Closely Studied Population of Polar Bears Drops 40 Percent in Number Over a Decade

Shrinking Arctic sea ice is likely responsible for a steep decline in polar bear numbers according to a new study from researchers at the U.S. Geological Survey (USGS), Environment Canada, and several other public and private research groups. From 2001 to 2010 the scientists captured, tagged, and released hundreds of bears in the South Beaufort Sea, an oft-frozen body of water above eastern Alaska and western Canada. By 2010 the population was about 900, down from 1,600 in 2004.

The middle years in that decade, 2004 to 2007, were especially hard on the marine mammals, long considered a poster species for the ravages of climate change. Over that period, just two of 80 cubs tracked by the research team survived. In a good year, around fifty percent of cubs make it.

Jeff Bromaghin, a research statistician at the USGS and the study's lead author, told VICE News that the bears were likely perishing due to starvation — probably because sea ice declines made it impossible for them to hunt their preferred food, ringed and bearded seals. Since satellite observations began in 1979, the annual low-point of Arctic sea ice extent has declined by about 12 percent per decade.

"A couple decades ago, ice used to stay fairly close to shore in the summer," Bromaghin said. "Now it's melting back hundreds of kilometers."

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Without near-shore ice on which to hunt seals, the bears have two summer options, both of them bad. First, they can remain at sea with the deteriorating ice, a decision that can lead to disaster if they can't find new floes. In 2011, researchers tracked one female that swam ceaselessly for nine days. By the time the bear reached ice 427 miles later, she had lost 22 percent of her body weight and her cub had died.

The creatures' second option is to stay on land and wait for ice to form again. But that's not ideal, either. While polar bears have been documented eating snow geese eggs and other terrestrial foods, Pete Ewins, Species Conservation Specialist at WWF Canada, told VICE News that those alternate sources of sustenance are no substitute for fat-filled seals.

"Rather than sitting on the beach conserving energy for three months, now they have to wait four or five months," Ewins said. "Unless you're a really fit polar bear who's been able to pack on a lot of fat in the spring, you're going to be in trouble."

Not only is the ice shrinking, it's also changing in shape and texture — another unfortunate development for polar bears. Now that ice is developing later in the year, said Bromaghin, it's thinner and lighter, and therefore more susceptible to being deformed by battering winds and waves. That can create sections of jumbled-up ice where hunting seals is difficult.

"During the period of low survival, we saw a number of instances where bears were clawing and chewing through really thick chunks of ice trying to get at seals," Bromaghin told VICE News. "That's not very energetically efficient, and it's probably a sign that they're hungry."

The study's news wasn't all bad: Polar bear survival ticked up again from 2008 to 2010, though not enough to offset the preceding years of population decline. Scientists aren't certain why the population stabilized when it did, though they have theories. For instance, changing patterns of "leads" — cracks in the ice where seals pop up to breathe — may have helped the bears to catch more of their favorite prey.

Another possibility is that bears learned to rely on the bowhead whales killed by indigenous subsistence hunters. "You see a growing number of bears coming to shore and making a beeline for those whale carcass sites," Bromaghin told VICE News. "That may be contributing to increased survival as well."

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So bears may be somewhat adaptable in the short term. And, indeed, they've historically proven resilient. Prior to recent declines, the South Beaufort Sea bears had enjoyed decades of population growth thanks to the passage of the 1972 Marine Mammal Protection Act, which banned hunting.

Still, barring swift and dramatic reductions in the greenhouse gas emissions that cause climate change, many scientists and conservationists forecast a grim future. One study projected the disappearance of two-thirds of the world's polar bears by 2050.

"For a specialist species like this, these population declines are exactly what you'd expect," the WWF's Ewins told VICE News. "We've basically melted its habitat away."

Follow Ben Goldfarb on Twitter: @ben_a_goldfarb

Image via Flickr