2/13/2015 5:22:25 PM
Seal Kills and Polar Bears
It is one seal kill observation I will not soon forget. It was early in the afternoon on April 25, 2010, and we were flying over the sea ice of the Beaufort Sea, following the tracks of an adult female polar bear with two yearling cubs. The tracks zigzagged through an active area of sea ice, where small to large floes of ice were interspersed with cracks and openings that revealed the seawater below.
As we followed the footprints across the white and blue canvas, we were led to the edge of a floe, where the tracks suddenly disappeared into the open water of a lead. The lead was a sharp tear in the sea ice, cutting in either direction as far as the eye could see, but less than five meters across.
We circled with the helicopter, rising higher in the air to try to see where the tracks started again. It was then that we spotted the seal kill, perhaps 250 meters down from where the mother had entered the water with her cubs. The kill was fresh, and located only a few meters from the floe/lead edge. We flew over, landed, and Dr. Andrew Derocher and I exited the helicopter to take a closer look.
The kill was a young bearded seal. Through laboratory analysis months later, we determined that the seal was a one-year-old male. The mother and her cubs had cleaned the blubber off the kill, taking the highest value food, leaving only the skin and viscera behind. As I crouched down to take samples from the remains, Andy investigated the site, and it soon became clear how the kill occurred.
The seal had hauled out onto the sea ice, basking in the mid-afternoon light. Likely recognizing the scent of the seal from a distance, the mother polar bear slipped into the open water of the lead and approached from the downwind side. She stalked the seal from the water, silently paddling until she was close enough to emerge onto the sea ice and ambush her prey, cutting off its only access to safety. The young seal was no match for the adult female and her two cubs, who were fast learning how to be skillful hunters themselves. The hunt made it clear how in such a dynamic environment, polar bears are able to use cunning tactics to feed.
For a young scientist, events like these can impress upon the conscience, and I soon found myself wondering: what is a good day for hunting seals for a polar bear? Did the sunny conditions and slight breeze help the mother that day? Why did that juvenile bearded seal haul out, when it was risky to do so? Why not stay in the water where the seal would have had the advantage in agility and speed, and likely would have escaped? Did the sea ice conditions play a part? Do larger scale climatic patterns influence the layout of the spring icescape, providing the types of sea ice advantageous for hunts like these?
I spent the last four years analyzing questions like these for my PhD. I recently defended my dissertation and published my final chapter. In this chapter I analyzed the fine- and large-scale factors that influence the frequency at which we saw seals being killed by polar bears in the spring. I found that hauling out for seals is indeed a risky behavior, as we were much more likely to see a kill on days with more seals visible on the sea ice. As spring progressed and air temperatures warmed, seals use the sea ice surface more often, routinely hauling out in the open.
Seal kill rates for polar bears followed a similar trend, becoming more frequent as spring progressed. Despite the risk, seals must haul out on the ice surface in May and June in order to molt their winter coats. For many arctic animals, overheating can be a more serious survival threat than freezing. Although hauling out may come with the risk of being eaten by a polar bear, it is a required trade-off for being able to survive the summertime heat.
Another interesting finding was the interplay between spring sea ice and climate. Kills were more frequent when sea ice concentration in the study area was higher. This is fairly intuitive: the less sea ice, the smaller the platform from which polar bears can hunt seals and the more widespread the access to the water column for seals to escape. But climate indices also suggested that kills were more frequent in years when the sea ice would begin to crack and become active earlier. Active ice is the preferred hunting habitat for polar bears in the spring. The dynamic between high ice concentration and active ice formation suggests polar bears like some, but not a lot of open water in the system. One can think of it a little like Goldilocks (if Goldilocks was a polar bear): polar bears prefer some active ice, not too much, not too little, just enough to entice seals to surface in open areas.
Part One of a two-part series. Read Part Two here.