7/15/2013 3:01:16 PM
As the Ice Leaves, the Polar Bears Come
The polar bears of Hudson Bay are leaving their home on the sea ice and heading to shore to wait out the ice-free period. The first of our satellite-collared bears decided to celebrate Independence Day (4th of July) by coming ashore north of Churchill (see map below). However, rather than celebrating with a BBQ and fireworks, coming ashore is a letdown for the bears. The ice-free period is also known as the fasting period for good reason. Summer is a time when the bears give up eating chubby seal pups and hunker down to live off their fat reserves until the ice forms again in late fall.
It’s a bit unusual for a polar bear to come ashore north of Churchill, but the ice is less predictable than it used to be. Scientists have predicted that site fidelity will break down as the ice melt and formation patterns change.
Data from the Canadian Ice Service show that Hudson Bay, Hudson Strait, and parts of Davis Strait have a mix of ice levels both well below and well above normal (see map below). The missing ice seems to have won, with large parts of Hudson Bay well below normal ice concentrations. The bears normally leave the ice when concentrations fall to 30-50% cover. We hit this level over the last week so we can expect all the bears to be ashore in the next couple of weeks (see chart below).
A polar bear coming ashore in early July is off the ice about a month earlier than bears used to come show up back in the 1980s. I can remember watching bears swimming ashore in early August in 1985 and seeing lots of ice floating just offshore. How such an early arrival will affect a polar bear will depend on a couple of things:
- First, the hunting conditions during spring that determined how much fat the bear was able to store.
- Second, for non-pregnant bears, the timing of freeze-up in autumn is important. Bears returning to the ice in autumn have a brief window of good hunting as the ice reforms. Autumn hunting doesn’t match spring hunting for caloric return, but it helps put the bears back into a positive energy balance (more calories in than out).
Sea ice in the Arctic as a whole is tracking well below the long-term average but also well above 2012, which was the lowest year on record (see graph below by the National Snow and Ice Data Center). The trajectory to the minimum ice cover is still difficult to determine and will depend on temperature and wind patterns over the next couple of months. Sea ice is a notoriously dynamic habitat. Polar bears are used to variation but the longer term downward trend in sea ice losses remains worrisome.
What summer 2013 will mean for the bears of western Hudson Bay remains to be seen. We can hope that it was a good spring feasting season and that they arrive onshore chubby. We can also hope for a cold summer so the Bay stays cool and freezes early.