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Federal Register / Vol. 86, No. 109 / Wednesday, June 9, 2021 / Notices deep. The resulting ocean depthinformed ensonification zone was then modified to remove land shadows marine areas behind land features. To do this, we created lines representing ensonification that radiated from each point along the proposed project transects. Lines were then clipped with a landform shapefile to identify areas where underwater sound will be absorbed by land features.
As we described in Description of Marine Mammals in the Specified Area, sea otters are overwhelmingly observed 95 percent within the 40-m 131-ft depth contour, although they can be found in areas with deeper water. Thus, high-density sea otter habitat was delineated by the 40-m 131-ft depth contour, and low-density otter habitat was between the 40-m and 100-m 131-

ft to 328-ft depth contours. Habitat was further divided into subregions established by Tinker et al. 2019 as densities of otters in these subregions differed. Otter densities for the affected subregions were determined using 2012
abundance estimates generated using the Bayesian hierarchical model developed by Tinker et al. 2019.
Abundance estimates are traditionally generated using aerial survey data from high-density habitat <40 m or 131 ft in depth. To calculate the density of otters in low-density habitat 40100 m or 131328 ft ocean depth, we multiplied the density of the adjacent high-density habitat by 0.05. The resulting density estimate accounts for the five percent of otters found in low-density areas.
The Level A ensonification zone did not overlap with either highor low-

density habitat areas. To determine the amount km2 of Level B ensonified habitat in each subregion, the highand low-density habitat shapefiles were clipped using the Level B ensonification shapefiles in ArcGIS Pro. The area impacted in each subregion was multiplied by the estimated otter density in that region to determine the number of otters that will experience Level B sound levels Table 2. The total number of takes was predicted by estimating the projected days of activity in each subregion using survey start points supplied by the applicant. In several areas, the length and direction of the proposed survey transects make it highly unlikely that impacts will last only one day. In these instances, we estimated two days of disturbance, and thus two takes for each otter.

TABLE 2ESTIMATED NUMBER OF OTTERS ENSONIFIED BY SOUND LEVELS GREATER THAN 160 dB DUE TO THE
PROPOSED ACTIVITIES
Level B take was calculated by multiplying the area ensonified in each subregion by that subregions modeled sea otter density, then multiplying by the projected days of ensonification Subreg.

lotter on DSK11XQN23PROD with NOTICES1

N06
S05
S12
N06
S01
S05
S12

Density otters/km2

Habitat type
Area impacted km2

Estimated take/day
Projected days of take
Estimated survey total takes

High <40 m
High <40 m
High <40 m
Low 40100 m
Low 40100 m
Low 40100 m
Low 40100 m

0.778
1.333
0.1748
0.034
0.084
0.123
0.0092

4.66
8.74
2.56
15.69
42.31
31.32
647.62

4
12
1 1
4 4
1

1
2 2
1 2
2 2

4
24
2 1
8 8
2

Total

27

49

Current Stock Total.
Percentage of Stock.

25,584

0.001

Critical Assumptions We estimate 49 takes of 27 sea otters by Level B harassment will occur due to NSF/LDEOs proposed high-energy seismic surveys. In order to conduct this analysis and estimate the potential amount of Level B take, several critical assumptions were made.
Otter density was calculated using a Bayesian hierarchical model created by Tinker et al. 2019, which includes assumptions that can be found in the original publication. The most recently available density estimates and those used for our analysis were for the year 2012. Low-density otter populations exhibit a growth rate that is typically directly related to resource availability, with growth rates slowing as the populations approach carrying capacity Estes 1990. The populations in Southeast Alaska vary in their densities and estimated carrying capacities
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Tinker et al. 2019, making it difficult to predict current density values. Thus, we relied on 2012 density estimates to calculate projected take. One subregion within the impact area, S12, was not included in the Tinker et al. 2019
published densities. To calculate otter density in this subregion, we used the 2012 aerial survey data that served as the models primary input. Thus, the S12 density estimate does not benefit from the additional information included in the Bayesian model provided by Tinker et al. 2019.
Estimation of ensonification zones used sound attenuation models that focused on absorption and dispersion rather than reflection and refraction.
Our models assumed that points of land intercepting high-level noise will effectively attenuate sound levels above 160 dB, and sea otters in areas behind those land features in land shadows
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will be exposed to sound less than 160
dB. This assumption is adequate for this analysis given the offshore location of the survey transects.
Finally, we estimated the repeated take of a portion of the otters affected by the proposed action due to the presence of the R/V Langseth for more than one day. We assume, due to the proposed survey transects, start points, and speed of the R/V Langseth, that otters within subregions S01, S05, and S12 will be ensonified for two days each. The applicant has listed a number of potential yet unanticipated reasons the R/V Langseth may remain in one area for an extended period of time, including poor data quality, inclement weather, or mechanical issues with the research vessel and/or equipment.
However, except for the case of a reshoot due to poor data quality, the vessels airgun array i.e., the source of
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