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Federal Register / Vol. 86, No. 148 / Thursday, August 5, 2021 / Rules and Regulations The estimated harassment values for the open-water 2021 and open-water 2026 seasons were adjusted to account for incomplete seasons as the regulations will be effective for only 85
and 15 percent of the open-water 2021
and 2026 seasons, respectively.

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Aircraft Impact to Surface Bears Polar bears in the project area will likely be exposed to the visual and auditory stimulation associated with AOGAs fixed-wing and helicopter flight plans; however, these impacts are likely to be minimal and not long-lasting to surface bears. Flyovers may cause disruptions in the polar bears normal behavioral patterns, thereby resulting in incidental Level B harassment. Sudden changes in direction, elevation, and movement may also increase the level of noise produced from the helicopter, especially at lower altitudes. This increased level of noise could disturb polar bears in the area to an extent that their behavioral patterns are disrupted and Level B harassment occurs.
Mitigation measures, such as minimum flight altitudes over polar bears and restrictions on sudden changes to helicopter movements and direction, will be required to reduce the likelihood that polar bears are disturbed by aircraft.
Once mitigated, such disturbances are expected to have no more than shortterm, temporary, and minor impacts on individual bears.
Estimating Harassment Rates of Aircraft Activities To predict how polar bears will respond to fixed-wing and helicopter overflights during North Slope oil and gas activities, we first examined existing data on the behavioral responses of polar bears during aircraft surveys conducted by the Service and U.S.
Geological Survey USGS between August and October during most years from 2000 to 2014 Wilson et al. 2017, Atwood et al. 2015, and Schliebe et al.
2008. Behavioral responses due to sight and sound of the aircraft have both been incorporated into this analysis as there was no ability to differentiate between the two response sources during aircraft survey observations. Aircraft types used for surveys during the study included a fixed-wing Aero-Commander from 2000
to 2004, a R44 helicopter from 2012 to 2014, and an A-Star helicopter for a portion of the 2013 surveys. During surveys, all aircraft flew at an altitude of approximately 90 m 295 ft and at a speed of 150 to 205 km per hour km/
h or 93 to 127 mi per hour mi/h.
Reactions indicating possible incidental Level B harassment were recorded when a polar bear was observed running from
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the aircraft or began to run or swim in response to the aircraft. Of 951 polar bears observed during coastal aerial surveys, 162 showed these reactions, indicating that the percentage of Level B harassments during these low-altitude coastal survey flights was as high as 17
percent.
Detailed data on the behavioral responses of polar bears to the aircraft and the distance from the aircraft each polar bear was observed were available for only the flights conducted between 2000 to 2004 n = 581 bears. The AeroCommander 690 was used during this period. The horizontal detection distance from the flight line was recorded for all groups of bears detected. To determine if there was an effect of distance on the probability of a response indicative of potential Level B harassment, we modeled the binary behavioral response by groups of bears to the aircraft with Bayesian probit regression Hooten and Hefley 2019.
We restricted the data to those groups observed less than 10 km from the aircraft, which is the maximum distance at which behavioral responses were likely to be reliably recorded.
In nearly all cases when more than one bear was encountered, every member of the group exhibited the same response, so we treated the group as the sampling unit, yielding a sample size of 346 groups. Of those, 63 exhibited behavioral responses. Model parameters were estimated using 10,000 iterations of a Markov chain Monte Carlo algorithm composed of Gibbs updates implemented in R R core team 2021, Hooten and Hefley 2019. Normal 0,1
priors, which are uninformative on the prior predictive scale Hobbs and Hooten 2015, were placed on model parameters. Distance to bear as well as squared distance to account for possible non-linear decay of probability with distance were included as covariates. However, the 95 percent confidence intervals for the estimated coefficients overlapped zero suggesting no significant effect of distance on polar bears behavioral responses. While it is likely that bears do respond differently to aircraft at different distances, the data available is heavily biased towards very short distances because the coastal surveys are designed to observe bears immediately along the coast. We were thus unable to detect any effect of distance. Therefore, to estimate a single rate of harassment, we fit an interceptonly model and used the distribution of the marginal posterior predictive probability to compute a point estimate.
Because the data from the coastal surveys were not systematically collected to study polar bear behavioral
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responses to aircraft, the data likely bias the probability of behavioral response low. We, therefore, chose the upper 99th percentile of the distribution as our point estimate of the probability of potential harassment. This equated to a harassment rate of 0.23. Because we were not able to detect an effect of distance, we could not correlate behavioral responses with profiles of sound pressure levels for the AeroCommander the aircraft used to collect the survey data. Therefore, we could also not use that relationship to extrapolate behavioral responses to sound profiles for takeoffs and landings nor sound profiles of other aircraft.
Accordingly, we applied the single harassment rate to all portions of all aircraft flight paths.
General Approach to Estimating Harassment for Aircraft Activities Aircraft information was determined using details provided in AOGAs Request, including flight paths, flight take-offs and landings, altitudes, and aircraft type. More information on the altitudes of future flights can be found in the Request. If no location or frequency information was provided, flight paths were approximated based on the information provided. Of the flight paths that were described clearly or were addressed through assumptions, we marked the approximate flight path start and stop points using ArcGIS Pro version 2.4.3, and the paths were drawn. For flights traveling between two airstrips, the paths were reviewed and duplicated as closely as possible to the flight logs obtained from www.FlightAware.com FlightAware, a website that maintains flight logs in the public domain. For flight paths where airstrip information was not available, a direct route was assumed. Activities such as pipeline inspections followed a route along the pipeline with the assumption the flight returned along the same route unless a more direct path was available.
Flight paths were broken up into segments for landing, take-off, and traveling to account for the length of time the aircraft may be impacting an area based on flight speed. The distance considered the landing area is based on approximately 4.83 km 3 mi per 305 m 1,000 ft of altitude descent speed. For all flight paths at or exceeding an altitude of 152.4 m 500
ft, the take-off area was marked as 2.41 km 1.5 mi derived from flight logs found through FlightAware, which suggested that ascent to maximum flight altitude took approximately half the time of the average descent. The remainder of the flight path that
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