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Melting Sea Ice Forcing Polar Bears to Swim Longer Distances, Linked to Increased Mortality of Cubs, Finds New Study

Study Represents the First Empirical Research to Find a Significant, Increasing Trend in Polar Bear Long-Distance Swimming

OTTAWA, CANADA – JULY 19, 2011 - Sea ice loss from climate change is causing polar bears to swim longer distances to find stable ice or to reach land, resulting in greater risk to their cubs, according to a new paper co-authored by a WWF expert.

“Climate change is pulling the sea ice out from under polar bears’ feet, forcing some to swim longer distances to find food and habitat,” said Geoff York, WWF Polar Bear Expert who is an author of the study.

“This research is the first analysis to identify a significant multi-year trend of increased long-distance swimming by polar bears. Prior research had only reported on single incidents,” said York.

U.S. Geological Survey biologist and lead author Anthony Pagano will present the study (“Long-distance swimming events by adult female polar bears in the southern Beaufort and Chukchi Seas”) on July 19 at the International Bear Association (IBA) Conference held in Ottawa, Canada.

Between 2004 and 2009 researchers collected data from 68 GPS collars deployed on adult female polar bears, in combination with satellite imagery of sea ice, to identify incidences of bears swimming more than 30 miles at a time. Researchers identified 50 long-distance swimming events during the six year period involving 20 polar bears. Swimming events ranged in distance up to 426 miles and in duration up to 12.7 days.

Eleven of the polar bears that swam long distances had young cubs at the time of collar deployment; five of those bears lost their cubs during swimming -- a 45% morality rate. In contrast, only 18% of cubs died that were not compelled to swim long distances with their mother.

The final report is not yet released but the full abstract is provided below.

High Resolution Photos and Video Available

Contact Rhys Gerholdt,, for a selection of high-resolution photos and video of polar bears swimming in the Arctic.

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.

Study Abstract
Long-distance swimming events by adult female polar bears in the southern Beaufort and Chukchi seas

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


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.