Census shows that U.S.-Russian polar bear population is stable … at least for now

Polar bears
An adult female polar bear and a cub stroll on Wrangel Island in the fall of 2017. Hundreds of Chukchi Sea polar bears spend the summer months on the island. (University of Washington Photo / Eric Regehr)

The first census of polar bears living around the Chukchi Sea, straddling Alaska and eastern Siberia, suggests that the population has been stable and healthy over the past decade.

That comes as a welcome contrast to the problems facing polar bears in other Arctic regions as their sea-ice habitat shrinks. The loss of  sea ice is an issue for the Chukchi Sea as well, but the nearly 3,000 bears in that region don’t seem to be feeling the strain as much.

“Despite having about one month less time on preferred sea-ice habitats to hunt compared with 25 years ago, we found that the Chukchi Sea subpopulation was doing well from 2008 to 2016,” Eric Regehr, a biologist at the University of Washington’s Polar Science Center, said today in a news release.

Regehr is the principal author of a study about the census published in the open-access journal Scientific Reports. The census, conducted by researchers from UW and federal agencies, chronicles a decade’s worth of observations — and delves into why the Chukchi Sea bears seem to be faring better than their cousins elsewhere.

“Sea-ice loss due to climate change remains the primary threat to the species but, as this study shows, there is variation in when and where the effects of sea-ice loss appear,” Regher said.

Conservationists recognize 19 distinct subpopulations of polar bears, ringing the Arctic from Alaska to Russia to Greenland to Canada. Just two of those subpopulations take in U.S. territory: the U.S.-Russian Chukchi bears, and the southern Beaufort Sea bears that range over Alaska and far northwestern Canada.

Polar Bear map
The International Union for Conservation of Nature’s Polar Bear Specialist Group has mapped out the territory for 19 subpopulations of polar bears. (IUCN Map)

“The southern Beaufort Sea subpopulation is well-studied, and a growing body of evidence suggests it’s doing poorly due to sea-ice loss,” Regehr said. They’re not the only bears at risk: Last year, a photo of an emaciated polar bear near Baffin Island in Canada’s northern Nunavut territory went viral and drew attention to that subpopulation’s plight.

In contrast, the Chukchi Sea is a “very rich area,” Regher said.

“Most of the Chukchi Sea is shallow, with nutrient-rich waters coming up from the Pacific. This translates into high biological productivity and, importantly for the polar bears, a lot of seals,” he explained.

The area is also heavily trafficked by whales, and washed-up whale carcasses provide a source of nourishment for the Chukchi Sea bears during the Arctic summer when the ice thaws.

Regehr and his colleagues gathered data for their survey by studying and tagging about 60 polar bears during the period between 2008 and 2016. GPS readings were fed into a computer model to estimate the subpopulation’s total size. The assessment of the bears’ health was based on data on reproductive rates and survival rates.

The assessment also drew upon the bears’ body-fat measurements as well as reports from Native Alaskan hunters. “It was important to bring our science together with the observations and expertise of people who live in polar bear country year-round and understand the animals in different ways,” Regehr said.

In 2008, polar bears were listed as threatened across their range under the U.S. Endangered Species Act, due to projected population declines associated with sea-ice loss.

Today, a joint U.S.-Russian commission is responsible for managing the Chukchi Sea subpopulations. Thanks to the newly published assessment, the commission raised next year’s subsistence harvest quotas for Chukchi Sea hunters from 58 to 85 bears. That’s good news for Alaskan tribes, who eat polar bear meat and make use of the hides for handicrafts.

Regehr said scientists will continue to monitor the Chukchi Sea situation as the effects of global climate change sink in.

“These findings are good news for now, but it doesn’t mean that bears in the Chukchi Sea won’t be affected by ice loss eventually,” he said. “Polar bears need ice to hunt seals, and the ice is projected to decline until the underlying problem of climate change is addressed.”

In addition to Regehr, the authors of the paper in Scientific Reports, “Integrated Population Modeling Provides the First Empirical Estimates of Vital Rates and Abundance for Polar Bears in the Chukchi Sea,” include Nathan Hostetter, Ryan Wilson, Karyn Rode, Michelle St. Martin and Sarah Converse.

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