23 November 2007Nature.comSusan Brown
census of polar bears in Canada’s Hudson Bay has lent some hard numbers to the long-held fear that retreating sea ice is causing some bears to starve or drown.
Biologists have predicted that polar bears will struggle to survive as summer comes sooner to the Arctic. Less time spent on icy hunting platforms means the bears are slimming down before winter sets in. And there were anecdotal reports in 2005 of more bears found swimming far out at sea; a few were found floating dead, presumably drowned. But so far no evidence has directly linked the trend of melting sea ice associated with climate change to bear deaths.
Now, looking at 20 years of data from bears captured along the coast of Hudson Bay, a team of scientists from the United States and Canada has found that fewer of the youngest and oldest bears survived in years when the ice broke early.
“Survivorship has dropped in the cubs, subadults and very old animals and is directly related to the date of break-up,” says Ian Stirling, a biologist with the Canadian Wildlife Service in Edmonton, Alberta, and an author on the report.
Because Hudson Bay is near the southern edge of polar bear habitat, what happens there may foreshadow the fate of more northern populations, the authors say.
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 1.
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 ice2.
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.”
Obbard studies bears farther south than the western population now experiencing a decline in numbers. Because wind and currents sweep ice into this part of the bay, the effect of earlier ice break-up has been less severe here. But Obbard’s group has also seen skinnier bears. In a recently completed study, they found that bears captured in the past five years weighed 15% less than bears of comparable length captured 20 years ago3.
“It seems to me that the southern Hudson Bay population is probably on the same trajectory as the western Hudson Bay population was,” Obbard says, “but there’s a lag.” They haven’t yet counted fewer bears, he says, but “it looks like we’re headed that way”.
The United States is considering listing polar bears as threatened under the Endangered Species Act; a decision on this is expected early next year.
