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Federal Register / Vol. 86, No. 148 / Thursday, August 5, 2021 / Rules and Regulations
small-scale alterations to bear movements.

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Polar Bear: Effects to Denning Bears Known polar bear dens in the Beaufort Sea ITR region, whether discovered opportunistically or as a result of planned surveys such as tracking marked bears or den detection surveys, are monitored by the Service.
However, these known denning sites are only a small percentage of the total active polar bear dens for the SBS stock in any given year. Each year, Industry coordinates with the Service to conduct surveys to determine the location of Industrys activities relative to known dens and denning habitat. Under past ITRs Industry activities have been required to avoid known polar bear dens by 1.6 km 1 mi. However, occasionally an unknown den may be encountered during Industry activities. When a previously unknown den is discovered in proximity to Industry activity, the Service implements mitigation measures such as the 1.6-km 1-mi activity exclusion zone around the den and 24hour monitoring of the site.
The responses of denning bears to disturbance and the consequences of these responses can vary throughout the denning process. Consequently, we divide the denning period into four stages when considering impacts of disturbance: Den establishment, early denning, late denning, and postemergence.
Den Establishment The den establishment period begins in autumn near the time of implantation when pregnant females begin scouting for, excavating, and occupying a den.
The timing of den establishment is likely governed by a variety of environmental factors, including snowfall events Zedrosser et al. 2006;
Evans et al. 2016; Pigeon et al. 2016, accumulation of snowpack Amstrup and Gardner 1994; Durner et al. 2003, 2006, temperature Rode et al. 2018, and timing of sea ice freeze-up Webster et al. 2014. Spatial and temporal variation in these factors may explain variability in the timing of den establishment, which occurs between October and December in the SBS stock Durner et al. 2001; Amstrup 2003.
Rode et al. 2018 estimated November 15 as the mean date of den entry for bears in the SBS stock.
The den establishment period ends with the birth of cubs in early to midwinter Ramsay and Stirling 1988 after a gestation period that is likely similar to the 60-day period documented for brown bears Tsubota et al. 1987. Curry et al. 2015 found the mean and median
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birth dates for captive polar bears in the Northern Hemisphere were both November 29. Similarly, Messier et al.
1994 estimated that most births had occurred by December 15 in the Canadian Arctic Archipelago based on activity levels recorded by sensors on females in maternity dens.
Much of what is known of the effects of disturbance during the den establishment period comes from studies of polar bears captured by researchers in autumn. Although capture is a severe form of disturbance atypical of events likely to occur during oil and gas activities, responses to capture can inform our understanding of how polar bears respond to substantial levels of disturbance. Ramsay and Stirling 1986 reported that 10 of 13
pregnant females that were captured and collared at dens in October or November abandoned their existing dens. Within 12 days after their release, these bears moved a median distance of 24.5 km and excavated new maternal dens. The remaining three polar bears reentered their initial dens or different dens <2 km from their initial den soon after being released.
Amstrup 1993, 2003 documented a similar response in Alaska and reported 5 of 12 polar bears abandoned den sites and subsequently denned elsewhere following disturbance during autumn, with the remaining 7 bears remaining at their original den site.
The observed high rate of den abandonment during autumn capture events suggests that polar bears have a low tolerance threshold for intense disturbance during den initiation and are willing to expend energy to avoid further disturbance. Energy expenditures during den establishment are not replenished because female ursids do not eat or drink during denning and instead rely solely on stored body fat Nelson et al.1983;
Spady et al. 2007. Consequently, because female body condition during denning affects the size and subsequent survival of cubs at emergence from the den Derocher and Stirling 1996;
Robbins et al. 2012, disturbances that cause additional energy expenditures in fall could have latent effects on cubs in the spring.
The available published research does not conclusively demonstrate the extent to which capture or den abandonment during den initiation is consequential for survival and reproduction. Ramsay and Stirling 1986 reported that captures also known as handling of females did not significantly affect numbers and mean weights of cubs, but the overall mean litter size and weights of cubs born to previously handled
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mothers consistently tended to be slightly lower than those of mothers not previously handled. Amstrup 1993
found no significant effect of handling on cub weight, litter size, or survival.
Similarly, Seal et al. 1970 reported no loss of pregnancy among captive ursids following repeated chemical immobilization and handling. However, Lunn et al. 2004 concluded that handling and observations of pregnant female polar bears in the autumn resulted in significantly lighter female, but not male, cubs in spring. Swenson et al. 1997 found that pregnant female grizzly bears U. arctos horribilis that abandoned excavated dens pre-birth lost cubs at a rate 10 times higher 60%
than bears that did not abandon dens 6%.
Although disturbances during the den establishment period can result in pregnant females abandoning a den site and/or incurring energetic or reproductive costs, fitness consequences are relatively small during this period compared to after the birth of cubs because females are often able to identify and excavate new sites within the temporal period that den establishment occurs under undisturbed conditions Amstrup 1993; Lunn et al.
2004. Consequently, prior to giving birth, disturbances are unlikely to result in injury or a reduction in the probability of survival of a pregnant female or her cubs. However, responses by polar bears to anthropogenic activities can lead to the disruption of biologically important behaviors associated with denning.
Early Denning The second denning period we identified, early denning, begins with the birth of cubs and ends 60 days after birth. Polar bear cubs are altricial and are among the most undeveloped placental mammals at birth Ramsay and Dunbrack 1986. Newborn polar bears weigh 0.6 kg, are blind, and have limited fat reserves and fur, which provides little thermoregulatory value Blix and Lentfer 1979; Kenny and Bickel 2005. Roughly 2 weeks after birth, their ability to thermoregulate begins to improve as they grow longer guard hairs and an undercoat Kenny and Bickel 2005. Cubs first open their eyes at approximately 35 days after birth Kenny and Bickel 2005 and achieve sufficient musculoskeletal development to walk at 6070 days Kenny and Bickel 2005, but movements may still be clumsy at this time Harington 1968.
At approximately 2 months of age, their capacity for thermoregulation may facilitate survival outside of the den and is the minimum time required for cubs
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