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Differing diets make polar bears more vulnerable to ice loss

Polar bears that rely on fewer prey animals to fill their bellies may be more vulnerable to starvation and stress as climate change ravages the extent of their summer ice hunting platform across the Arctic, according to a profile posted online about a longtime Canadian bear researcher.

Over the past 30 years, Dalhousie University biologist Sara Iverson has been part of an effort that collected fat samples from polar bears throughout the Canadian Arctic, producing a remarkable catalog of what bears eat for dinner and how that diet varies from population to population.

Finding out exactly what these top Arctic predators scarf down has become more important as summer sea ice shrinks back, forcing these ice-adapted carnivores to swim immense distances to find ice or retreat to the shore to wait for the onset of winter.

“Polar bears are absolutely dependent on sea ice to hunt seals, which use the ice as a platform to breed,” said Iverson in the story. “With the loss of ice, they’re having a difficult time. A major source of food has been removed and in some areas they’ve been forced ashore earlier in the spring in poor condition.”

Over the past couple of decades, the extent of summer ice in the Beaufort and Chukchi seas north of Alaska has shrunk to record and near record levels --matching ice shrinkage elsewhere in the Arctic. The fear that this loss will drive the U.S.-managed polar bear populations to extinction prompted the United States to list the bears as endangered, and to designate 187,000 square miles of North Slope and Arctic Ocean as habitat critical to their survival.

The legal battle over that habitat designation – an area the size of California – has only just begun. Lawsuits challenging the legality of the decision have been filed by the State of Alaska, the Alaska Oil and Gas Commission and the Arctic Slope Regional Corp. (along with 11 other Native corporations and the North Slope Borough.) They variously argue that the designation was not done correctly, that it ignored population data or the economic consequences, or actually violated the Endangered Species Act. Conservation groups say they will intervene to defend the designation.

The biological issue hinges on whether polar bears will be able to find enough to eat as they roam shrinking ice floes. The research by Iverson and other Canadian scientists focused on the basics: what do bears in various Canadian populations eat already?

Between 1972 and 2004, these Canadian scientists took 1,902 fat samples from 1,738 bears spread among 10 different Canadian populations, including two populations that live in the Canadian portion of the Beaufort Sea, just east of Alaska’s territory.

What they found suggests that the Beaufort bears might have a harder time adapting as ice extent shrinks because they rely on just three kinds of prey.

An analysis of 181 bears in the Northern Beaufort Sea showed that ringed seal dominates 70 percent of their menu, followed by bearded seals (20 percent) and beluga whales (10 percent,) according to a research paper published in Ecological Monographs. Southern Beaufort Sea bears ate along similar lines.

In contrast, bears that live other Canadian areas consumed a much wider cuisine,  presumably because more critters roamed their environment. Bears in Davis Strait, for instance, dined on five species of seals, walruses and beluga whales.

Relying on ringed seals with few other options could make Beaufort Sea bears more vulnerable during years when the ice disintegrates, the authors said. Bears snatch their seal snacks by busting into dens or pouncing after long waits on the ice surface.

“In areas where prey diversity is relatively rich, some bears appear to feed opportunistically on locally abundant species other than ringed and bearded seals,” they wrote here. “In regions where prey species are less diverse, the greater dependence of polar bears on ringed and bearded seals likely make them more sensitive to environmental change.”

Iverson -- scientific director of the Ocean Tracking Network Canada and a University Research Professor at Dalhousie -- has also studied Northern fur seals in Alaska’s Pribilof Islands, creeping into rock-beach rookeries amid hundreds of bellowing seals as large and toothed (and irritated) as cranky brown bears.

The story goes on:

To study fur seals, the researchers (three at a time) assemble in a plywood box with no ceiling or floor, and shuffle into the midst of the rookery. They bring females into the box one at a time, take samples and attach a tag, before releasing them again, all the while with one person on the lookout for the ornery males.

“You have to have a good sense of humor and there are times when you think, ‘what in the world am I doing here?’” (Iverson) says. “But it is absolutely incredible to see these animals in their natural environment.”

Contact Doug O'Harra at doug(at)alaskadispatch.com.