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Federal Register / Vol. 86, No. 103 / Tuesday, June 1, 2021 / Proposed Rules
polar bear denning behavior and effects of disturbance.
From these sources, we identified 57
case studies representing instances where polar bears at a maternal den may have been exposed to human activities.
For each den, we considered the four denning periods separately, and for each period, determined whether adequate information existed to document
whether 1 the human activity met our definition of an exposure and 2 the response of the bears could be classified according to our rules and definitions. From these 57 dens, 80
denning period-specific events met these criteria. For each event, we classified the type and frequency i.e., discrete or repeated of the exposure,
the response of the bears, and the level of take associated with that response.
From this information, we calculated the probability that a discrete or repeated exposure would result in each possible level of take during each denning period, which informed the probabilities for outcomes in the simulation model Table 7.

TABLE 7PROBABILITY THAT A DISCRETE OR REPEATED EXPOSURE ELICITED A RESPONSE BY DENNING POLAR BEARS
THAT WOULD RESULT IN LEVEL B HARASSMENT, LEVEL A HARASSMENT INCLUDING SERIOUS AND NON-SERIOUS INJURY, OR LETHAL TAKE
Level B harassment was applicable to both adults and cubs, if present; Level A harassment and lethal take were applicable to cubs only. Probabilities were calculated from the analysis of 57 case studies of polar bear responses to human activity. Cells with NAs indicate these types of take were not possible during the given denning period Exposure type
Period
Discrete

Den Establishment
Early Denning
Late Denning
Post-emergence
Den Establishment
Early Denning
Late Denning
Post-emergence

Repeated

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Case Study Analysis Definitions Below, we provide definitions for terms used in this analysis, a general overview of denning chronology and periods details are provided in the Potential Effects to Pacific Walrus, Polar Bears and Prey Species: Effects on denning bears, and the rules established for using the case studies to inform the model.
Exposure and Response Definitions Exposure: Any human activity within 1.6 km 1 mi of a polar bear den site.
In the case of aircraft, an overflight within 457 m 0.3 mi above ground level.
Discrete exposure: An exposure that occurs only once and of short duration <30 minutes. It can also be a shortduration exposure that happens repeatedly but that is separated by sufficient time that exposures can be treated as independent e.g., aerial pipeline surveys that occur weekly.
Repeated exposure: An exposure that occurs more than once within a time period where exposures cannot be considered independent or an exposure that occurs due to continuous activity during a period of time e.g., traffic along a road, or daily visits to a well pad.
Response probability: The probability that an exposure resulted in a response by denning polar bears.
We categorized each exposure into categories based on polar bear response:

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None
Level B

0.400
1.000
0.091
0.000
1.000
0.800
0.708
0.000

0.600
0.000
0.000
0.000
0.000
0.000
0.000
0.267

No response: No observed or presumed behavioral or physiological response to an exposure.
Likely physiological response: An alteration in the normal physiological function of a polar bear e.g., elevated heart rate or stress hormone levels that is typically unobservable but is likely to occur in response to an exposure.
Behavioral response: A change in behavior in response to an exposure.
Behavioral responses can range from biologically insignificant e.g., a resting bear raising its head in response to a vehicle driving along a road to substantial e.g., cub abandonment and concomitant levels of take vary accordingly.
Timing Definitions Entrance date: The date a female first enters a maternal den after excavation is complete.
Emergence date: The date a maternal den is first opened and a bear is exposed directly to external conditions.
Although a bear may exit the den completely at emergence, we considered even partial-body exits e.g., only a bears head protruding above the surface of the snow to represent emergence in order to maintain consistency with dates derived from temperature sensors on collared bears e.g., Rode et al.
2018b. For dens located near regularly occurring human activity, we considered the first day a bear was observed near a den to be the emergence
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Non-serious Level A
NA
NA
NA
0.750
NA
NA
NA
0.733

Serious Level A
NA
NA
0.909
NA
NA
NA
0.292
NA

Lethal NA
0.000
0.000
0.250
NA
0.200
0.000
0.000

date unless other data were available to inform emergence dates e.g., GPS collar data.
Departure date: The date when bears leave the den site to return to the sea ice. If a bear leaves the den site after a disturbance but later returns, we considered the initial movement to be the departure date.
Definition of Various Denning Periods Den establishment period: Period of time between the start of maternal den excavation and the birth of cubs. Unless evidence indicates otherwise, all dens that are excavated by adult females in the fall or winter are presumed to be maternal dens. In the absence of other information, this period is defined as denning activity prior to December 1
i.e., estimated earliest date cubs are likely present in dens Derocher et al.
1992, Van de Velde et al. 2003.
Early denning period: Period of time from the birth of cubs until they reach 60 days of age and are capable of surviving outside the den. In the absence of other information, this period is defined as any denning activity occurring between December 1
and February 13 i.e., 60 days after 15
December, the estimated average date of cub birth; Van de Velde et al. 2003, Messier et al. 1994.
Late denning period: Period of time between when cubs reach 60 days of age and den emergence. In the absence of other information, this period is defined
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