Experts believe that for millennia, polar bears have relied on the carcasses of large whales—which the bears find floating or washed up on Arctic shores—to sustain them during warming periods, when the loss of sea ice reduces access to their primary prey, seals.
The importance of whale carcasses to polar bears is highlighted by research published in the November issue of Frontiers in Ecology and the Environment and authored by a team that includes Kristin Laidre, associate professor at the University of Washington and a 2017 Pew marine fellow, and James A. Estes, professor emeritus at the University of California, Santa Cruz and a 1999 Pew fellow.
Although populations of large whales are increasing throughout much of the Arctic, human activities such as industrialized commercial whaling have reduced their abundance overall. In addition, coastal development can deter polar bears and make whale carcasses harder to access. Because of these factors, whale carcasses are a less reliable food source than they once were—potentially curtailing an important lifeline for polar bears in a region that is rapidly warming and may have summers free of sea ice by 2040.
For their research, the team developed estimates of the “bioenergetic value,” or the amount of nutrition that polar bears could gain from eating the various species of large whales that wash up on Arctic coasts. They compared these estimates with the bioenergetic values of ringed seals, the species that polar bears most commonly hunt. The scientists found that a population of bears would need to eat more than 1,300 adult ringed seals to access the same amount of energy contained in one bowhead whale carcass.
The team also estimated the number of large whale strandings in the Arctic each year based on a combination of physiological, historical, and oceanographic data. They hypothesized that stranded whales, which observers have recorded providing food for dozens of polar bears for more than a year at a time, were probably once common enough to help polar bear populations survive interglacial warming periods, when much of the Arctic was ice-free and hunting seals was more difficult.
But in some parts of today’s Arctic, the potential for stranded whales to replace seals as food source for polar bears in a future without sea ice is likely more limited than it was in the past. Populations of North Atlantic right whales and Svalbard and Barents Sea bowhead whales, for example, are still at only a fraction of their pre-commercial whaling numbers. Escalating human activities such as shipping and industrial development may also reduce the ability of bears to find stranded carcasses
“Scavenging on large whale carcasses is probably important for bears in some areas and may buffer them from sea-ice loss,” Laidre said. “However, carcasses of large whales are not expected to replace seals as nutritional resources as we move toward an ice-free Arctic. In most regions, the environmental changes are too large and the whale carcasses are too few.”
Yet, in some regions, whale stocks—as they are referred to by the International Whaling Commission—are likely healthy enough to once again offer a useful food resource for polar bears. In Alaska, effective conservation measures have helped drive a rapid rebound of Bering-Chukchi-Beaufort Sea bowhead whales, with population growth measured at more than 3.5 percent per year. This growth supports sustainable subsistence whaling by Alaska Natives, which—the research team notes—can also provide a food source for polar bears that scavenge from hunters’ kills.
Although the researchers are encouraged by these conservation successes, they caution that Arctic warming is progressing at an unprecedented rate.
“If the rate of sea-ice loss and warming continues unmitigated, what will happen to polar bear habitat will exceed anything documented over the last million years,” Laidre said. “The extremely rapid pace of this change makes it almost impossible for us to use history to predict the future.”
Polita Glynn is a project director for environmental research and science with The Pew Charitable Trusts.