Traditional Whale Harvest Feeds Polar Bears
As dwindling sea ice makes it harder for polar bears to hunt seals, the carcasses of bowhead whales left over from traditional Inupiat harvests may help sustain the endangered predators. Analysis of blood taken from polar bears living along the Beaufort Sea off Alaska's north coast confirms that bowhead whales average up to 18 percent of a bear's fall and winter diet.
The bears specialize at preying on seals, with ringed seals typically comprising well over half of a bear's food. Another 17 percent of their meat and blubber intake comes from bearded seals. Seals eat fish, placing them at a high trophic level on the food web. The significance of documenting baleen whales in a polar bear diet lies in demonstrating that the bears do feed further down the food web, on a mammal that eats zooplankton.
The bowhead whale component of a bear's winter diet varies considerably among individuals, ranging from 0 to 59 percent. Occasionally whales that die of natural causes become available to scavenging bears. The largest source of whale carcasses in northern Alaska, though, are the remains from subsistence hunting by indigenous people. From September to March, up to 60 polar bears at once hang around butchered bowhead carcasses.
The proportion of bowhead whale in the bears' diet was quite different for the two years studied. In 2003, whales made up about 18 percent of bear nutrition, whereas in 2004 the intake measured was much lower, at 6 percent.
While reasons for the difference could pertain to variables such as how sea ice conditions influence access to seals, the variation does correspond to changes in Inupiat whale harvesting. At Barrow, Alaska 19 whales were taken in fall 2002, compared with only six a year later. In 2003, the whale remains were pushed into the water, making them inaccessible to bears, rather than being left on beaches or ice floes as in the previous year. Considerably less harvested whale leftovers were available to polar bears in 2003 compared with 2004.
T.W. Bentzen, E.H. Follmann, S.C. Amstrup, G.S. York, M.J. Wooller and T.M. O'Hara. 2007. Variation in winter diet of southern Beaufort Sea polar bears inferred from stable isotope analysis. Canadian Journal of Zoology. 85(5): 596-608.