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Federal Register / Vol. 86, No. 148 / Thursday, August 5, 2021 / Rules and Regulations to be able to survive outside of the den.
However, further development inside the den greatly enhances the probability of survival Amstrup 1993, Amstrup and Gardner 1994, Smith et al. 2007, Rode et al. 2018. Cubs typically weigh 1012
kg upon emergence from the den in the spring at approximately 3.5 months old Harington 1968, LBased on these developmental milestones, we consider 60 days after birth to mark the end of the early denning period. Currently, we are not aware of any studies directly documenting birth dates of polar bear cubs in the wild; however, several studies have estimated parturition based on indirect metrics. Van de Velde et al.
2003 evaluated historic records of bears legally harvested in dens. Their findings suggest that cubs were born between early December and early January. Additionally, Messier et al.
1994 found that the activity levels of radio-collared females dropped significantly in mid-December, leading the authors to conclude that a majority of births occurred before or around 15
December. Because cub age is not empirically known, we consider early denning to end on 13 February, which is 60 days after the estimated average birth date of 15 December.
Although disturbance to denning bears can be costly at any stage in the denning process, consequences in early denning can be especially high because of the vulnerability of cubs early in their development Elowe and Dodge 1989, Amstrup and Gardner 1994, Rode et al.
2018. If a female leaves a den during early denning, cub mortality is likely to occur due to a variety of factors including susceptibility to cold temperatures Blix and Lentfer 1979, Hansson and Thomassen 1983, Van de Velde 2003, predation Derocher and Wiig 1999, Amstrup et al. 2006b, and mobility limitations Lentfer 1975.
Thus, we can expect a high probability that cubs will suffer lethal take if they emerge early during this stage. Further, adult females that depart the den site during early denning are likely to experience physiological stresses such as increased heart rate Craighead et al.
1976, Laske et al. 2011 or increased body temperature Reynolds et al. 1986
that can result in significant energy expenditures Karprovich et al. 2009, Geiser 2013, Evans et al. 2016 thus likely resulting in Level B harassment.
Late Denning The third denning period, late denning, begins when cubs are 60 days old and ends at den emergence in the spring, which coincides with increases in prey availability Rode et al. 2018b.

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In the SBS, March 15th is the median estimated emergence date for landdenning bears Rode et al. 2018b.
During late denning, cubs develop the ability to travel more efficiently and become less susceptible to heat loss, which enhances their ability to survive after leaving the den Rode et al. 2018b.
For example, date of den emergence was identified as the most important variable influencing cub survival in a study of marked polar bears in the CS
and SBS stocks Rode et al. 2018b. The authors reported that all females that denned through the end of March had one cub when re-sighted 100 days after den emergence. Conversely, roughly half of the females that emerged from dens before the end of February did not have cubs when resighted 100
days after emergence, suggesting that later den emergence likely results in a greater likelihood of cub survival Rode et al. 2018b. Rode et al. 2018b do note several factors that could affect their findings; for example, it was not always known whether a female emerged from a den with cubs i.e., cubs died before re-sighting during the spring surveys.
Although the potential responses of bears to disturbance events e.g., emerging from dens early, abandoning dens, physiological changes during early and late denning are the same, consequences to cubs differ based on their developmental progress. In contrast to emergences during early denning, which are likely to result in cub mortality, emergences during late denning do not necessarily result in cub mortality because cubs potentially can survive outside the den after reaching approximately 60 days of age. However, because survival increases with time spent in the den during late denning, disturbances that contribute to an early emergence during late denning are likely to increase the probability of cub mortality, thus leading to a serious injury Level A harassment. Similar to the early denning period, this form of disturbance would also likely lead to Level B harassment for adult females.
Post-Emergence The post-emergence period begins at den emergence and ends when bears leave the den site and depart for the sea ice, which can occur up to 30 days after emergence Harington 1968, Jonkel et al.
1972, Kolenoski and Prevett 1980, Hansson and Thomassen 1983, Ovsyanikov 1998, Robinson 2014.
During the post-emergence period, bears spend time in and out of the den where they acclimate to surface conditions and engage in a variety of activities, including grooming, nursing, walking, playing, resting, standing, digging, and
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foraging on vegetation Harington 1968;
Jonkel et al. 1972; Hansson and Thomassen 1983; Ovsyanikov 1998;
Smith et al. 2007, 2013. While mothers outside the den spend most of their time resting, cubs tend to be more active, which likely increases strength and locomotion Harington 1968, Lentfer and Hensel 1980, Hansson and Thomassen 1983, Robinson 2014.
Disturbances that elicit an early departure from the den site may hinder the ability of cubs to travel Ovsyanikov 1998, thereby increasing the chances for cub abandonment Haroldson et al.
2002 or susceptibility to predation Derocher and Wiig 1999, Amstrup et al.
2006b.
Considerable variation exists in the duration of time that bears spend at dens post-emergence, and the relationship between the duration and cub survival has not been formally evaluated. However, a maternal female should be highly motivated to return to the sea ice to begin hunting and replenish her energy stores to support lactation, thus, time spent at the den site post emergence likely confers some fitness benefit to cubs. A disturbance that leads the family group to depart the den site early during this period therefore is likely to lead to a nonserious Level A harassment for the cubs and a Level B harassment for the adult female.
Walrus: Human-Walrus Encounters Walruses do not inhabit the Beaufort Sea frequently and the likelihood of encountering walruses during Industry operations is low and limited to the open-water season. During the time period of this ITR, Industry operations may occasionally encounter small groups of walruses swimming in open water or hauled out onto ice floes or along the coast. Industry monitoring data have reported 38 walruses between 1995 and 2015, with only a few instances of disturbance to those walruses AES Alaska 2015, USFWS
unpublished data. From 2009 through 2014, no interactions between walrus and Industry were reported in the Beaufort Sea ITR region. We have no evidence of any physical effects or impacts to individual walruses due to Industry activity in the Beaufort Sea.
However, in the Chukchi Sea, where walruses are more prevalent, Level B
harassment is known to sometimes occur during encounters with Industry.
Thus, if walruses are encountered during the activities proposed in this ITR, the interaction it could potentially result in disturbance.
Human encounters with walruses could occur during Industry activities,
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