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Federal Register / Vol. 86, No. 106 / Friday, June 4, 2021 / Notices overestimated and not applicable. In fact, until one reaches a distance of approximately three or four times the near-field distance the average intensity of sound at any given distance from the array is still less than that based on calculations that assume a directional point source Lurton, 2002. The 6,600in3 airgun array planned for use during the proposed survey has an approximate diagonal of 28.8 m, resulting in a nearfield distance of 138.7 m at 1 kHz NSF
and USGS, 2011. Field measurements of this array indicate that the source behaves like multiple discrete sources, rather than a directional point source, beginning at approximately 400 m deep site to 1 km shallow site from the center of the array Tolstoy et al., 2009, distances that are actually greater than four times the calculated 140-m nearfield distance. Within these distances, the recorded received levels were always lower than would be predicted based on calculations that assume a directional point source, and increasingly so as one moves closer towards the array Tolstoy et al., 2009.
Given this, relying on the calculated distance 138.7 m as the distance at which we expect to be in the near-field is a conservative approach since even beyond this distance the acoustic modeling still overestimates the actual received level. Within the near-field, in order to explicitly evaluate the
likelihood of exceeding any particular acoustic threshold, one would need to consider the exact position of the animal, its relationship to individual array elements, and how the individual acoustic sources propagate and their acoustic fields interact. Given that within the near-field and dimensions of the array source levels would be below those assumed here, we believe exceedance of the peak pressure threshold would only be possible under highly unlikely circumstances.
In consideration of the received sound levels in the near-field as described above, we expect the potential for Level A harassment of mid-frequency cetaceans, otariid pinnipeds, and phocid pinnipeds to be de minimis, even before the likely moderating effects of aversion and/or other compensatory behaviors e.g., Nachtigall et al., 2018
are considered. We do not believe that Level A harassment is a likely outcome for any mid-frequency cetacean, otariid pinniped, or phocid pinniped and do not propose to authorize any Level A
harassment for these species.
Marine Mammal Occurrence In this section we provide the information about the presence, density, and group dynamics of marine mammals that will inform the take calculations. The Navys Marine Species Density Database DoN, 2019, 2021 is
currently the most comprehensive compendium for density data available for the Gulf of Alaska GOA and is the only source of density data available for southeast Alaska. Habitat-based stratified marine mammal densities developed by the U.S. Navy for assessing potential impacts of training activities in the GOA DoN, 2021; Rone et al., 2014, 2017 and at Behm Canal in southeast Alaska DoN, 2019 represent the best available information for estimating potential marine mammal exposures. The Navys GOA Temporary Marine Activities Area TMAA is situated south of Prince William Sound and east of Kodiak Island. The northern boundary of the TMAA is approximately 24 nautical miles south of the Kenai Peninsula. Behm Canal is approximately 45 km east of Ketchikan, AK, inshore of the proposed survey area in the same general part of southeast Alaska. In general, GOA density values were used for offshore deep water depths portions of the survey area, and Behm Canal density values were used for inshore shallow and intermediate water depths portions. For some species, no Behm Canal density information is available, and the GOA
density value was applied to all water depths. Density values are provided in Table 6 and discussed in greater detail below.

TABLE 6ESTIMATED DENSITY VALUES BY WATER DEPTH
/km2
Shallow depth <100 m 1

Species Gray whale 4
Humpback whale
Blue whale 4
Fin whale
Sei whale 4
Minke whale
Sperm whale 4
Bairds beaked whale 4
Stejnegers beaked whale 4
Cuviers beaked whale 4
Pacific white-sided dolphin
Northern right whale dolphin 5
Rissos dolphin 2
Killer whale
Dalls porpoise
Harbor porpoise 6
Northern fur seal 4
California sea lion 3
Steller sea lion
Northern elephant seal 4
Harbor seal

Intermediate depth 1001,000 m 1

0.0486

0.0486

0

3 0.0117

3 0.0117

4 0.0010

0.0001

0.0001

0.0005

3 0.0001

3 0.0001

4 0.0160

0.0004

0.0004

0.0004

3 0.0008

3 0.0008

4 0.0006

0
0 0
0 3 0.0075
0.0110
0.0000
3 0.0057
3 0.1210
0.0330
0.0661
0.0288
3 0.3162
0.0779
3 0.7811

0.0020
0 0
0 3 0.0075
0.0276
0.0000
3 0.0057
3 0.1210
0.0330
0.0661
0.0288
4 0.0570
0.0779
4 0.1407

0.0013
0.0005
0.0021
0.0020
4 0.0200
0.0367
0.0000
4 0.0020
4 0.0370
0 0.0661
0.0065
0 0.0779
0

1A

zero value indicates the species is not expected to occur in that depth stratum.
density value of 0.00001 applied to Rissos dolphin.
3 Source: DoN, 2019; 4 Source DoN, 2021; 5 Source: Becker et al. 2016; 6 Hobbs and Waite 2010.
2 Nominal
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Deep depth >1,000 m 1

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