Shallow and surrounded by land—yet part of the Arctic Ocean—Hudson Bay freezes over completely in winter and becomes ice-free for a period in summer and autumn. The sea ice typically melts away between June and August, and the bay begins to freeze over again in late October or November.
The ice’s distribution and rhythms play a central role in the lives of many animals, especially polar bears. When the bay is topped with ice, polar bears head out to hunt for ringed seals and other prey. When the ice melts, the bears retreat to shore, where they fast or feed on whatever bits of food they can find until the ice returns. Since bears burn about 1 kilogram (2 pounds) per day while on land, too much time on shore can lead to stress and starvation.
In early June 2023, warm weather accelerated Hudson Bay’s ice breakup, according to data from the Canadian Ice Service. This left much of the bay with less ice than usual by the end of the month, especially in the western and central parts of the bay. When the Visible Infrared Imaging Radiometer Suite (VIIRS) on the NOAA-20 satellite captured this image on June 28, 2023, there was still some sea ice drifting along the bay’s southwestern shore, but it was fragmented and unlikely to persist for much longer.
In 2023, some Western Hudson Bay polar bears started to return to shore in mid-June, but others lingered on the ice well into July. “Mid-June is early to have bears on land," said University of Alberta scientist Andrew Derocher. “When I started studying polar bears in Western Hudson Bay in the 1980s, we would have bears on the ice well into August.”
Derocher is part of a research group that monitors polar bear populations by tracking bears with GPS satellite collars. They typically conduct fieldwork in Churchill, Manitoba, a remote town on the west shore of Hudson Bay known for its concentrated but declining polar bear population. Polar bears tend to congregate and make dens near Churchill because nearby bays are typically some of the first places where ice freezes over in the winter.
The bears that returned to land in mid-June will have to fast for about 150 days if the timing of the freeze-up (usually mid-November) is normal. If freeze up is late (around late December), higher numbers of bears will starve. Despite some of the bears swimming ashore early, Derocher noted that many bears were still out on the ice in early July, despite how little ice was left and how fragmented the last bits of ice had become.
“The bears on ice are hanging on at extraordinarily low ice levels,” Derocher said. “But better for them to be there trying to kill another seal than on land with little or nothing to eat. Bears can feed and keep putting on weight well into the summer if they have the ice. For polar bears, it truly is survival of the fattest.”
NASA Earth Observatory image by Wanmei Liang, using VIIRS data from NASA EOSDIS LANCE, GIBS/Worldview, and the Joint Polar Satellite System (JPSS). Story by Adam Voiland.