At issue is not just a decline in the area of polar bear's sea ice habitat, but also changes in its quality -- "you could have 100 percent sea ice cover, and it might not be polar bear habitat, because it could be too thin," Derocher noted -- and also the time available for polar bears to hunt on the ice.
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"The ice-free period is increasing in much of the polar bear's range," Derocher said. "It's what we see in western Hudson Bay, the Beaufort Sea and many other parts. Svalbard is just a mess this year, as well. It's pretty grim." On average, Hudson Bay for example is ice-free one extra day each year: That's one more day every year that polar bears go without hunting, and over time, that adds up.
"The way that climate change is manifested in any polar bear population is: The first thing we see is a change in sea ice," explained Derocher. "The second thing we see is a change in the body condition of polar bears. When we see that change in body condition, the next things are changes in survival rates and changes in reproductive output. Ultimately, those two factors result in a population decline, which is hard and expensive to document."