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Abundance and Vital Rates

Male polar bear

Polar bear population status in the southern Beaufort Sea

Polar bears depend upon sea ice for survival.  In recent years, a warming climate has caused major changes in the Arctic sea ice environment, leading to concerns regarding the status of polar bear populations. We have conducted long-term studies of polar bears in the southern Beaufort Sea (SBS) region of the U.S. and Canada.  When we applied open population capture-recapture models to data collected from 2001-2006, we estimated there were 1526 (95% confidence interval (CI) is 1211- 1841) polar bears in the SBS region in 2006. The number of polar bears in this region was previously estimated to be approximately 1800.  Because precision of earlier estimates was low, our current estimate of population size and the earlier ones cannot be statistically differentiated. Analysis did reveal a decrease in the survival of cubs and a decrease in the skull size of both adult males and cubs.

Regehr, E. V., S. C. Amstrup, and I. Stirling. 2006. Polar bear population status in the southern Beaufort Sea. U.S. Geological Survey, Alaska Science Center, Anchorage, Alaska. USGS Open-File Report 2006-1337. 20 pp.


Survival and population size of polar bears in western Hudson Bay in relation to earlier sea ice breakup

Male polar bear - photo by USGSSome of the most pronounced responses to climatic warming are expected to occur in polar marine regions, where temperature increases have been the greatest, and sea ice provides a sensitive mechanism by which climatic conditions affect ice dwelling species. Population level effects of climatic change, however, remain difficult to quantify. We analyzed data for polar bears captured from 1984–2004 along the western coast of Hudson Bay, and in the community of Churchill, Manitoba, Canada. The size of the Western Hudson Bay polar bear population declined from 1194 (95% CI = 1020, 1368) in 1987, to 935 (95% CI = 794, 1076) in 2004. Total apparent survival of prime-adult polar bears (age 5–19 yr) was stable over the course of the study for both females (0.93; 95% CI = 0.91, 0.94) and males (0.90; 95% CI = 0.88, 0.91). Survival of juvenile, sub adult, and senescent-adult polar bears was correlated with spring sea ice breakup date, which was variable among years and occurred approximately 3 weeks earlier in 2004 than at the beginning of the study in 1984. We propose that this correlation provides evidence for a causal association between earlier sea ice breakup (due to climatic warming) and decreased polar bear survival. It may also explain why Churchill, like other communities along the western coast of Hudson Bay, has experienced an increase in the number of human–polar bear interactions in recent years. Earlier sea ice breakup may have resulted in a larger number of nutritionally-stressed polar bears, which are encroaching on human habitations in search of supplemental food. Because western Hudson Bay is near the southern limit of the polar bear’s range, our findings may foreshadow how more northerly polar bear populations will respond to the continued warming that is projected for many parts of the Arctic.

Regehr, E.V., Lunn, N.J., Amstrup, S.C., and I. Stirling. 2007. Effects of earlier sea ice breakup on survival and population size of polar bears in western Hudson Bay. Journal of Wildlife Management. 71(8):2673-2683.

Regehr, E. V., N. J. Lunn, S. C. Amstrup, and I. Stirling. 2007. Supplemental materials for the analysis of capture-recapture data for polar bears in western Hudson Bay, Canada, 1984-2004. U. S. Geological Survey Data Series 304. 13 p.

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