed seals, the Arctic ringed seal occurs in the Arctic Ocean and Bering Sea and is the only stock that occurs in U.S. waters. NMFS listed the Arctic ringed seal subspecies as threatened under the ESA on December 28, 2012 (77 FR 76706), primarily due to anticipated loss of sea ice through the end of the 21st century. Climate change presents a major concern for the conservation of ringed seals due to the potential for long-term habitat loss and modification (Muto et al., 2021). Based upon an analysis of various life history features and the rapid changes that may occur in ringed seal habitat, ringed seals are expected to be highly sensitive to climate change (Laidre et al., 2008; Kelly et al., 2010b).

Critical Habitat

Critical habitat for the ringed seal was designated in May 2022 and includes marine waters within one specific area in the Bering, Chukchi, and Beaufort Seas (87 FR 19232, April 1, 2022). Essential features established by NMFS for conservation of ringed seals are (1) snow-covered sea ice habitat suitable for the formation and maintenance of subnivean birth lairs used for sheltering pups during whelping and nursing, which is defined as waters 3 m (9.8 ft) or more in depth (relative to Mean Lower Low Water (MLLW)) containing areas of seasonal land-fast (shore-fast) ice or dense, stable pack ice, that have undergone deformation and contain snowdrifts of sufficient depth to form and maintain birth lairs (typically at least 54 cm (21.3 in) deep); (2) sea ice habitat suitable as a platform for basking and molting, which is defined as areas containing sea ice of 15 percent or more concentration in waters 3 m (9.8 ft) or more in depth (relative to MLLW); and (3) primary prey resources to support Arctic ringed seals, which are defined to be small, often schooling, fishes, in particular Arctic cod ( Boreogadus saida), saffron cod ( Eleginus gracilis), and rainbow smelt ( Osmerus dentex); and small crustaceans, in particular, shrimps and amphipods.

The Study Area does not overlap with ringed seal critical habitat (87 FR 19232, April 1, 2022). However, as stated in NMFS' final rule for the Designation of Critical Habitat for the Arctic Subspecies of the Ringed Seal (87 FR 19232, April 1, 2022), the area excluded from the critical habitat contains one or more of the essential features of the Arctic ringed seal's critical habitat, therefore, even though this area is excluded from critical habitat designation, habitat with the physical and biological features essential for ringed seal conservation is still available to the species, although data are limited to inform NMFS' assessment of the relative value of this area to the conservation of the species. As described later and in more detail in the Potential Effects of Specified Activities on Marine Mammals and Their Habitat section, we expect minimal impacts to marine mammal habitat as a result of the ONR's ARA, including impacts to ringed seal sea ice habitat suitable as a platform for basking and molting and impacts on prey availability.

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) and Southall et al. (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.). Subsequently, NMFS (2018) described generalized hearing ranges for these marine mammal hearing groups. Generalized hearing ranges were chosen based on the approximately 65 dB threshold from the normalized composite audiograms, with the exception for lower limits for low- ( print page 66077) 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 4.

Table 4—Marine Mammal Hearing Groups

[NMFS, 2018]

Hearing group	Generalized hearing range *
Low-frequency (LF) cetaceans (baleen whales)	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 approximately 65 dB threshold from normalized composite audiogram, with the exception for lower limits for LF cetaceans (Southall et al., 2007) and PW pinniped (approximation).

The pinniped functional hearing group was modified from Southall et al. (2007) on the basis of data indicating that phocid species have consistently demonstrated an extended frequency range of hearing compared to otariids, especially in the higher frequency range (Hemilä et al., 2006; Kastelein et al., 2009; Reichmuth et al., 2013). This division between phocid and otariid pinnipeds is now reflected in the updated hearing groups proposed in Southall et al. (2019).

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 Sound Sources

The marine soundscape is comprised of both ambient and anthropogenic sounds. Ambient sound is defined as the all-encompassing sound in a given place and is usually a composite of sound from many sources both near and far (ANSI, 1995). The sound level of an area is defined by the total acoustical energy being generated by known and unknown sources. These sources may include physical ( e.g., waves, wind, precipitation, earthquakes, ice, atmospheric sound), biological ( e.g., sounds produced by marine mammals, fish, and invertebrates), and anthropogenic sound ( e.g., vessels, dredging, aircraft, construction).

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 shipping 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 the 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 the specified activities may be a negligible addition to the local environment or could form a distinctive signal that may affect marine mammals.

Active acoustic sources and icebreaking, if necessary, are proposed for use in the Study Area. The sounds produced by these activities fall into one of two general sound types: impulsive and non-impulsive. Impulsive sounds ( e.g., ice explosions, gunshots, sonic booms, impact pile driving) are typically transient, brief (less than 1 second), broadband, and consist of high peak sound pressure with rapid rise time and rapid decay (ANSI, 1986; NIOSH, 1998; NMFS, 2018). Non-impulsive sounds ( e.g., aircraft, machinery operations such as drilling or dredging, vibratory pile driving, pile cutting, diamond wire sawing, and active sonar systems) can be broadband, narrowband, or tonal, brief or prolonged (continuous or intermittent), and typically do not have the high peak sound pressure with raid rise/decay time that impulsive sounds do (ANSI, 1986; NIOSH, 1998; NMFS, 2018). 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., Ward, 1997; Southall et al., 2007).

The likely or possible impacts of the ONR's proposed action on marine mammals involve both non-acoustic and acoustic stressors. Potential non-acoustic stressors could result from the physical presence of vessels, equipment, and personnel ( e.g., icebreaking impacts, vessel and in-water vehicle strike, and bottom disturbance); however, any impacts to marine mammals are expected to primarily be acoustic in nature ( e.g., non-impulsive acoustic sources, noise from icebreaking vessel (“icebreaking noise”), and vessel noise).

