9/2/2013 8:53:14 PM

Chukchi Sea Polar Bears

A new study of the remote Chukchi Sea population of polar bears, which is shared by Russia and Alaska, shows that the body condition and reproductive rates of polar bears in that area have held stable or have slightly increased over the past 20 years. It's a bright spot of news for at least one of the 19 polar bear populations—and contrasts with the adjoining Southern Beaufort Sea population, which has experienced recent drops in condition and survival.

"The findings are exactly in line with our predictions," says Dr. Steven C. Amstrup, our chief scientist, who led polar bear research in Alaska until 2010. "The near-term effects of global warming on sea ice and polar bears are expected to differ geographically, depending on variations in historic productivity and how winds and currents combine with persistent warming to alter the sea ice.

"As hypothesized, some populations are doing better than others, at least temporarily—and this gives us hope that we'll be able to address man-made climate change in time to prevent the extinction of the species."

He cautions, however, that without action to reduce greenhouse gas emissions, those polar bear populations still doing well will eventually face the same challenges as those experiencing declines.

Sea Ice Differences

A paper on the study is scheduled for publication in Global Change Biology. In it, the authors compare and contrast the habitat conditions of the two populations.

So what differences did they see?

To begin with, primary productivity in the Chukchi Sea is up to 10 times greater than in the adjacent Beaufort Sea, meaning more prey may be available. Second, although both the Chukchi and Beaufort experienced major sea ice losses during the study period (2008-2011), the Beaufort Sea had twice as many ice-free days as the Chukchi. And, finally, thinning of some of the thick, multi-year sea ice in the Chukchi may be providing better access to seals, at least temporarily.

"In the Beaufort Sea, the narrow continental shelf, low overall productivity, and proportionately greater number of days without sea ice over productive waters, apparently has led to significant nutritional stress. This in turn has resulted in reduced condition and survival rates in the bears there," says Amstrup. "In contrast, greater productivity in the Chukchi Sea—which may even have increased recently—and fewer days of ice absence from productive waters, may have allowed Chukchi Sea polar bears to avoid critical energetic thresholds. For now, they seem to be doing just fine."

Amstrup says the new results add to our understanding of how polar bears in different areas may respond to warming and sea ice losses. They also substantiate the fundamental dependence of polar bears on sea ice. 

"Polar bears in the Chukchi Sea continue to rely on seals that they catch from the surface of the ice as their primary prey. As sea ice declines ever further, there will be reduced access to those seals, just as there now appears to be in Hudson Bay and the Beaufort Sea. Therefore, without action, polar bears like those in the Chukchi Sea, which now may be deriving some transient benefit from a warmer Arctic, will eventually begin to suffer from it" Amstrup says. 

The study was conducted by employees of the U.S. Fish and Wildlife Service and the U.S. Geological Survey in collaboration with the State of Alaska and government, Native, and industry partners in western Alaska. Prior to the study, which began in 2008, little was known about the health and condition of the Chukchi Sea polar bears due to its remote, harsh location and the fact that the population is shared by the U.S. and Russia.

For more about the four sea-ice eco-regions in the Arctic and how seal-hunting conditions vary in each, visit our Polar Bears and Sea Ice page. A preview of the paper, "Variation in the response of an Arctic top predator experiencing habitat loss: feeding and reproductive ecology of two polar bear populations," is available via the Wiley Online Library.

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