Whales not enough sustenance for polar bears in fast-changing climate

Dead whales are a nutritional boon for polar bears, and they’ve likely helped bears survive in the past.

Dead whales are a nutritional boon for polar bears, and they’ve likely helped bears survive lean periods during warm spells in the past when much of the Arctic was ice-free. In the future, however, most bears won’t be so lucky, writes a team of scientists in the journal Frontiers in Ecology and the Environment on Oct. 9.

“[When] we look at the situation now, ecologically, with respect to food sources, it’s a very different picture,” Ian Stirling, a biologist at the University of Alberta in Canada and a co-author of the paper, said in a statement. “The potential of whale carcasses to bail bears out may still be important in a few areas but, quite simply, their overall availability is going to be substantially less than before humans invaded the Arctic.”

Polar bears (Ursus maritimus) depend on sea ice, which allows them to stalk seals, their primary prey. But as the climate heats up, sea ice is disappearing, and ice-free Arctic summers could become a reality by 2040.

Right now, when warmer temperatures carve up Arctic ice, some bears fast until the ice appears again, unless they come across another source of food. A whale carcass that washes ashore, for example, is a great source of fat and protein, and scientists figure that this strategy may have helped polar bears get through temperature spikes in the past.

“I think this is likely one of the most probable explanations for how polar bears made it through previous warm interglacial periods,” Stirling said.

The carcass of a bowhead whale (Balaena mysticetus), weighing in at perhaps 100 metric tons (110 tons) can provide bears with about the same amount of nourishment as 1,300 ringed seals (Pusa hispida), the authors write. Such a windfall can feed dozens of bears, sometimes for years at a time.

But do enough whales wash up to sustain polar bears? Stirling and his colleagues wanted to find out. They estimated that a population of 1,000 polar bears would need around 28 whales to die, float to the surface and wash ashore every year to meet their caloric needs. In a place like the Chukchi Sea, which sits north of the Bering Strait between Russia and Alaska and has stable numbers of bowhead and gray whales (Eschrichtius robustus), the team found that there are likely enough that die and come ashore to support that many polar bears.

“Scavenging on large whale carcasses is probably important for bears in some areas and may buffer them from sea ice loss,” Kristin Laidre, a marine biologist at the University of Washington and the study’s lead author, said in the statement.

“However, carcasses of large whales are not expected to replace seals as nutritional resources as we move towards an ice-free Arctic,” Laidre added. “In most regions, the environmental changes are too large and the whale carcasses are too few.”

Most subpopulations of bears don’t live in places where as many whale carcasses appear each year. Whale numbers used to be much higher, prior to the whaling of the past few centuries. And where they do wash up, the carcasses are only valuable to polar bears if they can find them and if bears aren’t kept from accessing them by human settlements, industry or shipping.

Those complicating factors mean that the opportunistic strategy that helped polar bears survive ice-free periods in the past might not work in the coming decades.

“If the rate of sea ice loss and warming continues unmitigated, what is going to happen to polar bear habitat will exceed anything documented over the last million years,” Laidre said. “The extremely rapid pace of this change makes it almost impossible for us to use history to predict the future.”

Banner image of polar bears feeding on the carcass of a fin whale in Norway by Daniel J. Cox/Arctic Documentary Project.

Citation

Laidre, K. L., Stirling, I., Estes, J. A., Kochnev, A., & Roberts, J. (2018). Historical and potential future importance of large whales as food for polar bears. Frontiers in Ecology and the Environment.

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