Climate Fact: Ice Break-Up Dates and Bears
Polar bears, Earth’s largest land predator, are most common on annual sea ice that sits over shallow seas. This ice provides the bears with a platform from which they can hunt for food. In Canada’s western Hudson Bay region, which is at the southernmost extent of the polar bear’s range, winter and spring are the best times to hunt, as that is when there is the most ice cover and spring is when the most seals are available. By the time late spring and summer arrive, the Hudson Bay is ice-free, and the bears are essentially stranded from their prey until the ice freezes-up again. The bears must live on their fat reserves during this period, which generally lasts about four months. The earlier the ice breaks up in the spring, the less time the bears have to build fat reserves to survive the hungry periods. Warmer temperatures in the region mean that the ice is now breaking up an average of three weeks earlier than it did 30 years ago, and a correlation exists between the date of the break-up and starvation induced mortality in young and older bears. Overall, the region’s Polar Bear population declined from 1194 in 1987 to 935 in 2004. While healthy adult bears can usually survive the extra stress of earlier ice break-up, years when ice breaks-up especially early generally correspond to more human-bear encounters, as the bears stray from their usual territories to find food.
Seasons: Winter, Spring, Summer
Source: Regehr, EV et al. “Effects of Earlier Sea Ice Breakup on Survival and Population Size of Polar Bears in Western Hudson Bay.” The Journal of Wildlife Management 71 (2007): 2673-2683.