 By JANE KAY San Francisco Chronicle July 13, 2007
Data from northern Alaska show that the proportion of the bears' dens that are on pack ice declined from 62 percent between 1985 and 1994 to 37 percent from 1998 to 2004, according to the study, which was published Thursday in the journal Polar Biology. The scientists based their findings on 89 females that were captured and collared and then followed using satellite technology. They ruled out hunting and attraction to bowhead whale bones as other possible causes for changing denning locations. Ice floating in the Arctic Ocean will continue to melt, making polar bear dens there too unstable to survive the winter, according to global warming scenarios. At the same time, the sea ice in autumn will retreat so far from shore that the bears cannot swim across the expanse of water to reach land, which in recent years has been as far as 125 miles, the study said. Unlike other bears, polar bears don't hibernate through the winter but continue to hunt seals. The pregnant females, however, dig dens into snow either on the ice or on the land. In northern Alaska, in the Beaufort Sea in the Arctic Ocean, most prefer ice dens, some 400 miles offshore. But during the past 30 years, summer ice has been shrinking 9 percent per decade, according to NASA satellite data. In the winter of 2005 and 2006, the sea ice reformed to 5 to 6 percent below its average size. Overall, the ice and snow cover is less stable for the bear dens, where the mother bears remain from autumn or early winter through spring nursing and warming cubs. "The ice is thinner all over, there is less older and thicker multiyear ice, and it's freezing up later," said Steve Amstrup, a USGS biologist and a study author. "It's probably those changes in the nature of the sea ice that are responsible for the decrease in the amount of denning in the pack ice," said Amstrup, who is based in Anchorage and has studied polar bears since 1980. But when the bears try to head toward land, already they're having trouble, Amstrup said. "Right now, pregnant females foraging offshore in summer must wait up to a month longer than they did even 10 years ago for new sea ice to form so they can travel to denning areas on land," he said. Research published last year showed that the survival of cubs and size of adult male polar bears have declined in the Beaufort Sea region, suggesting that foraging has fallen off in recent years. The polar bear's global population has remained stable at roughly 22,000 bears. The USGS has forwarded the findings to the U.S. Fish and Wildlife Service. The agency is considering a petition to list the polar bear as a threatened species because of the effects of global warming. The other authors are Anthony Fischbach and D.C. Douglas, all with the USGS Alaska Science Center in Anchorage.
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