Scientists try to assess potential impact of shrinking sea ice
By Bob Berwyn
SUMMIT COUNTY — Given the steady decline of sea ice in the Arctic and the status of polar bears as threatened under the Endangered Species Act, biologists are trying to understand how the top-level predators might respond to those changing conditions.
It’s important because some of the most recent studies link declining sea ice with dropping polar bear survival and reproduction rates in the Southern Beaufort Sea and around Hudson Bay.
“With the sea ice retreating earlier and coming back later, there’s less time for them to hunt in the spring, when put on their fat,” said Alaska-based U.S. Geological Survey researcher Anthony Pagano. “They end up in poor body condition,” Pagano said, adding that some of the studies suggest a downward trend in average body weight.
So Pagano and other researchers decided to get more data on how far polar bears actually swim. The bears spend much of their lives in and around water, and they are well adapted for swimming. But the results of the study show they are even better swimmers than many imagined: In years of extreme sea-ice retreat in the southern Beaufort Sea region of Alaska, polar bears have been documented taking very long swims, many in excess of 30 miles.
The findings suggest that bears may not be as susceptible to at-sea stranding as has been portrayed in some media reports, but biologists still wonder how much essential energy the bears use during their long-distance swims.
The USGS study tracked 52 adult female polar bears outfitted with global positioning system collars from 2004 to 2009, locating bears during helicopter flights and tranquilizing them from the air. While the bears are tranquilized, scientists attach a radio collar with multiple antennae and give them a small identifying tattoo on the inside of the upper lip.
The tags are positioned so that when the bears are swimming, one of the antennae is submerged so the data appear as gaps in the transmissions. Matching the data against maps of sea ice shows the trackers where the bears are swimming.
Researchers documented 50 swims with an average length of 96 miles. While long-distance swims were relatively uncommon, 38 percent of the collared bears took at least one long swim.
“We only looked at swims that were greater than 30 miles … some of the swims that we looked at included brief rests. They may haul out briefly on a small ice floe, but for the most part, it appears they were swimming continuously,” he said.
The longest documented swim on record for a polar bear is 425 miles, he added.
Since this type of study is new, scientists aren’t sure if such long-distance swims are a new feature of polar bear life.
“We did not have the GPS technology on collars to document this type of swimming behavior in polar bears in prior decades,” said Karen Oakley, of the USGS Alaska Science Center. “However, summer sea ice conditions in the southern Beaufort Sea have changed considerably over the last 20 to 30 years, such that there is much more open water during summer and fall. Historically, there had not been enough open water for polar bears in this region to swim the long distances we observed in these recent summers of extreme sea ice retreat,” Oakley said.
While their swimming prowess may give polar bears a fighting chance as sea ice dwindles, it’s also a potential risk.
The energy and physical costs of such long-distance swimming are unknown, but scientists did dcument that polar bears moved, on average, 2.3 times more than when the same individuals were on sea ice.
The movement data also suggest the bears were not pausing to rest or feed during long-distance swims. Twelve of the twenty documented swimming bears were adult females that had yearlings or cubs-of-the-year at the time they were outfitted with the GPS collar.
“We were able to recapture or observe 10 of these females within a year of collaring, and 6 of these females still had their cubs,” Pagano said. “These observations suggest that some cubs are also capable of swimming long distances. For the other four females with cubs, we don’t know if they lost their cubs before, during, or at some point after their long swims.”
Results from the study appear in the current issue of the Canadian Journal of Zoology.
the biggest concern at this pt – the pops in alaska – sea ice retreating earlier coming back later – less time in the spring to hunt in the spring when they put on fat – their oppty to hunt and put on weight – end up in poor body condition, reproduction rates
starting to show declining survival rates in S Beaufort Sea, similar hudson bay, declining body weights, survival rates -
we’re interested in trying to understand the energetic effect – how it compares to their more normal movements
USGS studying in S Beaufort sea for 30 years 1,500 bears in 2007 – 2010 – survival rates appear to be declining, linked with more ice-free days in the summer – right now developing a population estimate
if they’re not in good enough condition when they go into dens – better body condition, higher cub survival rates
Filed under: biodiversity, climate and weather, endangered species, Environment, global warming Tagged: | Arctic sea ice, Beaufort Sea, endangered species, global warming, polar bears, polar bears swimming, US Geological Survey