Polar Bears Change Diet
But over the last decade, the bay has been freezing later and melting earlier. Polar bear biologist Dr. Ian Stirling, with the Canadian Wildlife Service, says climate warming is making for shorter wintersand that means polar bears aren't finding enough seals to eat.
STIRLING: "The climate has been warming quite a bit in western Hudson Bay, and it's causing the ice to break up a little bit earlier. So it shortens the time that the bears have to feed on ringed seals, at the very best time of the year, which is late June and early July. So the bears, over the last 20 years, have lost approximately 15 percent of their condition (body weight). And so one of the things we forecasted a number of years ago was that if the climate warmed and we had more open waterespecially in the winterthat we might expect to see changes in the number of bearded seals and of harbor seals. We think we're seeing some of that and we thought that one way to try and measure it would be to look at the long-chain fatty acids in bear fat. It's the kind of work that Sara Iverson does, in which she can look at the fat of a predator and determine what species of prey it's eating."
Dr. Sara Iverson, a physiologist at Dalhousie University in Nova Scotia, Canada, has been working with Stirling to understand how polar bears are responding to a warmer Arctic. She says fatty acids found in seals accumulate in the tissues of polar bears. By identifying these fatty acids, she can tell what species of seals the bears have been eating.
IVERSON: "I've found that fatty acids, which are the building blocks of fat, can reveal a great deal of information about an animal's diet. In any marine sample there are probably about 70 fatty acids that can be identified. They differ according to ecosystem, according to species' feeding habits. Because fatty acids travel up the food chain intact, the fat that is stored in the predator begins to reflect and look like a mixture of what it's consuming in the diet."
Early in her work, Iverson found that polar bears were, as expected, eating mostly ringed seals. More recently, however, Iverson says polar bears have begun eating other seals, especially harbor seals and bearded seals.
IIVERSON: "It's been presumed for a long time that they concentrate on one or a few seal species. And what we're finding is that, depending on the location and changes in the ecosystem, their diets are changing dramatically. We're seeing bears having to shift to things like bearded seals and harbor seals, because the ringed seals, which are ice-dependent, aren't quite as available. That's because when the ice cover isn't there, the bears can't get to them."
Ian Stirling says the change in the polar bear diet is due to warmer Arctic winters, which has reduced the bay's ice cover, something that has made for better habitat for bearded seals and harbor seals. But he says the benefits are likely to be short-term, and that polar bears may one day be a rare sight around the bay.
STIRLING: "In Hudson bay, all the ice melts completely in the summertime, so every bear in the population has to come ashore and live on its stored fat reserves for four months, and the pregnant females for eight months. So being able to prey on harbor seals and bearded seals will help them out in the short term. But ultimately if the climate warms sufficiently for Hudson Bay to be ice-free most of the year, then it's likely that polar bears will disappear from that part of the world."
OUTRO: This is Arctic Science Journeys Radio, a production of the Alaska Sea Grant Program and the University of Alaska Fairbanks. I'm Doug Schneider.
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Thanks to the following individuals for help preparing this script:
Dr. Ian Stirling
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