Forced to migrate farther as sea ice thins, these polar bears may starve

April 2, 2021

Thinning sea ice is going to make life tougher for polar bears (and their cubs). (AP Photo/Ronald Zak)

A unique group of migrating polar bears will face added pressures as the world warms, according to a new analysis of satellite data suggesting that shrinking ice coverage will force the top predators to travel farther for food, ultimately leading to higher mortality.

Researchers from the University of Alberta created statistical models to predict polar bear movement in relation to a range of variables, such as wind speed and ice concentration, in Canada's Hudson Bay. The scientists outline their methods and findings in a study published March 9 in Behavioral Ecology

"At some point, I would expect that we're probably going to see more bears not able to meet their nutritional needs," said lead author Alyssa Bohart, who conducted the research as a University of Alberta master's student in ecology. "Then we're likely going to see decreases in the population from that."

"We know that their access to prey will not be as good, and it's going to get worse," Bohart continued, in an interview with The Academic Times. "Basically what's going to happen is there's probably going to be this energetic or nutritional stress on bears."

The polar bears of Hudson Bay are one of a kind: They are the only nonaquatic apex carnivores to migrate for food, sometimes moving as many as 300,000 square kilometers across the sea ice, which fluctuates with changing seasons. These bears would not migrate if ice levels remained more constant. 

Drawing on 12 years of satellite-tracked migration data from female polar bears in western Hudson Bay, Bohart and her colleagues devised a set of multivariate response regression models. Similar to typical linear regression models, these models allow researchers to calculate relationships across different combinations of variables, such as wind direction and ice coverage, against arrival date and the trajectory of a bear's path, for example. 

From these models, Bohart could discern that polar bears will likely migrate farther when there are lower ice concentrations during freeze-up and breakup periods on Hudson Bay, suggesting that, as climate change worsens, polar bears will start moving even more to find food. 

According to Bohart, these findings drive home the importance of ice in polar bear survival and ecology. In the Arctic, polar bears are already threatened by retreating sea ice, forcing them to swim more and expend three times more energy than they would on foot. 

"If they're increasing their movement, then we know that's a dire situation for bears," Bohart said. "If we had seen that their movement isn't higher with lower ice concentrations, then it might've changed the story."

According to Bohart, this Hudson Bay population can serve as a good model to help predict the effects of climate change on Arctic polar bears and study how they are adapting, because some have begun to build dens in the ground rather than in snow. This behavior is similar to that of the Hudson Bay bears, which live at lower latitudes, as well as grizzly bears. 

In future research, Bohart wants to incorporate seal population data, as difficult and expensive as it is to acquire. Polar bears need lots of fat, which, in their environment, can be derived from only ringed and bearded seals. Therefore, these seals likely play a significant role in how polar bears migrate and the variables studied in Bohart's multivariate response regression models. 

She would also like to study the migration of males, rather than only females with cubs, because their movements could be completely different. 

While Bohart and her colleagues did not observe any significant change to polar bear migration over their 12 years of data, they believe their findings signal a need to keep tracking these vulnerable bears.

"We need long-term monitoring for these migrating species to actually detect and to be able to see how they are adapting to climate change," Bohart said. "Our study really highlights that, because we had what some would consider a big timeframe, and we didn't detect the change, but we know that if we had a longer time frame we would have."

The study, "Migration dynamics of polar bears (Ursus maritimus) in western Hudson Bay," published March 9 in Behavioral Ecology, was authored by Alyssa M. Bohart, Nicholas J. Lunn, Andrew E. Derocher and David McGeachy, University of Alberta.

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