Polar bears off the northern coasts of Alaska and Norway are breaking away from their traditional seal-based diets, behavior that correlates with reduced sea ice and tracked through chemistry, according to research presented at the 24th International Conference of Bear Research and Management being held in Anchorage this week.
GPS tracking collars and isotope analysis are documenting how animals from the southern Beaufort Sea population off northern Alaska and northwestern Canada are eating bowhead-whale scraps and how bears around the Svalbard Archipelago are dining on a variety of nontraditional food sources, from bird eggs to reindeer.
"We're seeing bears almost act like scavengers. They're really starting to show this ability to diversify their diet," Jeff Welker, a biology professor at the University of Alaska Anchorage, said in a Wednesday presentation of the research. "This is something that may lead to their success, the ability to adapt to these new conditions and these other resources."
Hair, blood and tissue samples from bears on both sides of the Arctic are revealing chemical fingerprints of those alternative food sources, Welker said.
Bears that come to the Beaufort Sea shore in autumn are feeding from the pile of bowhead scraps left by local Inupiat whale hunters, a contrast with Beaufort Sea bears that stay out on sea ice farther north, he said.
Around Svalbard, where ice conditions vary widely from year to year and from locality to locality, bears in low-ice areas have turned to eggs, birds, whales, walruses and even reindeer.
The trends are not uniform, Welker said. For example, Svalbard adult females without cubs are more likely to stick to the traditional seal diet, as they likely have the time and energy to hunt. Females with cubs are more "opportunistic," he said.
Declining mercury levels in Beaufort Sea polar bears
Meat and blubber scraps at the bowhead bone piles might provide another benefit to southern Beaufort Sea whales, according to other research presented at the conference: lower mercury levels than would be ingested from a seal-only diet.
Mercury is a contaminant of special concern in the Arctic. Emissions from coal burning and other industrial operations to the south are carried northward on atmospheric and oceanic currents, and there are signs that warming in the far north is releasing additional mercury into the ecosystem.
But hair samples from polar bears in the southern Beaufort Sea population show a decided decline in mercury levels, according to a presentation by Melissa McKinney of the University of Connecticut. By 2011, average mercury readings from sampled bears were less than half the levels measured in 2004 and 2005, when southern Beaufort Sea polar bears registered readings that exceeded thresholds of concern, McKinney said in a presentation on Tuesday.
One possible factor is the diversification of diet and the use of something other than seal meat. Fish-eating ringed seals, the traditional prey for Beaufort Sea polar bears, are higher on the food-web level than zooplankton-eating bowhead whales and even higher than bearded seals, which eat a slightly different fish-based diet, McKinney said. Mercury levels accumulate up the food chain.
The research, by McKinney and partners at the U.S. Geological Survey, found a varying pattern, with adult males showing more of a decline in mercury than adult females.
It is not yet clear why mercury levels are dropping for these polar bears, McKinney said. Body condition — higher body mass and more body fat — rather than individual animals' choice of prey is the factor most closely linked to lower mercury levels in individual animals, she said. Thinner bears have higher levels, she said. Yet other factors may be at play, she said.
To get a more complete explanation, "we're going to need additional years of data," she said.
Starvation odds in Hudson Bay
How long can polar bears go without food? Some answers come from research on temporarily captive polar bears near Churchill, Manitoba, an area where annual ice-free periods have lengthened over time, according to research presented at the conference.
Bears detained by Manitoba environmental officials from 2009 to 2014 were monitored at the time of capture and the time of release to see how their bodies changed during a period when they were given water but no food.
Median time of detainment was 17 days, and the median loss was 1 kilogram per day, or 0.5 percent of body mass. From those numbers, Canadian university and government scientists figured how long it would take for the animals to starve. After 180 days, they calculated, 56 percent to 63 percent of subadults and 18 percent to 24 percent of adult males would die of starvation.
The bears of James Bay in the southernmost part of Hudson Bay, the world's southernmost habitat for polar bears, have evolved to survive long periods of fasting. But even they are at increased risk of starvation if current low-ice trends continue, according to research by scientists from Canada's Trent University and the Ontario Ministry of Natural Resources and Forestry.