Acoustic Impacts

The introduction of anthropogenic noise into the aquatic environment from active acoustic sources and noise from icebreaking is the means by which marine mammals may be harassed from the ONR's specified activity. In general, animals exposed to natural or anthropogenic sound may experience behavioral, physiological, and/or physical effects, ranging in magnitude from none to severe (Southall et al., 2007). In general, exposure to pile driving noise has the potential to result in behavioral reactions ( e.g., avoidance, temporary cessation of foraging and vocalizing, changes in dive behavior) and, in limited cases, an auditory threshold shift (TS). Exposure to anthropogenic noise can also lead to non-observable physiological responses such an increase in stress hormones. Additional noise in a marine mammal's habitat can mask acoustic cues used by marine mammals to carry out daily functions such as communication and predator and prey detection. The effects ( print page 66078) of pile driving noise on marine mammals are dependent on several factors, including, but not limited to, sound type ( e.g., impulsive versus non-impulsive), the species, age and sex class ( e.g., adult male versus mother with calf), duration of exposure, the distance between the pile and the animal, received levels, behavior at time of exposure, and previous history with exposure (Wartzok et al., 2004; Southall et al., 2007). Here we discuss physical auditory effects ( i.e., TS) followed by behavioral effects and potential impacts on habitat.

NMFS defines a noise-induced TS as a change, usually an increase, in the threshold of audibility at a specified frequency or portion of an individual's hearing range above a previously established reference level (NMFS, 2018). The amount of TS is customarily expressed in dB and TS can be permanent or temporary. As described in NMFS (2018), there are numerous factors to consider when examining the consequence of TS, including, but not limited to, the signal temporal pattern ( e.g., impulsive or non-impulsive), likelihood an individual would be exposed for a long enough duration or to a high enough level to induce a TS, the magnitude of the TS, time to recovery (seconds to minutes or hours to days), the frequency range of the exposure ( i.e., spectral content), the hearing and vocalization frequency range of the exposed species relative to the signal's frequency spectrum ( i.e., how animal uses sound within the frequency band of the signal) (Kastelein et al., 2014), and the overlap between the animal and the source ( e.g., spatial, temporal, and spectral).

Permanent Threshold Shift (PTS)—NMFS defines PTS as a permanent, irreversible increase in the threshold of audibility at a specified frequency or portion of an individual's hearing range above a previously established reference level (NMFS, 2018). Available data from humans and other terrestrial mammals indicate that a 40 dB TS approximates PTS onset (see Ward et al., 1958; Ward et al., 1959; Ward, 1960; Kryter et al., 1966; Miller, 1974; Ahroon et al., 1996; Henderson et al., 2008). PTS levels for marine mammals are estimates as, with the exception of a single study unintentionally inducing PTS in a harbor seal ( e.g., Kastak et al., 2008), there are no empirical data measuring PTS in marine mammals largely due to the fact that, for various ethical reasons, experiments involving anthropogenic noise exposure at levels inducing PTS are not typically pursued or authorized (NMFS, 2018).

Temporary Threshold Shift (TTS)—TTS is a temporary, reversible increase in the threshold of audibility at a specified frequency or portion of an individual's hearing range above a previously established reference level (NMFS, 2018). Based on data from cetacean TTS measurements (see Southall et al., 2007), a TTS of 6 dB is considered the minimum TS clearly larger than any day-to-day or session-to-session variation in a subject's normal hearing ability (Finneran et al., 2000; Schlundt et al., 2000; Finneran et al., 2002). As described in Finneran (2016), marine mammal studies have shown the amount of TTS increases with cumulative sound exposure level (SEL_cum) in an accelerating fashion: At low exposures with lower SEL_cum, the amount of TTS is typically small and the growth curves have shallow slopes. At exposures with higher SEL_cum, the growth curves become steeper and approach linear relationships with the noise SEL.

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 (similar to those discussed in the Auditory Masking section). 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 takes place during a time when the animal is traveling through the open ocean, 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. We note that reduced hearing sensitivity as a simple function of aging has been observed in marine mammals, as well as humans and other taxa (Southall et al., 2007), so we can infer that strategies exist for coping with this condition to some degree, though likely not without cost.

Many studies have examined noise-induced hearing loss in marine mammals (see Finneran, 2015; Southall et al., 2019 for summaries). TTS is the mildest form of hearing impairment that can occur during exposure to sound (Kryter et al., 1966). 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. For cetaceans, published data on the onset of TTS are limited to captive bottlenose dolphin ( Tursiops truncatus), beluga whale, harbor porpoise ( Phocoena phocoena), and Yangtze finless porpoise ( Neophocoena asiaeorientalis) (Southall et al., 2019). For pinnipeds in water, measurements of TTS are limited to harbor seals ( Phoca vitulina), elephant seals ( Mirounga angustirostris), bearded seals, and California sea lions ( Zalophus californianus) (Kastak et al., 1999; Kastak et al., 2008; Kastelein et al., 2020b; Reichmuth et al., 2013; Sills et al., 2020). TTS was not observed in spotted and ringed seals exposed to single airgun impulse sounds at levels matching previous predictions of TTS onset (Reichmuth et al., 2016). These studies examine hearing thresholds measured in marine mammals before and after exposure to intense or long-duration sound exposure. The difference between the pre-exposure and post-exposure thresholds can be used to determine the amount of threshold shift at various post-exposure times.

The amount and onset of TTS depends on the exposure frequency. Sounds at low frequencies, well below the region of best sensitivity for a species or hearing group, are less hazardous than those at higher frequencies, near the region of best sensitivity (Finneran and Schlundt, 2013). At low frequencies, onset-TTS exposure levels are higher compared to those in the region of best sensitivity ( i.e., a low frequency noise would need to be louder to cause TTS onset when TTS exposure level is higher), as shown for harbor porpoises and harbor seals (Kastelein et al., 2019a; Kastelein et al., 2019b; Kastelein et al., 2020a; Kastelein et al., 2020b). Note that in general, harbor seals and harbor porpoises have a lower TTS onset than other measured pinniped or cetacean species (Finneran, 2015). In addition, TTS can accumulate across multiple exposures but the resulting TTS will be less than the TTS from a single, continuous exposure with the same SEL (Mooney et al., 2009; Finneran et al., 2010; Kastelein et al., 2014; Kastelein et al., 2015). This means that TTS predictions based on the total SEL_cum will overestimate the amount of TTS from intermittent exposures, such as sonars and impulsive sources. Nachtigall et al. (2018) describe measurements of hearing sensitivity of multiple odontocete species (bottlenose dolphin, harbor porpoise, beluga whale, and false killer whale ( Pseudorca crassidens)) when a relatively loud sound was preceded by a warning ( print page 66079) sound. These captive animals were shown to reduce hearing sensitivity when warned of an impending intense sound. Based on these experimental observations of captive animals, the authors suggest that wild animals may dampen their hearing during prolonged exposures or if conditioned to anticipate intense sounds. Another study showed that echolocating animals (including odontocetes) might have anatomical specializations that might allow for conditioned hearing reduction and filtering of low-frequency ambient noise, including increased stiffness and control of middle ear structures and placement of inner ear structures (Ketten et al., 2021). Data available on noise-induced hearing loss for mysticetes are currently lacking (NMFS, 2018). Additionally, the existing marine mammal TTS data come from a limited number of individuals within these species.

