As Arctic sea ice dwindles, polar bears are spending more time on land. Now a long-term study tracks the dramatic change in behavior over the recent warming in the Beaufort Sea off northern Alaska and northwestern Canada.
Polar bears in the beleaguered southern Beaufort Sea population are now three times as likely to come ashore in summer and fall as they were in the mid-1980s, according to the study, by scientists from the U.S. Geological Survey, the U.S. Fish and Wildlife Service and two universities. The bears that come ashore are also staying there much longer than bears did in the past, according to the study.
The big changes, the study found, have happened over the past decade and a half — corresponding to big reductions in summer and fall sea ice.
“That’s where we found a turning point in the data series. Prior to 2000, there really was no trend,” said Todd Atwood, a USGS wildlife biologist and the study’s lead author.
Since the late 1990s, the open-water season in the southern Beaufort Sea has expanded by 36 days on average, and polar bears have responded in kind, said the study, published online in the journal PLOS ONE. Since that time, the study said, the polar bears that swim to shore are spending 31 more days a year there.
Before then, the polar bears that went ashore in the late summer and fall tended to make brief exploratory forays, not the extended stays that are happening now, Atwood said.
The study uses data from movements of 228 adult female polar bears that, over the period from 1986 to 2014, were fitted with 389 radio collars. Only adult females can wear the tracking collars; males’ necks are too wide to allow collars to stay in place.
The study also uses data from aerial surveys conducted from 2010 to 2013. Those surveys tracked all polar bears — male and female, adult and subadult.
The newly open Beaufort Sea water, a trend continuing this year, is forcing polar bears in summer and fall to make a choice — swim north to the receding edge of the pack ice, or swim south to land, Atwood said.
The bears that come ashore still make up only a minority of the population, but they may be making the better choice, Atwood said. They seem to be making a beeline for piles of meat- and blubber-laden bones left after local Inupiat hunters butcher bowhead whales on the beaches, according to scientists’ data.