Study Abstract
July 2011
Long-distance swimming events by adult female polar bears in the southern Beaufort and Chukchi seas By Anthony M. Pagano, Kristin S. Simac, George M. Durner, and Geoff S. York U.S. Geological Survey, Alaska Science Center, 4210 University Drive, Anchorage, Alaska 99508 Present address: WWF Global Arctic Program, 30 Metcalfe Street, Suite 400, Ottawa, ON K1P 5L4 Abstract The final report is not yet released but the full abstract is provided below.
Polar bears (Ursus maritimus) are dependent on sea ice for their survival and reductions in sea ice have been linked to population declines. In the southern Beaufort and Chukchi seas, the duration between melt and freeze onset has increased and summertime sea ice extent has decreased. As summer ice habitats melt in this region, polar bears that do not follow the receding pack ice may be forced to swim long distances to areas of higher sea ice concentration or to land. We used data from 68 Global Positioning System collars deployed on adult female polar bears between 2004 and 2009, in combination with satellite imagery of sea ice, to identify swimming events >50 km. During summer and autumn (June through October) we identified 50 swimming events from 20 polar bears. We found a significant increase (r2 = 0.83, P = 0.03) in the proportion of GPS collared bears that swam over the 6 years of this study, but variations in the number of collar deployments per year and improvements in collar technology over the course of the study preclude us from fully evaluating this trend. Swimming duration and distances traveled ranged from 0.7 to 12.7 days (mean: 4.0 days) and 53.7 to 687.1 km (mean: 167.3 km). Most bears swam from areas of unconsolidated sea ice to the main pack ice (mean distance: 178.1 km; n = 25). Average movement rates during swimming (1.9 km/hr) were 2.2 times higher than movement rates on sea ice >50% concentration (0.9 km/hr). We identified 6 bears whose dependent cubs survived long distance swimming events and 5 bears that may have lost their cubs as a consequence of swimming.
Despite the ability of polar bears to swim long distances, this behavior places them at risk of drowning and imposes greater energy expenditure, which could have negative impacts on recruitment. Long distance swimming is likely an additional indicator of the negative effects of sea ice loss on polar bears in the southern Beaufort and Chukchi seas.
Further Key Facts • Arctic sea ice extent has dropped to record low levels in July 2011; sea ice volume is now 47% lower than 1979 levels when satellite records began (Source: University of Washington Polar Science Center). • The annual Arctic sea ice minimum will be reached in mid-September. September 2010 saw the third lowest sea ice extent on record. The lowest and second-lowest extents occurred in 2007 and 2008 (Source: National Snow and Data Center). • Long-distance swimming puts polar bears at risk of drowning due to fatigue or rough seas. Like humans, polar bears can't close off their nasal passages so they are at risk of drowning in rough water. Cubs are at even greater risk. Their smaller body size and limited body fat leaves them more prone to hypothermia, and they don’t have the energy reserves of an adult bear. • On June 30, 2011 a federal judge upheld the George W. Bush administration decision to list the polar bear as threatened under the Endangered Species Act. In July 2011, Canada placed the polar bear on their Species at Risk list. • Adult male polar bears were not studied because they cannot retain GPS collars because their muscular necks are larger than their heads.
2