Relationships between TTS and PTS thresholds have not been studied in marine mammals and 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 decibels above that inducing mild TTS ( e.g., a 40-dB threshold shift approximates PTS onset (Kryter et al., 1966; Miller, 1974), while a 6-dB threshold shift approximates TTS onset (Southall et al., 2007; Southall et al., 2019). Based on data from terrestrial mammals, a precautionary assumption is that the PTS thresholds for impulsive sounds (such as impact pile driving 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; Southall et al., 2019). 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.

Activities for this project include active acoustics, equipment deployment and recovery, and, potentially, icebreaking. For the proposed action, these activities would not occur at the same time and there would likely be pauses in activities producing the sound during each day. Given these pauses and that many marine mammals are likely moving through the Study Area and not remaining for extended periods of time, the potential for TS declines.

Behavioral Harassment—Exposure to noise from pile driving and drilling also has the potential to behaviorally disturb marine mammals. Generally speaking, NMFS considers a behavioral disturbance that rises to the level of harassment under the MMPA a non-minor response—in other words, not every response qualifies as behavioral disturbance, and for responses that do, those of a higher level, or accrued across a longer duration, have the potential to affect foraging, reproduction, or survival. 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 may include changing durations of surfacing and dives, changing direction and/or speed; reducing/increasing vocal activities; changing/cessation of certain behavioral activities (such as socializing or feeding); eliciting a visible startle response or aggressive behavior (such as tail/fin slapping or jaw clapping); avoidance of areas where sound sources are located. Pinnipeds may increase their haul out time, possibly to avoid in-water disturbance (Thorson and Reyff, 2006). Behavioral responses to sound are highly variable and context-specific 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., 2004; Southall et al., 2007; Southall et al., 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). In general, pinnipeds seem more tolerant of, or at least habituate more quickly to, potentially disturbing underwater sound than do cetaceans, and generally seem to be less responsive to exposure to industrial sound than most cetaceans. Please see Appendices B and C of Southall et al. (2007) and Gomez et al. (2016) for reviews 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., 2004). 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 above, 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; Wartzok et al., 2004; NRC, 2005). Controlled experiments with captive marine mammals have showed pronounced behavioral reactions, including avoidance of loud sound sources (Ridgway et al., 1997; Finneran et al., 2003). Observed responses of wild marine mammals to loud pulsed sound sources ( e.g., seismic airguns) have been varied but often consist of avoidance behavior or other behavioral changes (Richardson et al., 1995; Morton and Symonds, 2002; Nowacek et al., 2007).

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 does react briefly to an underwater sound by changing its behavior or moving a small distance, the impacts of the 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). However, there are broad categories of potential response, which we describe in greater detail here, that include alteration of dive behavior, alteration of foraging behavior, effects to 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; Nowacek et al., 2004; Goldbogen et al., 2013a; Goldbogen et al., 2013b). Variations in dive behavior may reflect interruptions ( print page 66080) 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 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 incur fitness consequences 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.

Variations 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., 2005; Kastelein et al., 2006). For example, harbor porpoise' respiration rate increased in response to pile driving sounds at and above a received broadband SPL of 136 dB (zero-peak SPL: 151 dB re 1 μPa; SEL of a single strike: 127 dB re 1 μPa² -s) (Kastelein et al., 2013).

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 (Miller et al., 2000; Fristrup et al., 2003) or vocalizations (Foote et al., 2004), respectively, while North Atlantic right whales ( Eubalaena glacialis) 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).

Avoidance is the displacement of an individual from an area or migration path as a result of the presence of a sound or other stressors, and is one of the most obvious manifestations of disturbance in marine mammals (Richardson et al., 1995). Avoidance may be short-term, with animals returning to the area once the noise has ceased ( e.g., Bowles et al., 1994; Morton and Symonds, 2002). 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., Blackwell et al., 2004; Bejder et al., 2006).

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; Bowers et al., 2018). 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 solitary 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 fishes and terrestrial animals have shown that increased vigilance may substantially reduce feeding rates ( e.g., Beauchamp and Livoreil, 1997; Purser and Radford, 2011; Fritz et al., 2002). 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., 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 ( i.e., meaningful) 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.

Behavioral Responses to Icebreaking Noise—Ringed seals on pack ice showed various behaviors when approached by an icebreaking vessel. A majority of seals dove underwater when the ship was within 0.93 km (0.5 nm) while others remained on the ice. However, as icebreaking vessels came closer to the seals, most dove underwater. Ringed seals have also been observed foraging in the wake of an icebreaking vessel (Richardson et al., 1995) and may have preferentially established breathing holes in the ship tracks after the ice-breaker moved through the area. Previous observations and studies using icebreaking ships provide a greater understanding in how seal behavior may be affected by a vessel transiting through the area.

