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Federal Register / Vol. 86, No. 148 / Thursday, August 5, 2021 / Rules and Regulations
fractures in the ice, and over relatively shallow continental shelf waters with high marine productivity Durner et al.
2004. This sea-ice habitat favors foraging for their primary prey, ringed seals Pusa hispida, and other species such as bearded seals Erignathus barbatus Thiemann et al. 2008, Cherry et al. 2011, Stirling and Derocher 2012.
Although over most of their range polar bears prefer to remain on the sea-ice year-round, an increasing proportion of stocks are spending prolonged periods of time onshore Rode et al. 2015, Atwood et al. 2016b. While time spent on land occurs primarily in late summer and autumn Rode et al. 2015, Atwood et al. 2016b, they may be found throughout the year in the onshore and nearshore environments. Polar bear distribution in coastal habitats is often influenced by the movement of seasonal sea ice Atwood et al. 2016b, Wilson et al. 2017 and its direct and indirect effects on foraging success and, in the case of pregnant females, also dependent on availability of suitable denning habitat Durner et al. 2006, Rode et al. 2015, Atwood et al. 2016b.
In Alaska during the late summer/fall period July through November, polar bears from the Southern Beaufort Sea stock often occur along the coast and barrier islands, which serve as travel corridors, resting areas, and to some degree, foraging areas. Based on Industry observations and coastal survey data acquired by the Service Wilson et al. 2017, encounter rates between humans and polar bears are higher during the fall July to November than in any other season, and an average of 140 polar bears may occur on shore during any week during the period July through November between Utqiagvik and the Alaska Canada border Wilson et al. 2017. The length of time bears spend in these coastal habitats has been linked to sea ice dynamics Rode et al. 2015, Atwood et al. 2016b. The remains of subsistence-harvested bowhead whales at Cross and Barter islands provide a readily available food attractant in these areas Schliebe et al. 2006. However, the contribution of bowhead carcasses to the diet of SBS polar bears varies annually e.g., estimated as 1126
percent and 014 percent in 2003 and 2004, respectively and by sex, likely depending on carcass and seal availability as well as ice conditions Bentzen et al. 2007.
Polar bears have no natural predators though cannibalism is known to occur;
Stirling et al. 1993, Amstrup et al.
2006b. However, their life-history e.g., late maturity, small litter size, prolonged breeding interval is
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conducive to low intrinsic population growth i.e., growth in the absence of human-caused mortality, which was estimated at 6 percent to 7.5 percent for the SBS stock during 20042006 Regehr et al. 2010; Hunter et al. 2010. The lifespan of wild polar bears is approximately 25 years Rode et al.
2020. Females reach sexual maturity at 36 years old giving birth 1 year later Ramsay and Stirling 1988. In the SBS
region, females typically give birth at 5
years old Lentfer & Hensel 1980. On average, females in the SBS produce litter sizes of 1.9 cubs SD=0.5; Smith et al. 2007, 2010, 2013; Robinson 2014 at intervals that vary from 1 to 3 or more years depending on cub survival Ramsay and Stirling 1988 and foraging conditions. For example, when foraging conditions are unfavorable, polar bears may delay reproduction in favor of survival Derocher and Stirling 1992;
Eberhardt 2002. The determining factor for growth of polar bear stocks is adult female survival Eberhardt 1990. In general, rates above 90 percent are essential to sustain polar bear stocks Amstrup and Durner 1995 given low cub litter survival, which was estimated at 50 percent 90 percent CI: 3367
percent for the SBS stock during 2001
2006 Regehr et al. 2010. In the SBS, the probability that adult females will survive and produce cubs-of-the-year is negatively correlated with ice-free periods over the continental shelf Regehr et al. 2007a. In general, survival of cubs-of-the-year is positively related to the weight of the mother and their own weight Derocher and Stirling 1996; Stirling et al. 1999.
Females without dependent cubs typically breed in the spring Amstrup 2003, Stirling et al. 2016. Pregnant females enter maternity dens between October and December Durner et al.
2001; Amstrup 2003, and young are usually born between early December and early January Van de Velde et al.
2003. Only pregnant females den for an extended period during the winter Rode et al. 2018. Other polar bears may excavate temporary dens to escape harsh winter conditions; however, shelter denning is rare for Alaskan polar bear stocks Olson et al. 2017.
Typically, SBS females denning on land emerge from the den with their cubs around mid-March median emergence: March 11, Rode et al. 2018, USGS 2018, and commonly begin weaning when cubs are approximately 2.32.5 years old Ramsay and Stirling 1986, Arnould and Ramsay 1994, Amstrup 2003, Rode 2020. Cubs are born blind, with limited fat reserves, and are able to walk only after 6070
days Blix and Lentfer 1979; Kenny and
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Bickel 2005. 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.
Therefore, it is thought that successful denning, birthing, and rearing activities require a relatively undisturbed environment. A more detailed description of the potential consequences of disturbance to denning females can be found below in Potential Effects of Oil and Gas Industry Activities on Pacific Walrus, Polar Bear, and Prey Species: Polar Bear: Effects to Denning Bears. Radio and satellite telemetry studies indicate that denning can occur in multiyear pack ice and on land Durner et al. 2020. The proportion of dens on land has been increasing along the Alaska region 34.4 percent in 1985
1995 to 55.2 percent in 20072013;
Olson et al. 2017 likely in response to reductions in stable old ice, which is defined as sea ice that has survived at least one summers melt Bowditch 2002, increases in unconsolidated ice, and lengthening of the melt season Fischbach et al. 2007, Olson et al.
2017. If sea-ice extent in the Arctic continues to decrease and the amount of unstable ice increases, a greater proportion of polar bears may seek to den on land Durner et al. 2006, Fischbach et al. 2007, Olson et al. 2017.
In Alaska, maternal polar bear dens occur on barrier islands linear features of low-elevation land adjacent to the main coastline that are separated from the mainland by bodies of water, river bank drainages, and deltas e.g., those associated with the Colville and Canning Rivers, much of the North Slope coastal plain in particular within the 1002 Area, i.e., the land designated in section 1002 of the Alaska National Interest Lands Conservation Actpart of ANWR in northeastern Alaska;
Amstrup 1993, Durner et al. 2006, and coastal bluffs that occur at the interface of mainland and marine habitat Durner et al. 2006, 2013, 2020; Blank 2013;
Wilson and Durner 2020. These types of terrestrial habitat are also designated as critical habitat for the polar bear under the Endangered Species Act 75
FR 76086, December 7, 2010.
Management and conservation concerns for the SBS and Chukchi/Bering Seas CS polar bear stocks include sea-ice loss due to climate change, humanbear conflict, oil and gas industry activity, oil spills and contaminants, marine shipping, disease, and the potential for
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