In September 2009, after a summer of warm weather and dwindling ice, a young polar bear slipped into the frigid waters of the Beaufort Sea and began to swim.

She didn't stop for food or rest until nine days later, when she finally encountered a slab of sea ice large enough to sustain her. The journey was some 250 miles.

That female polar bear was one of more than 100 monitored by biologist Andrew Derocher, a researcher at the University of Alberta who spent six years tracking bears in the waters off the northern coasts of Alaska and Canada. He found that, as sea ice in those areas fractured and melted away, the bears were making longer and longer swims across the open ocean — journeys that taxed their already limited resources and proved perilous to vulnerable cubs.

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"Ice is changing so quickly that we’re finding the bears are getting caught in places where they’re finally coming to the realization, 'I just can’t stay here,' " Derocher said in a phone interview. "... These kinds of long-distance swims are not what they evolved to undergo." 

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The results of Derocher's study, published in the most recent issue of the journal Ecography, show a dramatic increase in the number of polar bears paddling across vast expanses of ocean to find suitable ice to stand on. In 2004, just a quarter of the bears monitored performed a long-distance swim (defined as more than 50 kilometers, or about 31 miles). By 2012, that proportion had ballooned to 69 percent.

The number of bears making such a swim was directly proportional to the loss of sea ice in the area, Derocher said.

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These journeys are hard on polar bears. Though they're good swimmers, they're not adapted to long trips and can only paddle about 2 kilometers an hour. A 30-mile journey to find new ice takes an entire day, during which the bears can't eat or rest. Adult bears are likely to lose weight, and their cubs tend to get hypothermic. In 2009, a mother bear who swam for nine days straight off the coast of Alaska (who was not part of Derocher's study) lost 22 percent of her body weight, biologists with the U.S. Geological Survey reported. Her year-old cub died during the journey.

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"With cubs, if they have to undergo a long distance swim, it's basically a death sentence," Derocher said.

Mothers with cubs were much less likely to swim, he found. Instead, "they will walk for hundreds of kilometers to keep their cubs out of water."

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In part because of the difficulty of monitoring bears (collars can fall off or malfunction, bears seem to drop off the map), the amount of data Derocher and his colleagues collected varied from year to year. But the trend is pretty clear, Derocher said. In the 1980s, when he first began studying polar bears, it would have been unheard of for any bears to make a long distance swim, let alone dozens. In those days there was no need — the Beaufort Sea and Hudson Bay (the two areas covered by the study) were clogged with ice even in the height of summer.

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That's changed now, especially in the Beaufort Sea above Alaska and the Yukon, which has seen an especially large decline in sea ice compared with the Hudson Bay. Satellite images taken earlier this month show that the ice there is already beginning to break up. That's bad news during prime hunting season for polar bears, which rely on sea ice as a platform from which to dive for seals and other prey.

"None of this is what I would call a smoking gun as to what is happening with polar bear abundance, but the signs are all pointing in the same direction," Derocher said. "We're seeing bears with lower body fat, fewer cubs, changing hunting behavior."

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Pair that with a 2014 study that found that the polar bear population in the southern Beaufort Sea dropped between 25 and 50 percent from 2001 to 2010, and the picture becomes pretty clear.

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"The Beaufort population is one of the ones that will more than likely be extirpated probably by mid-century," Derocher said.

The best-case scenario is that these bears will travel south and find a way to hunt on land during the summers, the way some of their Hudson Bay cousins do. So much depends on their ability to adapt.

"We’re really changing the rules on the bears with the warming that we’re observing," he said. 

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