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Federal Register / Vol. 86, No. 38 / Monday, March 1, 2021 / Notices Services calculation of estimated incidental take is limited to the specified activity occurring in United States jurisdictional waters within the stocks range.
The survey would include several strike lines, parallel including one continuous line along the continental shelf and perpendicular to the coast.
The R/V Langseth will tow 4 strings containing an array of 36 airguns at a depth of 12 m 39 ft, creating a discharge volume of approximately 6,600 cubic inches in3 or 0.11 cubic meter m3 at a shot interval of 37.5 m 123 ft. The 36-airgun array could operate 24 hours a day, except during mitigation shutdowns, for the entirety of the 37 days of survey. The energy produced by the seismic array is broadband and ranges from a few hertz Hz to kilohertz kHz; however, all but a small fraction of the energy is focused in the 10300 Hz range Tolstoy et al.
2009. The receiving system would consist of one 15-km 9.3-mi long hydrophone streamer, Ocean Bottom Seismometers OBSs, and Ocean Bottom Nodes OBNs deployed within the survey area. In addition to the operations of the airgun array, a multibeam echosounder, a single-beam dual-frequency echosounder 4 and 12
kHz, a sub-bottom profiler SBP, and an Acoustic Doppler Current Profiler ADCP would be operated. Further information and technical specifications can be found in NSFs IHA application and the Services draft EA available at:
http www.regulations.gov, Docket No.
FWSR1ES20202012;0131.
Description of Northern Sea Otters in the Specified Activity Area The proposed area of specified activity occurs within the range of the Washington stock of the northern sea otter, a portion of the species range that is not listed under the Endangered Species Act of 1973, as amended ESA.
This stock primarily occurs along the Washington coast between Cape Flattery and Grays Harbor, but small groups have been reported in the Straits of Juan de Fuca and individual sea otters have been reported in Puget Sound and along the Oregon coast as far south as Cape Blanco Jeffries et al. 2019, USFWS
2018, unpublished observations J. Rice OSU. Among the largest members of the family Mustelidae but one of the smallest of marine mammals, northern sea otters exhibit limited sexual dimorphism males are larger than females and can attain weights and lengths up to 40 kg 110 lb and 1.4 m 4.6 ft, respectively. They have a typical life span of 1115 years Riedman and Estes 1990. Unlike most other marine
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mammals, sea otters have little subcutaneous fat. They depend on their clean, dense, water-resistant fur for insulation against the cold and maintain a high level of internal heat production to compensate for their lack of blubber.
Consequently, their energetic requirements are high, and they consume an amount of food equivalent to approximately 23 to 33 percent of their body weight per day Riedman and Estes 1990.
Northern sea otters forage in both rocky and soft-sediment communities in water depths of 40 m 131 ft or less Laidre et al. 2009, although otters have been documented along the Washington coast as far as 58 km 36 mi offshore in waters deeper than 200 m 656 ft Pearson 2019; supplemental data provided to USFWS. They tend to be found closer to shore during storms, but they venture farther out during good weather and calm seas Kenyon 1975.
Sea otters occasionally make dives of up to 100 m 328 ft Newby 1975, but the vast majority of feeding dives more than 95 percent occur in waters less than 40 m 131 ft in depth Tinker et al. 2006. Therefore, sea otter habitat is typically defined by the 40-m 131-ft depth contour Laidre et al. 2011.
The number of sea otters in this stock, for the purposes of this analysis, was estimated to be approximately 3,000, based on survey count data and projections for areas not surveyed. The estimated minimum abundance of the stock, based on survey count data, was 2,785 sea otters within the area between Cape Flattery and Grays Harbor, Washington, between shore and the 40m 131-ft depth contour Jeffries et al.
2019. While systematic surveys farther offshore have not been conducted in Washington or Oregon, otters have been documented farther offshore Pearson 2019. Surveys conducted in Southeast Alaska found 95 percent of northern sea otters were found in areas shallower than 40-m 131 ft and 5 percent farther offshore Tinker et al. 2019. Therefore, assuming a similar proportion of sea otters in Washington occur offshore, we added 5 percent 139 sea otters to the minimum abundance to account for otters farther offshore than 40-m 131-ft depth contour, to get a total population estimate of 2,924 for the area between Cape Flattery and Grays Harbor. Based on best professional judgment and limited anecdotal observations, we estimate two sea otters would be somewhere along the coast between Grays Harbor and the Washington/
Oregon border and two sea otters would be somewhere along the Oregon coast.
Otter densities were calculated for the area between Cape Flattery and Grays
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Harbor, broken down to north and south of the Quillayute River. Surveys indicate the otter population is not evenly distributed throughout the area surveyed Jeffries et al. 2019, and the distribution of the population during the proposed project is likely to be similar to that detected during surveys, as work will occur during the same time of year as the surveys were conducted.
See Table 2 for density estimations. A
density was not estimated for the area between Grays Harbor and the southern end of the project; rather, we assumed that the four sea otters estimated to occur there would be exposed.
Further biological information on this stock can be found in the Washington Department of Fish and Wildlifes Periodic Status Review Sato 2018 and Recovery Plan Lance et al. 2004. The sea otters in this stock have no regulatory status under the ESA. The potential biological removal PBR for this stock is 18 sea otters USFWS
2018. 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. While no mortality is anticipated or authorized here, PBR is included as a gross indicator of the status of the species.
Sea Otter Hearing Controlled sound exposure trials on a single older male southern sea otter E.
l. nereis indicate that otters can hear frequencies between 125 Hz and 38 kHz with best sensitivity between 1.2 and 27
kHz in air and 2 to 26 kHz underwater;
however, these thresholds may underrepresent best hearing capabilities in younger otters Ghoul and Reichmuth 2014. Aerial and underwater audiograms for a captive adult 14-yearold male southern sea otter in the presence of ambient noise suggest the sea otters hearing was less sensitive to high-frequency greater than 22 kHz and low-frequency less than 1 kHz sound than terrestrial mustelids, but was similar to that of a California sea lion Zalophus californianus. However, the subject otter was still able to hear low-frequency sounds, and the detection thresholds for sounds between 0.1251 kHz were between 116101 dB, respectively. Dominant frequencies of southern sea otter vocalizations are between 3 and 8 kHz, with some energy extending above 60 kHz McShane et al.
1995; Ghoul and Reichmuth 2012.

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