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Warming Arctic: Receding ice leaves Hudson Bay polar bears less time to eat

Polar bears' territorial tendencies and the diminishing ice season on Hudson Bay are conspiring to leave the animals less time to eat, researchers say. This bodes ill for their ability to reproduce, and survive.

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The bay boasts high biological productivity, so polar bears can gorge themselves in anticipation of the melt season. But the bay's melt season also has experienced some of the fastest growth.

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Between 1979 and 2009, the melt season Arctic-wide expanded by at least 20 days, according to a study published four years ago by scientists from NASA and the National Snow and Ice Data Center in Boulder, Colo. That's a pace of 6.6 days per decade.

Hudson Bay, by contrast, has seen the melt season grow by more than 10 days per decade – among the highest rates for any region in the Arctic. It's also one of the southernmost habitats for polar bears, so in effect they live on a climatological knife's edge compared with their northern kin. Any effects of warming are likely to show up there earlier than at higher latitudes, Cherry says.

Between 2004 and 2009 the team tagged 68 female polar bears with GPS collars. The collars were programmed to phone home every 4 hours. The researchers tagged only females, because unlike males, their necks are narrower than their skulls. Males lose collars with a nod.

They compared migration patterns on and off the ice with a similar study done in the area between 1991 and 1997. In comparing polar-bear migration patterns from one period to the next, the researcher found that the bears remained landlubbers for increasingly long periods during the melt season.

The bears showed a surprisingly consistent sense of timing from one study period to the next, based on ice extent. They returned to land an average of 28.3 days after sea-ice cover had shrunk to 30 percent of its winter extent. They headed back onto the ice an average of 2.5 days after sea-ice had grown to about 10 percent of its winter expanse – reluctant to leave the restaurant and raring to return.

After looking at previous studies of the food supply's effect on polar bear reproduction and survival, the team suggests that the increase in the bears' fasting period likely is taking a toll on both. The bears likely to be hit hardest are the bears still shy of adulthood, because they can't hold as much body fat as an adult, the scientists suggest.

Indeed, some studies have tied a post-fasting hunger among bears to increasing run-ins between bears and humans in the western Hudson Bay.

Over the long term, the prognosis for polar-bear populations isn't encouraging, other research suggests.

A study formally published in the journal Ecology in 2010 – work influential in the process leading to listing polar bears as endangered – projected that if carbon dioxide emissions from burning fossil fuel and from land-use changes take a business-as-usual path through the end of the century, global warming would lead "to drastic declines in the polar bear population by the end of the 21st century."

Different populations in regions around the Arctic would be affected at different rates, the researchers noted. But even by mid-century, "the effects on polar bears will be severe."

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