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Polar bear on land in the western Hudson Bay region

Photo:

David McGeachy / springernature

Hudson Bay polar bears typically roam ice floes from late spring to early summer. They hunt seals there. As soon as the sea ice retreats, they have to go ashore. The ice-free period in the Canadian Bay has already increased by three weeks between 1970 and 2015. For the polar bears this means less time to hunt. They now spend around 130 days a year on land.

A team led by biologist Anthony Pagano wanted to find out how the predators deal with the fact that their hunting grounds are dwindling. The researchers were able to observe how the animals behaved using GPS trackers equipped with a camera. What they ate and how much they moved. From 2019 to 2022, they accompanied 20 polar bears in the sea ice-free months of August and September. Pagano and his team measured the daily energy consumption of animals in western Hudson Bay and changes in their body mass. Their results were published today in the specialist magazine “Nature Communications”.

"We observed very different behaviors among the polar bears," said Pagano, according to a press release from Washington State University. "Some bears simply lie down and use up a similar amount of energy as when hibernating, others actively look for food and feed on bird and caribou carcasses, seaweed and berries." Three animals even swam several kilometers across the sea to get there to look for food.

Depending on the activity, the researchers were able to observe large differences in the animals' daily energy consumption. Overall, 19 of the 20 polar bears lost weight: 0.4 to 1.7 kilograms per day and thus 8 to 36 kilograms within the three-week observation period. Some animals could have found more food. “But they ultimately used more energy searching for food than they could get back through eating,” explains Pagano.

Although polar bears can adapt their behavior to a great extent, the results show how much a longer period without sea ice increases the risk of the animals starving. “Because polar bears have to retreat to the mainland earlier, they also have less time to build up energy reserves that are essential for survival,” says Pagano. "We assume that more animals will starve to death in the future, especially younger polar bears and females with cubs."

Hardly any room for maneuver when the ice is gone

“Basically, polar bears are very good at making the best of the situation they find themselves in,” says Jon Aars, polar bear expert at the Norwegian Polar Institute. But if there is no more sea ice, they would have little room for maneuver. They actually need hunting in spring and early summer to build up their fat resources. “We need a seal every ten days,” says Aars.

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Polar bear camera collar

Photo: Joy Erlenbach / springernature

How much polar bears are affected by climate change depends largely on where they live, says Aars. “The polar bears in Hudson Bay are doing particularly badly.” They are in one of the southernmost places where polar bears live. In other places, such as northeastern Russia, polar bear populations are currently increasing again. The polar bears are also doing well in Svalbard, Norway - especially because their hunting was banned in 1973.

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