© Dmytro Cherkasov
11/10/2017 10:15:41 PM
Polar Bears Returning to the Ice
This year’s Polar Bear Week brought good news for the Western Hudson Bay polar bears, with frigid temperatures and blizzard conditions setting the stage for the earliest freeze-up seen in recent years.
“That’s a positive sign for this threatened population,” said Geoff York, senior director of conservation for Polar Bears International. “Being back on the ice will start them on a longer seal-hunting season and allow them to replenish their fat reserves.”
“So far, there’s thin ice forming over shallow tidal flats near shore,” he added. “Many of the bears have departed the coast near Churchill for that newly frozen rim, which is much closer to Hudson Bay proper and their seal prey. We’re cautiously optimistic that the bay will freeze earlier than last year’s very delayed freeze-up, giving the bears the first good start they’ve had in quite a few years.”
Sea ice loss over time
But does this mean we no longer need to worry about the Western Hudson Bay polar bears? York said that it’s important to look at this year’s freeze-up within the context of natural variation and long-term trends.
Last year at the same time, for example, the bears looked out to an ice-free bay and the tundra was only lightly dusted with snow. Freeze-up didn’t occur until late December, prolonging the fasting period for the population.
While this year’s closer to average freeze-up is good news for the Western Hudson Bay bears, it’s part of natural, year-to-year variation, as the chart below shows. Across the Arctic this year, sea ice was at its fifth lowest since satellite tracking began and the long-term trend remains sharply downward for both sea ice extent and volume or thickness.
Polar bear fasting limits
The Western Hudson Bay polar bears live in a seasonal ice area, where the ice melts each summer and re-forms in the fall. Polar bears have adapted to these feast and famine cycles by storing up fat during times of plenty. They live off these reserves during food-deprived periods, but there are limits to how many days they can go without food.
In the 1980s, the fasting period for the Western Hudson Bay polar bears was about 120 days. That period is now up to 177 days (2015) and is expected to increase beyond the 180-day threshold as the Arctic continues to warm. Past that mark we expect to see increased mortality for both young and old bears and dramatically lower reproductive rates.
A 2010 study led by Dr. Peter Molnar showed that 3% of Western Hudson Bay’s adult male polar bears would die with a 120-day fast; if the ice-free period extends to 180 days, however, the mortality rates increase to 28%.
“While we will always have up years and down years, the trend is very clear. There’s a limit to how long the bears can go without food,” York said, “and that’s why we monitor sea ice loss so closely and advocate for action on climate.”
The long term research of Environment and Climate Change Canada clearly shows that the adult females in this region are coming ashore with lower body weights over time, reproductive success rates are falling, and the population has been steadily declining as that fasting window grows longer.