Polar Bears Dying In Years Of Early Ice Melt

Missing cubs Although the timing of ice break-up varies from year to year, the trend has been towards more days of open water. Historically, ice has filled Hudson Bay for eight months each year. Now the ice is clearing nearly three weeks earlier than it did 30 years ago. Since 1984, wildlife managers have captured a few per cent of the bears that spend their summers on the western shore of the bay, releasing them with ear tags and lip tattoos. The marks allow biologists to recognize individual bears, track their fate and estimate how many survive each winter. Over two decades, this population has declined by more than 20%. Eric Regehr of the US Geological Survey in Anchorage, Alaska, and colleagues compared the bear numbers with available data on sea-ice extent. Adult bears in their prime — those between 5 and 19 years old — seem unaffected by the changes in ice cover. But many of the cubs, subadults and old bears captured in years when the ice broke the earliest weren’t spotted again. “The earlier the break-up, the poorer the survival,” Stirling says. For each additional ice-free week, survival of these most vulnerable ages decreased by 2–5%. That’s the main reason behind the population decline, Stirling and colleagues report in the November issue of the Journal of Wildlife Management. Thin ice, thin bears Biologists were already worried about this population of bears. Back in 1999, before the population started to decline, the same census found that bears became thin in years with less ice. Polar bears fast through their summer by stocking up on weanling ring seals. In early spring, the bears feast on unwary seal pups until the ice breaks up, storing up much of the energy they’ll need to get by. In years when the ice breaks early, the less-able hunters catch fewer seals than they need. Now it seems the hunger is leading to deaths. “It’s scary,” says Martyn Obbard, who keeps tabs on polar bears at the southern edge of Hudson Bay for the Ontario Ministry of Natural Resources in Peterborough, but did not participate in this study. “The evidence and their conclusions are solid.” http://www.nature.com/news/2007/071123/full/news.2007.282.html

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