EDMONTON -- Research suggests polar bears are being forced to take long swims more often as climate change reduces Arctic sea ice.
It's particularly hard on mothers with cubs -- a possible explanation for the declining numbers of bears in the southern Beaufort Sea population, said Andrew Derocher, a University of Alberta bear biologist.
"We're quite certain that one of the mortality factors for cubs is if (mothers) have to take their cubs in open water," he said.
In a paper published Monday, Derocher and his co-authors write about the effect of declining sea ice on polar bear populations in the Beaufort Sea and Hudson Bay. The bears use the ice as a platform to hunt seals, the fat-rich prey they depend on to get through the long Arctic winter.
Arctic sea ice has been declining at about 12 per cent a decade since satellite monitoring began in the late 1970s.
Derocher and his colleagues worked with data from GPS collars between 2004 and 2012 that detailed the bears' movements and compared them with satellite imagery detailing the extent of sea ice.
Polar bears are amazingly capable in the water and the team chose to concentrate on swims of longer than 50 kilometres. Of the 115 animals that met that requirement, the median swim was 92 kilometres long and lasted 3.4 days.
One bear paddled for 404 kilometres.
Derocher said that although the data swung widely from year to year, more bears are undertaking such aquatic marathons. Only one-quarter of the bears had done so in 2004. That rose to two-thirds by 2012.
The great majority of the long-distance swimmers were from the Beaufort -- largely because of differences in how the sea ice fluctuates, Derocher said.
"The Beaufort Sea ice is almost like Venetian blinds -- they go up and down. If the bears want to stay with the ice they have to go north.
"In the Hudson Bay system, it's more like watching your laundry go round in your washing machine -- it's going around in a circle."
That means while Hudson Bay bears rarely get too far from land, Beaufort bears can find themselves a long way from shore during spring break-up. While healthy bears aren't too troubled by that, it's hard on the old, the sick and the young.
"Little cubs at this time of year are incredibly vulnerable to hypothermia."
By the fall, cubs are strong enough to take the frigid seas. But in spring, mother bears will take long detours on the ice to look for bridges to stay out of the water.
Derocher said the Beaufort Sea bears -- which he said have declined by about 50 per cent -- are likely to have to learn to start coming off the ice before it gets too far from shore. That will expose them to the same problem faced by their Hudson Bay cousins: finding enough fat-rich food to sustain them.
"The question becomes, is that a viable strategy for those bears? Can they make a living?" Derocher asked.
"It works in Hudson Bay, but that's a much different system. All indications are the Beaufort Sea is going to become a really challenging place to maintain polar bears."