Adult ringed seals spend up to 20 percent of the time in subnivean lairs during the winter season (Kelly et al., ( print page 66081) 2010a). Ringed seal pups spend about 50 percent of their time in the lair during the nursing period (Lydersen and Hammill, 1993). During the warm season ringed seals haul out on the ice. In a study of ringed seal haul out activity by Born et al. (2002), ringed seals spent 25-57 percent of their time hauled out in June, which is during their molting season. Ringed seal lairs are typically used by individual seals (haulout lairs) or by a mother with a pup (birthing lairs); large lairs used by many seals for hauling out are rare (Smith and Stirling, 1975). If the non-impulsive acoustic transmissions are heard and are perceived as a threat, ringed seals within subnivean lairs could react to the sound in a similar fashion to their reaction to other threats, such as polar bears (their primary predators), although the type of sound would be novel to them. Responses of ringed seals to a variety of human-induced sounds ( e.g., helicopter noise, snowmobiles, dogs, people, and seismic activity) have been variable; some seals entered the water and some seals remained in the lair. However, in all instances in which observed seals departed lairs in response to noise disturbance, they subsequently reoccupied the lair (Kelly et al., 1988).

Ringed seal mothers have a strong bond with their pups and may physically move their pups from the birth lair to an alternate lair to avoid predation, sometimes risking their lives to defend their pups from potential predators. If a ringed seal mother perceives the proposed acoustic sources as a threat, the network of multiple birth and haulout lairs allows the mother and pup to move to a new lair (Smith and Stirling, 1975; Smith and Hammill, 1981). The acoustic sources from this proposed action are not likely to impede a ringed seal from finding a breathing hole or lair, as captive seals have been found to primarily use vision to locate breathing holes and no effect to ringed seal vision would occur from the acoustic disturbance (Elsner et al., 1989; Wartzok et al., 1992). It is anticipated that a ringed seal would be able to relocate to a different breathing hole relatively easily without impacting their normal behavior patterns.

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., Selye, 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 sufficient to restore normal function.

Relationships between these physiological mechanisms, animal behavior, and the costs of stress responses are well-studied through controlled experiments 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 vessel 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), however, distress is an unlikely result of the proposed project based on observations of marine mammals during previous, similar projects in the region.

Auditory Masking—Since many marine mammals rely on sound to find prey, moderate social interactions, and facilitate mating (Tyack, 2008), noise from anthropogenic sound sources can interfere with these functions, but only if the noise spectrum overlaps with the hearing sensitivity of the receiving marine mammal (Southall et al., 2007; Clark et al., 2009; Hatch et al., 2012). Chronic exposure to excessive, though not high-intensity, noise could cause masking at particular frequencies for marine mammals that utilize sound for vital biological functions (Clark et al., 2009). Acoustic masking is when other noises such as from human sources interfere 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). Therefore, under certain circumstances, marine mammals whose acoustical sensors or environment are being severely masked could also be impaired from maximizing their performance fitness in survival and reproduction. 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 (Hotchkin and Parks, 2013).

Under certain circumstances, marine mammals experiencing significant masking could also be impaired from maximizing their performance fitness in survival and reproduction. Therefore, when the coincident (masking) sound is human-made, it may be considered harassment when disrupting or altering critical behaviors. 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 ( print page 66082) (though not necessarily one that would be associated with harassment).

The frequency range of the potentially masking sound is important in determining 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, 2010; Holt et al., 2009). Masking can be reduced 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 (Hotchkin and Parks, 2013). 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).

Marine mammals at or near the proposed project site may be exposed to anthropogenic noise which may be a source of masking. Vocalization changes may result from a need to compete with an increase in background noise and include increasing the source level, modifying the frequency, increasing the call repetition rate of vocalizations, or ceasing to vocalize in the presence of increased noise (Hotchkin and Parks, 2013). For example, in response to loud noise, beluga whales may shift the frequency of their echolocation clicks to prevent masking by anthropogenic noise (Eickmeier and Vallarta, 2023).

Masking is more likely to occur in the presence of broadband, relatively continuous noise sources such as vibratory pile driving. Energy distribution of pile driving covers a broad frequency spectrum, and sound from pile driving would be within the audible range of pinnipeds and cetaceans present in the proposed action area. While icebreaking during the ONR's proposed action may mask some acoustic signals that are relevant to the daily behavior of marine mammals, the short-term duration (up to 8 days) and limited areas affected make it very unlikely that the fitness of individual marine mammals would be impacted.

Potential Effects on Prey—The marine mammal species in the Study Area feed on marine invertebrates and fish. Studies of sound energy effects on invertebrates are few, and primarily identify behavioral responses. It is expected that most marine invertebrates would not sense the frequencies of the acoustic transmissions from the acoustic sources associated with the proposed action. Although acoustic sources used during the proposed action may briefly impact individuals, intermittent exposures to non-impulsive acoustic sources are not expected to impact survival, growth, recruitment, or reproduction of widespread marine invertebrate populations.

The fish species residing in the study area include those that are closely associated with the deep ocean habitat of the Beaufort Sea. Nearly 250 marine fish species have been described in the Arctic, excluding the larger parts of the sub-Arctic Bering, Barents, and Norwegian Seas (Mecklenburg et al., 2011). However, only about 30 are known to occur in the Arctic waters of the Beaufort Sea (Christiansen and Reist, 2013). Although hearing capability data only exist for fewer than 100 of the 32,000 named fish species, current data suggest that most species of fish detect sounds from 50 to 100 Hz, with few fish hearing sounds above 4 kHz (Popper, 2008). It is believed that most fish have the best hearing sensitivity from 100 to 400 Hz (Popper, 2003). Fish species in the study area are expected to hear the low-frequency sources associated with the proposed action, but most are not expected to detect sound from the mid-frequency sources. Human generated sound could alter the behavior of a fish in a manner than would affect its way of living, such as where it tries to locate food or how well it could find a mate. Behavioral responses to loud noise could include a startle response, such as the fish swimming away from the source, the fish “freezing” and staying in place, or scattering (Popper, 2003). Misund (1997) found that fish ahead of a ship showed avoidance reactions at ranges of 49-149 m (160-489 ft). Avoidance behavior of vessels, vertically or horizontally in the water column, has been reported for cod and herring, and was attributed to vessel noise. While acoustic sources associated with the proposed action may influence the behavior of some fish species, other fish species may be equally unresponsive. Overall effects to fish from the proposed action would be localized, temporary, and infrequent.

