Polar bears missing after long swims

ANCHORAGE, Alaska -- Polar bears forced to swim longer distances because of diminished sea ice off Alaska's coast may be paying a price in lost cubs or precious calories, according to a study by the U.S. Geological Survey.

The study reviewed data from female bears in the Chukchi and southern Beaufort seas that wore GPS collars and took swims of at least 30 miles between 2004 and 2009.

Eleven were mothers with cubs. In six cases, dependent cubs had survived the swim two months to a year later. But in five cases, cubs could not be located after the long swim.

USGS research zoologist George Durner said researchers cannot say for sure that the missing cubs drowned, but the evidence suggests long-distance swimming may be risky.

Cub survival rate was higher for bears recorded not taking long-distance swims. "There were seven of those individuals from which we had a re-sighting, and only two of those had lost their cubs," Durner said.

Diminished sea ice habitat was the reason cited by former Interior Department Secretary Dirk Kempthorne in 2008 for listing polar bears as a threatened species.

The National Snow and Ice Data Center at the University of Colorado found that the summer low for sea ice, measured each September, averaged 2.7 million square miles from 1979 to 2000. Sea ice in recent years has fallen far below that, including a record low 1.65 million square miles in summer 2007.

Pack ice used to remain relatively close to Alaska's shore, but in recent years it has receded far off the relatively shallow continental shelf, the resource-rich habitat of ringed seals, the main prey of polar bears.

Polar bears use sea ice for hunting. Their most important feeding time is mid-spring to early summer, when ringed seal pups are born and weaned in snow lairs on sea ice.

Not all polar bears attempt long-distance swims. Some ride ice pack beyond the shallow, nearshore water as temperatures rise. Some spend summers on land. But some on pack ice, land or remnant ice, for reasons unknown, take a notion to begin swimming and have been recorded paddling 100 miles or more.

One female tracked two years ago left a Beaufort Sea beach near Barrow, Alaska, and swam 426 miles over nine days without a break to pack ice. She walked or swam another 1,118 miles, eventually looping back to soil near the Canada border.

When recaptured two months later, her body mass was reduced 22 percent to 389.4 pounds and her internal temperature had dropped. Her yearling cub had disappeared.