Polar bears threatened if climate change warms Hudson Bay: study

The population of the North’s iconic polar bear could be dealt yet another blow due to climate change, results of a new Canadian study suggest.

The study, by biological science professor Andrew Derocher from the University of Alberta in Edmonton, predicts that anywhere from 40 to 73 per cent of pregnant bears would fail to give birth to cubs if the thaw in Hudson Bay occurs one month earlier.

And, if the ice breakup occurs two months earlier, 55 to 100 per cent of potential mothers would not give birth. At the same time, the average litter size could drop, with triplets becoming more rare and single births the new norm, the study says.

While those numbers will vary from year to year, Derocher said, they also will determine how quickly the polar bear population could drop.

"This is another piece of the puzzle of looking at how climate change could affect polar bears. What should we be watching for in these populations? There’s no indication this population is sustaining itself."

For roughly eight months of the year, free to roam the Hudson Bay ice, female polar bears gorge on seals and other prey. The less time that ice holds, the less time bears have to accumulate the more than 440 pounds of fat needed to carry a cub to term.

Polar bears mate in the spring, but the fertilized egg isn’t implanted in the womb until the fall. If a mother cannot carry the cub, or lacks the weight to lose four pounds a day to feed one, her body will abort the pregnancy by reabsorbing energy that would have been directed to the fetus.

A healthy mother can carry two to three cubs to term, while one that had a shorter feeding year may have one or none at all.

"This is not a mating issue," Derocher said. "Time without the ice, and the food, has a direct effect here."

The potential impact on birthrates could add to an already declining population in the Hudson Bay area, the study states, where 28 per cent of pregnant females failed to give birth as early as the 1990s.

Research was conducted by capturing and tagging 28 female polar bears, then determining if they had cubs in the next year. A study that looks at individual bears is more accurate than one done on the full population, Derocher said.

"If you could count all the cubs and mothers, you would get the same information," he said. "However, the same mother and cubs could be counted twice, and it’s very hard to get a gender of a bear unless they’re caught or have cubs with them."

The study is not a snapshot of the current situation, Derocher said, but a prediction that holds only if the Hudson Bay ice lasts less time over the years.

Female polar bears begin to reproduce at six years old, and can live to be 30.

Derocher has been studying polar bears for 28 years, in both Norway and Canada. He has been on more than 40 research excursions.

The article will be published in the online journal Nature Communications on Tuesday.