Effects to Physical and Foraging Habitat—Ringed seals haul out on pack ice during the spring and summer to molt (Reeves et al., 2002; Born et al., 2002). Additionally, some studies suggested that ringed seals might preferentially establish breathing holes in ship tracks after vessels move through the area (Alliston, 1980; Alliston, 1981). The amount of ice habitat disturbed by activities is small relative to the amount of overall habitat available and there will be no permanent or longer-term loss or modification of physical ice habitat used by ringed seals. Vessel movement would have minimal effect on physical beluga habitat as beluga habitat is solely within the water column. Furthermore, the deployed sources that would remain in use after the vessels have left the survey area have low duty cycles and lower source levels, and any impacts to the acoustic habitat of marine mammals would be minimal.

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 NMFS' consideration of the negligible impact determinations and impacts on subsistence uses.

Harassment is the only type of take expected to result from these activities. For this military readiness activity, the MMPA defines “harassment” as (i) Any act that injures or has the significant potential to injure a marine mammal or marine mammal stock in the wild (Level A harassment); or (ii) Any act that disturbs or is likely to disturb a marine mammal or marine mammal stock in the wild by causing disruption of natural behavioral patterns, including, but not limited to, migration, surfacing, nursing, breeding, feeding, or sheltering, to a point where the behavioral patterns are abandoned or significantly altered (Level B harassment).

Authorized takes would be by Level B harassment only, in the form of direct behavioral disturbances and/or TTS for individual marine mammals resulting from exposure to active acoustic transmissions and icebreaking. Based on the nature of the activity, Level A harassment is neither anticipated nor proposed to be authorized.

As described previously, no serious injury or mortality is anticipated or proposed to be authorized for this activity. Below we describe how the proposed take numbers are estimated.

For acoustic impacts, generally speaking, we estimate take by considering: (1) acoustic thresholds ( print page 66083) 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). Thresholds have also been developed identifying the received level of in-air sound above which exposed pinnipeds would likely be behaviorally harassed.

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; Southall et al., 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 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.

In this case, NMFS is proposing to adopt the ONR's approach to estimating incidental take by Level B harassment from the active acoustic sources for this action, which includes use of dose response functions. The ONR's dose response functions were developed to estimate take from sonar and similar transducers, but are not applicable to icebreaking. Multi-year research efforts have conducted sonar exposure studies for odontocetes and mysticetes (Miller et al., 2012; Sivle et al., 2012). Several studies with captive animals have provided data under controlled circumstances for odontocetes and pinnipeds (Houser et al., 2013b; Houser et al., 2013a). Moretti et al. (2014) published a beaked whale dose-response curve based on passive acoustic monitoring of beaked whales during U.S. Navy training activity at Atlantic Underwater Test and Evaluation Center during actual Anti-Submarine Warfare exercises. This information necessitated the update of the behavioral response criteria for the U.S. Navy's environmental analyses.

Southall et al. (2007), and more recently (Southall et al., 2019), synthesized data from many past behavioral studies and observations to determine the likelihood of behavioral reactions at specific sound levels. While in general, the louder the sound source the more intense the behavioral response, it was clear that the proximity of a sound source and the animal's experience, motivation, and conditioning were also critical factors influencing the response (Southall et al., 2007; Southall et al., 2019). After examining all of the available data, the authors felt that the derivation of thresholds for behavioral response based solely on exposure level was not supported because context of the animal at the time of sound exposure was an important factor in estimating response. Nonetheless, in some conditions, consistent avoidance reactions were noted at higher sound levels depending on the marine mammal species or group allowing conclusions to be drawn. Phocid seals showed avoidance reactions at or below 190 dB re 1 μPa at 1 m; thus, seals may actually receive levels adequate to produce TTS before avoiding the source.

Odontocete behavioral criteria for non-impulsive sources were updated based on controlled exposure studies for dolphins and sea mammals, sonar, and safety (3S) studies where odontocete behavioral responses were reported after exposure to sonar (Miller et al., 2011; Miller et al., 2012; Antunes et al., 2014; Miller et al., 2014; Houser et al., 2013b). For the 3S study, the sonar outputs included 1-2 kHz up- and down-sweeps and 6-7 kHz up-sweeps; source levels were ramped up from 152-158 dB re 1 μPa to a maximum of 198-214 re 1 μPa at 1 m. Sonar signals were ramped up over several pings while the vessel approached the mammals. The study did include some control passes of ships with the sonar off to discern the behavioral responses of the mammals to vessel presence alone versus active sonar.

The controlled exposure studies included exposing the Navy's trained bottlenose dolphins to mid-frequency sonar while they were in a pen. Mid-frequency sonar was played at six different exposure levels from 125-185 dB re 1 μPa (RMS). The behavioral response function for odontocetes resulting from the studies described above has a 50 percent probability of response at 157 dB re 1 μPa. Additionally, distance cutoffs (20 km for MF cetaceans) were applied to exclude exposures beyond which the potential of significant behavioral responses is considered to be unlikely.

The pinniped behavioral threshold was updated based on controlled exposure experiments on the following captive animals: hooded seal ( Cystophora cristata), gray seal ( Halichoerus grypus), and California sea lion (Götz et al., 2010; Houser et al., 2013a; Kvadsheim et al., 2010). Hooded seals were exposed to increasing levels of sonar until an avoidance response was observed, while the grey seals were exposed first to a single received level multiple times, then an increasing received level. Each individual California sea lion was exposed to the same received level ten times. These exposure sessions were combined into a single response value, with an overall response assumed if an animal responded in any single session. The resulting behavioral response function for pinnipeds has a 50 percent probability of response at 166 dB re 1 ( print page 66084) μPa. Additionally, distance cutoffs (10 km for pinnipeds) were applied to exclude exposures beyond which the potential of significant behavioral responses is considered unlikely. For additional information regarding marine mammal thresholds for PTS and TTS onset, please see NMFS (2018) and table 6.

