Five meters of ice - about 16 feet thick - is threatening the survival of polar bears in the Southern Beaufort Sea region along Alaska's Arctic coast, according to Dr. Susan J. Crockford, an evolutionary biologist in British Columbia who has studied polar bears for most of her 35-year career.


Female polar bear with cubs.
That's because the thick ice ridges could prevent ringed seals, the bears' major prey, from creating breathing holes they need to survive in the frigid waters, Crockford told

"Prompted by reports of the heaviest sea ice conditions on the East Coast 'in decades' and news that ice on the Great Lakes is, for mid-April, the worst it's been since records began, I took a close look at the ice thickness charts for the Arctic," Crockford noted in her Polar Bear Science blog on April 18th.

"Sea ice charts aren't a guarantee that this heavy spring ice phenomenon is developing in the Beaufort, but they could be a warning," she wrote, noting that they "don't bode well" for the Beaufort bears.

"What happens is that really thick ice moves in because currents and winds from Greenland and the Canadian islands push it against the shore," Crockford told

"The male seals arrive in the area in early spring to set up breeding territories. They drill a hole through the ice to maintain breathing holes close to the shore. But there's a limit. They can drill through two meters (about seven feet) of ice. But too much beyond that and they're in trouble."

"The reason that's important is that seals mate right after the pups, who are born in April, are weaned. So the male seal wants to be there, but he has to have breathing holes. If the ice is too thick, he has to move off someplace else," she explained.

But this is the same time that female polar bears are just emerging with their newborn cubs from maternity dens either on or near the shore.

"When those bears come out of their dens in the spring, they need to find seals right away because they will have gone six months without eating," Crockford said. "If there are no seals, they have to go further out, where there's thinner ice."

"Spring and early summer are really a critical time for polar bears. That's when they need to eat as many seals as they can because that's when they put on fat for the rest of the year. If they have trouble doing that in the spring, they're in big trouble."

There were comparably high levels of spring ice in the Beaufort Sea in 2004 and 2006, when bear counts were "one of the pieces of evidence used to have the bears listed as 'threatened' in the U.S.," Crockford pointed out.

"Polar bear biologists were finding some bears quite thin and found a population decline," she said, which they attributed to melting summer ice caused by global warming.

"But the biologists were not there to see the thick [spring] ice. All they saw was thin bears," she pointed out. "They blamed the poor condition of the bears on summer ice, instead of acknowledging that it was likely the condition of the ice in the spring that was the cause of the problem."

"Female [polar bears] with cubs having trouble feeding are one aspect of the repercussions of thick ice," Crockford added. "The other repercussion is that other bears, instead of hanging around and starving, probably left the area. They could have gone to the Chukchi Sea, which is located between the U.S. and Russia near the Bering Strait."

The international IUCN Polar Bear Specialist Group (PBSG) conducted a polar bear population survey for the area in 2006. It reported a decline in the adult polar bear population and reduced cub survival rates, which was used to list the bears as a "threatened species" in the U.S. in 2008.

But the PBSG did not take into account the fact that polar bears "can just move" to other areas if their food supply is limited, Crockford told "If some of those bears were part of that count, it would look like they died," she pointed out.

In its 2013 status update, released on February 14th, the PBSG repeated its 2006 "reduced" population estimate, putting the Southern Beaufort Sea at 1,526 bears and "declining due to a negative trend in sea ice conditions, particularly over the continental shelf, resulting from the continuing effects of climate warming."

However, in what Crockford characterizes as an "astonishing admission," the update also stated that "it is important to note that there is the potential for un-modeled spatial heterogeneity in mark-recapture sampling that could bias survival and abundance estimates. A thorough re-assessment of survival and abundance is underway and a final result is anticipated in 2014."

"What's shocking is that the PBSG have now admitted that the 'movement of bears' issue essentially invalidates the 2006 population estimate and the much-touted 'reduced survival of cubs'," Crockford said in a March 24th blog post.

"This is a cyclical pattern that is quite specific to that part of Alaska, which has been known about since the 1970s," when wildife biologists noticed "ten times as many seals as usual in the Chukchi Sea. There were more bears, too," Crockford told

"It seems to happen every 10 years, so it should be expected by people who work in the area. And not just by people who study polar bears, but also people who study seals."

"It looks like similar conditions are setting up now, and we know the timing is right," she added. "We're keeping an eye on it."