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Polar bears avoiding sea ice for cub dens

Polar bears avoiding sea ice for cub dens

Polar bears avoiding sea ice for cub dens
July 15, 2007

Polar bears in Alaska are increasingly setting up dens on sea on land because sea ice is thinning, reports a new study by U.S. Geological Survey (UCGS) researchers.

Over a 20-year period, scientists used satellite telemetry to track the den-making habits of bears in northern Alaska. They found the proportion of dens on ice compared with land fell from 62 percent in the 1985-1994 period to 37 percent between 1998 and 2004. The researchers say that declining quality sea ice makes ice less stable to mothers to raise their cubs. In recent years Arctic sea ice has been forming later and melting earlier, leaving it thinner.

USGC said it would pass the research on to the U.S. Fish and Wildlife Service, as it considers listing the polar bear as a threatened animal under the Endangered Species Act, a move that could require the government to take measures to protect the species. Since the biggest threat facing the species is melting sea ice, protection could require cutting greenhouse gas emissions contributing to global warming — a step the Bush administration has strongly opposed.

The research follows a number of other studies warning that polar bears could be increasingly threatened by global climate change.

Polar bear
Courtesy of the U.S. Fish and Wildlife Service

A November 2006 study published by scientists from the U.S. Geological Survey and the Canadian Wildlife Service, found a 22 percent decline in the size of the western Hudson Bay polar bear population, from 1,194 in 1987 to 935 in 2004. The research also found that only 43 percent of polar bear cubs in the surveyed area survived their first year, compared to a 65 percent survival rate in the late 1980s and early 1990s. Meanwhile in September, Ian Stirling, a research scientist with the Canadian Wildlife Service, reported that the average weight of adult female polar bears in western Hudson Bay have fallen from 650 pounds in 1980 to just 507 pounds in 2004, a 22 percent reduction.

Other reports indicate that drowned polar bears are being found for the first time in Alaska. Researchers speculate that greater distances between ice sheets could be taking a toll on the bears. While bears are capable of swimming long distances—up to 60 miles (100 km) without stopping—it is conceivable that they could suffer from exhaustion during an unexpectedly arduous swim. A shorter spring hunting season caused by progressively earlier breakup of sea ice, reduces the chances of reproductive success for female polar bears.

The loss of ice also makes it more difficult for bears to find food. Unlike grizzly bears, polar bears aren’t adapted to hunting land animals like caribou, instead feeding primarily on seals. However, recent aerial surveys by the U.S. Fish and Wildlife Service show that, over the past five years, polar bears are changing their habits and spending more time on land, congregating on beaches and scavenging whale carcasses.

Extrapolating from these developments, some scientists believe that polar bears could be extinct in the wild within the next century. While the last survey in 1997 suggested that polar bears in Alaska were not endangered, next year’s update might come to a different conclusion. Scientists estimate there are currently 20,000 to 25,000 polar bears world-wide.

The new USGC study is published online in the journal Polar Biology.

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