Melting Arctic ice forces polar bears to swim for more than a week without rest, study finds
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 400km.
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.
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 50km). By 2012, that proportion had ballooned to 69 per cent.
The number of bears making such a swim was directly proportional to the loss of sea ice in the area, Derocher said.
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 50km 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 US Geological Survey reported. Her year-old cub died during the journey.