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Federal Register / Vol. 86, No. 148 / Thursday, August 5, 2021 / Rules and Regulations
and CS stocks, the Eastern Polar Basin, the Western and Eastern Canadian Archipelago, and Norwegian Bay Malenfant et al. 2016. There is moderate genetic structure among these clusters, suggesting polar bears broadly remain in the same cluster when breeding. While there is currently no evidence for strong directional gene flow among the clusters Malenfant et al. 2016, migrants are not uncommon and can contribute to gene flow across clusters Kutschera et al. 2016.
Changing sea-ice conditions will make these cross-cluster migrations and the resulting gene flow more difficult in the future Kutschera et al. 2016.
Additionally, habitat loss from decreased sea-ice extent may impact polar bear reproductive success by reducing or altering suitable denning habitat and extending the polar bear fasting season Rode et al. 2018, Stirling and Derocher 2012, Molnar et al. 2020.
In the early 1990s, approximately 50
percent of the annual maternal dens of the SBS polar bear stock occurred on land Amstrup and Gardner 1994.
Along the Alaskan region the proportion of terrestrial dens increased from 34.4
percent in 19851995 to 55.2 percent in 20072013 Olson et al. 2017. Polar bears require a stable substrate for denning. As sea-ice conditions deteriorate and become less stable, seaice dens can become vulnerable to erosion from storm surges Fischbach et al. 2007. Under favorable autumn snowfall conditions, SBS females denning on land had higher reproductive success than SBS females denning on sea-ice. Factors that may influence the higher reproductive success of females with land-based dens include longer denning periods that allow cubs more time to develop, higher snowfall conditions that strengthen den integrity throughout the denning period Rode et al. 2018, and increased foraging opportunities on land e.g., scavenging on Bowhead whale carcasses Atwood et al. 2016b. While SBS polar bear females denning on land may experience increased reproductive success, at least under favorable snowfall conditions, it is possible that competition for suitable denning habitat on land may increase due to sea-ice decline Fischbach et al. 2007 and landbased dens may be more vulnerable to disturbance from human activities Linnell et al. 2000.
Polar bear reproductive success may also be impacted by declines in sea ice through an extended fasting season Molnar et al. 2020. By 2100, recruitment is predicted to become jeopardized in nearly all polar bear stocks if greenhouse gas emissions
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remain uncurbed RCP8.5
Representative Concentration Pathway 8.5 scenario as fasting thresholds are increasingly exceeded due to declines in sea-ice across the Arctic circumpolar range Molnar et al. 2020. As the fasting season increases, most of these 12
stocks, including in the SBS, are expected to first experience significant adverse effects on cub recruitment followed by effects on adult male survival and lastly on adult female survival Molnar et al. 2020. Without mitigation of greenhouse gas emissions and assuming optimistic polar bear responses e.g., reduced movement to conserve energy, cub recruitment in the SBS stock has possibly been already adversely impacted since the late 1980s, while detrimental impacts on male and female survival are forecasted to possibly occur in the late 2030s and 2040s, respectively.
Extended fasting seasons are associated with poor body condition Stirling and Derocher 2012, and a females body condition at den entry is a critical factor that determines whether the female will produce cubs and the cubs chance of survival during their first year Rode et al. 2018.
Additionally, extended fasting seasons will cause polar bears to depend more heavily on their lipid reserves for energy, which can release lipid-soluble contaminants, such as persistent organic pollutants and mercury, into the bloodstream and organ tissues. The increased levels of contaminants in the blood and tissues can affect polar bear health and body condition, which has implications for reproductive success and survival Jenssen et al. 2015.
Changes in sea-ice can impact polar bears by altering trophic interactions.
Differences in sea-ice dynamics, such as the timing of ice formation and breakup, as well as changes in sea-ice type and concentration, may impact the distribution of polar bears and/or their preys occurrence and reduce polar bears access to prey. A climate-induced reduction in overlap between female polar bears and ringed seals was detected after a sudden sea-ice decline in Norway that limited the ability of females to hunt on sea-ice Hamilton et al. 2017. While polar bears are opportunistic and hunt other species, their reliance on ringed seals is prevalent across their range Thiemann et al. 2007, 2008; Florko et al. 2020;
Rode et al. 2021. Male and female polar bears exhibit differences in prey consumption. Females typically consume more ringed seals compared to males, which is likely related to more limited hunting opportunities for females e.g., prey size constraints
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McKinney et al. 2017, Bourque et al.
2020. Female body condition has been positively correlated with consumption of ringed seals, but negatively correlated with the consumption of bearded seals Florko et al. 2020. Consequently, females are more prone to decreased foraging and reproductive success than males during years in which unfavorable sea-ice conditions limit polar bears access to ringed seals Florko et al. 2020.
In the SBS stock, adult female and juvenile polar bear consumption of ringed seals was negatively correlated with winter Arctic oscillation, which affects sea-ice conditions. This trend was not observed for male polar bears.
Instead, male polar bears consumed more bowhead whale as a result of scavenging the carcasses of subsistenceharvested bowhead whales during years with a longer ice-free period over the continental shelf. It is possible that these alterations in sea-ice conditions may limit female polar bears access to ringed seals, and male polar bears may rely more heavily on alternative onshore food resources in the southern Beaufort Sea region McKinney et al. 2017.
Changes in the availability and distribution of seals may influence polar bear foraging efficiency. Reduction in sea ice is expected to render polar bear foraging energetically more demanding, as moving through fragmented sea ice and open-water swimming require more energy than walking across consolidated sea ice Cherry et al. 2009, Pagano et al.
2012, Rode et al. 2014, Durner et al.
2017. Inefficient foraging can contribute to nutritional stress and poor body condition, which can have implications for reproductive success and survival Regehr et al. 2010.
The decline in Arctic sea ice is associated with the SBS polar bear stock spending more time in terrestrial habitats Schliebe et al. 2008. Recent changes in female denning habitat and extended fasting seasons as a result of sea-ice decline may affect the reproductive success of the SBS polar bear stock Rode et al. 2018; Stirling and Derocher 2012; Molnar et al. 2020.
Other relevant factors that could negatively affect the SBS polar bear stock include changes in prey availability, reduced genetic diversity through limited population connectivity and/or hybridization with other bear species, increased exposure to disease and parasite prevalence and/or dissemination, impacts of human activities oil and gas exploration/
extraction, shipping, harvesting, etc.
and pollution Post et al. 2013;
Hamilton and Derocher 2019. Based on the projections of sea-ice decline in the
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