Empirical evidence has not shown responses to non-impulsive acoustic sources that would constitute take beyond a few km from a non-impulsive acoustic source, which is why NMFS and the Navy conservatively set distance cutoffs for pinnipeds and mid-frequency cetaceans (U.S. Department of the Navy, 2017a). The cutoff distances for fixed sources are different from those for moving sources, as they are treated as individual sources in ONR's modeling given that the distance between them is significantly greater than the range to which environmental effects can occur. Fixed source cutoff distances used were 5 km (2.7 nm) for pinnipeds and 10 km (5.4 nm) for beluga whales (table 5). As some of the on-site drifting sources could come closer together, the drifting source cutoffs applied were 10 km (5.4 nm) for pinnipeds and 20 km (10.8 nm) for beluga whales (table 5). Regardless of the received level at that distance, take is not estimated to occur beyond these cutoff distances. Range to thresholds were calculated for the noise associated with icebreaking in the study area. These all fall within the same cutoff distances as non-impulsive acoustic sources; range to behavioral threshold for both beluga whales and ringed seal were under 5 km (2.7 nm), and range to TTS threshold for both under 15 m (49.2 ft) (table 5).

Table 5—Cutoff Distances and Acoustic Thresholds Identifying the Onset of Behavioral Disturbance, TTS, and PTS for Non-Impulsive Sound Sources

Hearing group	Species	Fixed source behavioral threshold cutoff distance a	Drifting source behavioral threshold cutoff distance a	Behavioral criteria: Non-impulsive acoustic sources	Icebreaking source behavioral threshold cutoff distance a b	Behavioral criteria: icebreaking sources	Physiological criteria: onset TTS	Physiological criteria: onset PTS
Mid-frequency cetaceans	Beluga whale	10 km (5.4 nm)	20 km (10.8 nm)	Mid-frequency BRF dose-response function *	5 km (2.7 nm)	120 dB re 1 µPa step function	178 dB SEL cum	198 dB SEL cum .
Phocidae (in water)	Ringed seal	5 km (2.7 nm)	10 km (5.4 nm)	Pinniped dose-response function *	5 km (2.7 nm)	120 dB re 1 µPa step function	181 dB SEL cum	201 dB SEL cum .
Note: The threshold values provided are assumed for when the source is within the animal's best hearing sensitivity. The exact threshold varies based on the overlap of the source and the frequency weighting (see figure 6-1 in IHA application).
a  Take is not estimated to occur beyond these cutoff distances, regardless of the received level.
b  Range to TTS threshold for both hearing groups for the noise associated with icebreaking in the Study Area is under 15 m (49.2 ft).

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). The ONR's proposed action includes the use of non-impulsive (active sonar and icebreaking) sources; however, Level A harassment is not expected as a result of the proposed activities based on modeling, as described below, nor is it proposed to be authorized by NMFS.

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-mammal-acoustic-technical-guidance.

Table 6—Thresholds Identifying the Onset of Permanent Threshold Shift

Hearing group	PTS onset acoustic thresholds * (received level)
	Impulsive	Non-impulsive
Low-Frequency (LF) Cetaceans	Cell 1:Lpk,flat : 219 dB; LE,LF,24h : 183 dB	Cell 2:LE,LF,24h : 199 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 1 μPa2 s. In this table, thresholds are abbreviated to reflect American National Standards Institute (ANSI) standards. 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.

( print page 66085)

Quantitative Modeling

The Navy performed a quantitative analysis to estimate the number of marine mammals likely to be exposed to underwater acoustic transmissions above the previously described threshold criteria during the proposed action. Inputs to the quantitative analysis included marine mammal density estimates obtained from the Kaschner et al. (2006) habitat suitability model and (Cañadas et al., 2020), marine mammal depth occurrence (U.S. Department of the Navy, 2017b), oceanographic and mammal hearing data, and criteria and thresholds for levels of potential effects. The quantitative analysis consists of computer modeled estimates and a post-model analysis to determine the number of potential animal exposures. The model calculates sound energy propagation from the proposed non-impulsive acoustic sources, the sound received by animat (virtual animal) dosimeters representing marine mammals distributed in the area around the modeled activity, and whether the sound received by animats exceeds the thresholds for effects.

The Navy developed a set of software tools and compiled data for estimating acoustic effects on marine mammals without consideration of behavioral avoidance or mitigation. These tools and data sets serve as integral components of the Navy Acoustic Effects Model (NAEMO). In NAEMO, animats are distributed non-uniformly based on species-specific density, depth distribution, and group size information and animats record energy received at their location in the water column. A fully three-dimensional environment is used for calculating sound propagation and animat exposure in NAEMO. Site-specific bathymetry, sound speed profiles, wind speed, and bottom properties are incorporated into the propagation modeling process. NAEMO calculates the likely propagation for various levels of energy (sound or pressure) resulting from each source used during the training event.

NAEMO then records the energy received by each animat within the energy footprint of the event and calculates the number of animats having received levels of energy exposures that fall within defined impact thresholds. Predicted effects on the animats within a scenario are then tallied and the highest order effect (based on severity of criteria; e.g., PTS over TTS) predicted for a given animat is assumed. Each scenario, or each 24-hour period for scenarios lasting greater than 24 hours is independent of all others, and therefore, the same individual marine mammal (as represented by an animat in the model environment) could be impacted during each independent scenario or 24-hour period. In few instances, although the activities themselves all occur within the proposed study location, sound may propagate beyond the boundary of the study area. Any exposures occurring outside the boundary of the study area are counted as if they occurred within the study area boundary. NAEMO provides the initial estimated impacts on marine species with a static horizontal distribution ( i.e., animats in the model environment do not move horizontally).

There are limitations to the data used in the acoustic effects model, and the results must be interpreted within this context. While the best available data and appropriate input assumptions have been used in the modeling, when there is a lack of definitive data to support an aspect of the modeling, conservative modeling assumptions have been chosen ( i.e., assumptions that may result in an overestimate of acoustic exposures):

• Animats are modeled as being underwater, stationary, and facing the source and therefore always predicted to receive the maximum potential sound level at a given location (i.e., no porpoising or pinnipeds' heads above water);
• Animats do not move horizontally (but change their position vertically within the water column), which may overestimate physiological effects such as hearing loss, especially for slow moving or stationary sound sources in the model;
• Animats are stationary horizontally and therefore do not avoid the sound source, unlike in the wild where animals would most often avoid exposures at higher sound levels, especially those exposures that may result in PTS;
• Multiple exposures within any 24-hour period are considered one continuous exposure for the purposes of calculating potential threshold shift, because there are not sufficient data to estimate a hearing recovery function for the time between exposures; and
• Mitigation measures were not considered in the model. In reality, sound-producing activities would be reduced, stopped, or delayed if marine mammals are detected by visual monitoring.

