Shrinking sea ice on Hudson Bay threatens the ability of its resident polar bears to produce new cubs, a new study suggests.
Researchers at the University of Alberta looked at the relationship between the energy a pregnant polar bear needs to give birth to healthy young and the increasingly early breakup of the ice the bears use as a hunting platform.
"Based on the best scenarios we've got for sea ice in the Hudson Bay, the situation looks pretty grim," said Andrew Derocher, one of the authors of the report, which is published in the journal Nature Communications.
"It is quite subtle, because if you were out just watching polar bears and just counting mothers with cubs, you wouldn't see it. But you would find the actual number of cubs produced would decline and this is directly related to the amount of time that the sea ice exists in the Hudson Bay ecosystem."
Polar bear reproduction depends on the ability of female bears to get fat. That requires a good, thick layer of sea ice, allowing the animals to go after seals.
Female bears mate on the ice in March or April and then begin packing on the pounds - as much as 200 kilograms of extra body fat. The bear embryos don't develop until the fall, when the mother is back on shore - a process called delayed implantation - and that won't happen at all if the mother isn't fat enough.
"The basic deal here is, the fatter she gets, the more cubs she is going to produce," Prof. Derocher said. "If she comes on shore and she is not really fat, she might be able to produce one cub. A little fatter, she should produce two. If she is really fat, she could produce three. If she is not very fat at all, she produces none."
The extra fat gives the mother bear enough energy for an eight-month fast while she births and nurses her young.
The problem, Prof. Derocher said, is the time the bears can spend out on the ice getting fat is shrinking because the ice is breaking up earlier in the year. In the past, bears have typically stayed on the ice until as late as August. Last year, some were forced off as early as the end of June.
"What we are asking them to do over time is not go for eight months, but to go nine months or perhaps even longer," he said. "They are just going to run out of gas."
Data collected in the early 1990s suggest that, in any given year, 28 per cent of female polar bears in the region don't give birth.
The University of Alberta study found that, if the ice breakup happens one month earlier than it did back then, the number of cubless females could rise as high as 73 per cent. And if the breakup happens two months earlier, it's possible no cubs would be born at all.
Prof. Derocher said it will only take a few years of bad ice conditions to have a serious impact on the Hudson Bay population, which numbers about 900, down from 1,200 in the past decade.
The worldwide population of polar bears is estimated at between 20,000 and 25,000. Of the 19 populations around the globe, eight are considered to be declining, three are stable and one is increasing. There isn't enough known about the other seven to assess their status.
While the study looked only at the bears along the western shores of Hudson Bay near communities such as Churchill, Man., the numbers could be indicative of what will happen elsewhere in the Arctic if sea ice continues to shrink, Prof. Derocher said.
The National Snow and Ice Data Center in Boulder, Colo. reported that sea ice across the Arctic was at its lowest level in January since satellite records began in 1979. The area of the ice was 50,000 square kilometres under the record low set in 2006 and 1.27 million square kilometres less than the average.
"It's telling us a lot about what we can expect in other areas," he said. "The simplest way to look at this is … a habitat loss issue. Losing the sea ice is simply no different than any other animal losing its habitat."
The Canadian Press