Polar bears swimming longer distances, cubs dying in the process

Less Arctic summer ice means polar bears need to swim further to get to the sea ice they use as a platform to catch seals. And cubs are dying in the process.

With the Arctic’s summer ice quickly disappearing, many experts believe the world’s polar bear population, which numbers about 25,000, could dwindle to just a few thousand by the end of the century. Some new evidence suggests how this could potentially unfold.

A joint study by the United States Geological Survey (USGS) and the World Wildlife Fund (WWF) whose conclusions were released yesterday, July 19, indicates that polar bears are swimming longer distances to get to the sea ice they use as a platform to catch seals. And, cubs are dying in the process.

The USGS/WWF joint study tracked a group of 68 female polar bears in the southern Beaufort and Chukchi seas. Researchers tracked the bears with the use of radio collars between 2004 and 2009. They found that polar bear swims now average about 110 miles. According to the press release:

Researchers identified 50 long-distance swimming events [more than 30 miles long] 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.

Image Credit: Ansgar Walk

Can you imagine swimming 426 miles? That’s a heart attack waiting to happen! The harder news to take is that 5 of the 11 mothers swimming with cubs lost those cubs along the way; that’s a 45% mortality rate. By contrast, when cubs didn’t have to go the distance with mom, their mortality rate was only 18%.

Geoff York, WWF Polar Bear expert and study co-author, wrote, “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.” The press release noted:

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.

The future of the Arctic, therefore, doesn’t bode well for polar bears. According to the University of Washington Polar Science Center, Arctic sea ice extent dropped to record low levels in July 2011; sea ice volume is now 47% lower than in was 1979. That’s when satellites first began recording Arctic sea levels and ice.

The full results of the WWF/USGS polar bear study were presented on July 19 at the International Bear Association (IBA) Conference held in Ottawa, Canada.

Bottom line: A joint study by the United States Geological Survey (USGS) and the World Wildlife Fund (WWF) found that polar bears are swimming longer distances to get to the sea ice they use as a platform to catch seals, which is contributing to cub mortality.

EarthSky chats with Kieran Mulvaney, author of The Great White Bear, on EarthSky 22

Beth Lebwohl