Due to these inherent model limitations and simplifications, model-estimated results should be further analyzed, considering such factors as the range to specific effects, avoidance, and the likelihood of successfully implementing mitigation measures. This analysis uses a number of factors in addition to the acoustic model results to predict acoustic effects on marine mammals, as described below in the Marine Mammal Occurrence and Take Estimation section.

The underwater radiated noise signature for icebreaking in the central Arctic Ocean by CGC HEALY during different types of ice-cover was characterized in Roth et al. (2013). The radiated noise signatures were characterized for various fractions of ice cover. For modeling, the 8/10 and 3/10 ice cover were used. Each modeled day of icebreaking consisted of 16 hours of 8/10 ice cover and 8 hours of 3/10 ice cover. The sound signature of the 5/10 icebreaking activities, which would correspond to half-power icebreaking, was not reported in Roth et al. (2013); therefore, the full-power signature was used as a conservative proxy for the half-power signature. Icebreaking was modeled for 8 days total. Since ice forecasting cannot be predicted more than a few weeks in advance, it is unknown if icebreaking would be needed to deploy or retrieve the sources after 1 year of transmitting. Therefore, the potential for an icebreaking cruise on CGC HEALY was conservatively analyzed within the ONR's request for an IHA. As the R/V Sikuliaq is not capable of icebreaking, acoustic noise created by icebreaking is only modeled for the CGC HEALY. Figures 5a and 5b in Roth et al. (2013) depict the source spectrum level versus frequency for 8/10 and 3/10 ice cover, respectively. The sound signature of each of the ice coverage levels was broken into 1-octave bins (table 7). In the model, each bin was included as a separate source on the modeled vessel. When these independent sources go active concurrently, they simulate the sound signature of CGC HEALY. The modeled source level summed across these bins was 196.2 dB for the 8/10 signature and 189.3 dB for the 3/10 ice signature. These source levels are a good approximation of the icebreaker's observed source level (provided in figure 4b of Roth et al. (2013). Each frequency and source level was modeled as an independent source, and applied simultaneously to all of the animats within NAEMO. Each second was summed across frequency to estimate SPL_RMS. Any animat exposed to sound levels greater than 120 dB was considered a take by Level B harassment. For PTS and TTS, determinations, sound exposure levels were summed over the duration of the ( print page 66086) test and the transit to the deep water deployment area. The method of quantitative modeling for icebreaking is considered to be a conservative approach; therefore, the number of takes estimated for icebreaking are likely an overestimate and would not be expected to reach that level.

Table 7—Modeled Bins for 8/10 Ice Coverage (Full Power) and 3/10 Ice Coverage (Quarter Power) Icebreaking on CGC HEALY

Frequency (Hz)	8/10 source level (dB)	3/10 source level (dB)
25	189	187
50	188	182
100	189	179
200	190	177
400	188	175
800	183	170
1,600	177	166
3,200	176	171
6,400	172	168
12,800	167	164

Non-Impulsive Acoustic Analysis

Most likely, individuals affected by acoustic transmission would move away from the sound source. Ringed seals may be temporarily displaced from their subnivean lairs in the winter, but a pinniped would have to be within 5 km (2.7 nm) of a moored source or within 10 km (5.4 nm) of a drifting source for any behavioral reaction. Any effects experienced by individual pinnipeds are anticipated to be short-term disturbance of normal behavior, or temporary displacement or disruption of animals that may be near elements of the proposed action.

Of historical sightings registered in the Ocean Biodiversity Information System Spatial Ecological Analysis of Megavertebrate Populations (OBIS-SEAMAP database) (Halpin et al., 2009) in the ARA Study Area, nearly all (99 percent) occurred in summer and fall seasons. However, there is no documentation to prove that this is because ringed seals would all move out of the Study Area during the cold season, or if the lack of sightings is due to the harsh environment and ringed seal behavior being prohibitive factors for cold season surveying. OBIS-SEAMAP reports 542 animals sighted over 150 records in the ARA Study Area across all years and seasons. Taking the average of 542 animals in 150 records aligns with survey data from previous ARA cruises that show up to three ringed seals (or small, unidentified pinnipeds assumed to be ringed seals) per day sighted in the Study Area. To account for any unsighted animals, that number was rounded up to 4. Assuming that four animals would be present in the Study Area, a rough estimate of density can be calculated using the overall Study Area size:

4 ringed seals ÷ 48,725 km² = 0.00008209 ringed seals/km²

The area of influence surrounding each moored source would be 78.5 km² , and the area of influence surrounding each drifting source would be 314 km² . The total area of influence on any given day from non-impulsive acoustic sources would be 942 km² . The number of ringed seals that could be taken daily can be calculated:

0.00008209 ringed seals/km² × 942 km² = 0.077 ringed seals/day

To be conservative, the ONR has assumed that one ringed seal would be exposed to acoustic transmissions above the threshold for Level B harassment, and that each would be exposed each day of the proposed action (365 days total). Unlike the NAEMO modeling approach used to estimate ringed seal takes in previous ARA IHAs, the occurrence method used in this ARA IHA request does not support the differentiation between behavioral or TTS exposures. Therefore, all takes are classified as Level B harassment and not further distinguished. Modeling for all previous years of ARA activities did not result in any estimated Level A harassment. NMFS has no reason to expect that the ARA activities during the effective dates of this IHA would be more likely to result in Level A harassment. Therefore, no Level A harassment is anticipated due to the proposed action.

Marine Mammal Occurrence and Take Estimation

In this section we provide information about the occurrence of marine mammals, including density or other relevant information which will inform the take calculations. We also describe how the marine mammal occurrence information is synthesized to produce a quantitative estimate of the take that is reasonably likely to occur and proposed for authorization.

