11/13/2014 3:01:42 PM
Of Polar Bears and Seals
Here I am again on the coast of Hudson Bay, but this is not my usual time to be up here. Normally, I come to Churchill as the ice is melting in late May. It is now November and we are seeing the start of ice forming along the shore. As we drive around in Buggy One and talk about the polar bears, I keep thinking about the seals I study when I am here in the spring.
Hudson Bay is home to three resident species of seals: bearded seals, ringed seals, and harbor seals. Polar bears are the primary predators of ringed and bearded seals, and they probably take harbor seals during the winter months. My research focuses on ringed and harbor seals—and how the abundance of these two closely related seals might be changing.
Ringed seals are the most common Arctic seal. They are highly adapted to life in sea ice. As the ice forms, ringed seals establish breathing holes that they maintain throughout the winter. To create a den for their pups, females will expand several breathing holes up into the snowdrifts that form over them. In late March and early April, they will give birth to a single pup in one of the dens they have dug. The pups nurse on high energy milk and are on their own by May and June. Adult seals start to haul out on the ice as spring progresses in order to moult their fur before they spend the summer foraging in the open water.
Harbor seals on the other hand are not well adapted to sea ice, but they still can be found in many parts of the Arctic. Harbor seals in Hudson Bay spend the winters in the cracks and leads in the ice that form predictably at the 50-meter depth contour because they don't maintain breathing holes in the ice. As soon as the ice leaves the Churchill River Estuary, harbor seals move in and haul-out in groups on the warm rocks. It is there that they give birth to pups and rest between trips out into the bay to feed. As the open water season gets longer and as spring conditions change we expect it to be more favorable for harbor seals but less favorable for ringed seals. What will it mean for polar bears is an ongoing question.