The beluga whale density numbers utilized for quantitative acoustic modeling are from the Navy Marine Species Density Database (U.S. Department of the Navy, 2014). Where available ( i.e., June through 15 October over the continental shelf primarily), density estimates used were from Duke density modeling based upon line-transect surveys (Cañadas et al., 2020). The remaining seasons and geographic area were based on the habitat-based modeling by Kaschner (2004) and Kaschner et al. (2006). Density for beluga whales was not distinguished by stock and varied throughout the project area geographically and monthly; the range of densities in the Study Area is shown in table 8. The density estimates for ringed seals are based on the habitat suitability modeling by Kaschner (2004) and Kaschner et al. (2006) and shown in table 8.

Table 8—Density Estimates of Impacted Species

Common name	Stock	Density (animals/km2 )
Beluga whale	Beaufort Sea	0.000506 to 0.5176
Beluga whale	Eastern Chukchi Sea	0.000506 to 0.5176
Ringed seal	Arctic	0.1108 to 0.3562

Take of all species would occur by Level B harassment only. NAEMO was previously used to produce a qualitative estimate of PTS, TTS, and behavioral exposures for ringed seals. For this proposed action, a new approach that utilizes sighting data from previous surveys conducted within the Study Area was used to estimate Level B harassment associated with non-impulsive acoustic sources (see section 6.4.3 of the IHA application). NAEMO modeling is still used to provide estimated takes of beluga whales associated with non-impulsive acoustic sources, as well as provide take estimations associated with icebreaking for both species. Table 9 shows the total number of requested takes by Level B harassment that NMFS proposes to authorize for both beluga whale stocks and the Arctic ringed seal stock based upon NAEMO modeled results. ( print page 66087)

Density estimates for beluga whales are equal as estimates were not distinguished by stock (Kaschner, 2004; Kaschner et al., 2006). The ranges of the Beaufort Sea and Eastern Chukchi Sea beluga whales vary within the study area throughout the year (Hauser et al., 2014). Based upon the limited information available regarding the expected spatial distributions of each stock within the study area, take has been apportioned equally to each stock (table 9). In addition, in NAEMO, animats do not move horizontally or react in any way to avoid sound, therefore, the current model may overestimate non-impulsive acoustic impacts.

Table 9—Proposed Take by Level B Harassment

Species	Stock	Active acoustics	Icebreaking (behavioral)	Icebreaking (TTS)	Total proposed take	SAR abundance	Percentage of population
Beluga whale	Beaufort Sea	a  177	a  21	0	99	39,258	<1
Beluga whale	Chukchi Sea	a  177	a  21	0	99	13,305	<1
Ringed seal	Arctic	365	538	1	904	b  UND (171, 418)	<1
a  Acoustic and icebreaking exposures to beluga whales were not modeled at the stock level as the density value is not distinguished by stock in the Arctic for beluga whales (U.S. Department of the Navy, 2014). Estimated take of beluga whales due to active acoustics is 177 and 21 due to icebreaking activities, totaling 198 takes of beluga whales. The total take was evenly distributed among the two stocks.
b  A reliable population estimate for the entire Arctic stock of ringed seals is not available and NMFS SAR lists it as Undetermined (UND). Using a sub-sample of data collected from the U.S. portion of the Bering Sea (Conn et al., 2014), an abundance estimate of 171,418 ringed seals has been calculated but this estimate does not account for availability bias due to seals in the water or in the shore-fast ice zone at the time of the survey. The actual number of ringed seals in the U.S. portion of the Bering Sea is likely much higher. Using the minimum population size (Nmin = 158,507) based upon this negatively biased population estimate, the PBR is calculated to be 4,755 seals, although this is also a negatively biased estimate.

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. 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)). The 2004 NDAA amended the MMPA as it relates to military readiness activities and the incidental take authorization process such that “least practicable impact” shall include consideration of personnel safety, practicality of implementation, and impact on the effectiveness of the military readiness activity.

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 or stocks, and their habitat, as well as subsistence uses. 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, impact on operations, and, in the case of a military readiness activity, personnel safety, practicality of implementation, and impact on the effectiveness of the military readiness activity.

The following measures are proposed for this IHA:

• All vessels operated by or for the Navy must have personnel assigned to stand watch at all times while underway. Watch personnel must employ visual search techniques using binoculars. While underway and while using active acoustic sources/towed in-water devices, at least one person with access to binoculars is required to be on watch at all times.
• Vessel captains and vessel personnel must remain alert at all times, proceed with extreme caution, and operate at a safe speed so that the vessel can take proper and effective action to avoid any collisions with marine mammals.
• During moored and drifting acoustic source deployment and recovery, ONR must implement a mitigation zone of 55 m (180 ft) around the deployed source. Deployment and recovery must cease if a marine mammal is visually deterred within the mitigation zone. Deployment and recovery may recommence if any one of the following conditions are met:

○ The animal is observed exiting the mitigation zone;

○ The animal is thought to have exited the mitigation zone based on a determination of its course, speed, and movement relative to the sound source;

○ The mitigation zone has been clear from any additional sightings for a period of 15 minutes for pinnipeds and 30 minutes for cetaceans.

• Vessels must avoid approaching marine mammals head-on and must maneuver to maintain a mitigation zone of 457 m (500 yards) around all observed cetaceans and 183 m (200 yards) around all other observed marine mammals, provided it is safe to do so.
• Activities must cease if a marine mammal species for which take was not authorized, or a species for which authorization was granted but the authorized number of takes have been met, is observed approaching or within the mitigation zone (table 10). Activities must not resume until the animal is confirmed to have left the area.
• Vessel captains must maintain at-sea communication with subsistence hunters to avoid conflict of vessel transit with hunting activity.

Table 10—Proposed Mitigation Zones

Activity and/or effort type	Species	Mitigation zone
Acoustic source deployment and recovery, stationary	Beluga whale	55 m (180 ft).
( print page 66088)
Acoustic source deployment and recovery, stationary	Ringed seal	55 m (180 ft).
Transit	Beluga whale	457 m (500 yards).
Transit	Ringed seal	183 m (200 yards).