With the summer sea ice melting faster and moving farther from shore in the Arctic, polar bears are forced to swim for several days in the open water in order to find food and a place to rest, according to a new Ecography study published last week.
Polar bears are ice-obligate, apex predators who prefer to hunt seals in sea ice habitat on the continental shelf. In seasonal-ice ecosystems, feeding is especially intense in springtime when seals are more vulnerable, allowing the bears to accumulate their energetic stores. When sea ice begins to melt in the late spring, polar bears migrate to land or offshore pack ice for the summer.
But environmental changes are occurring in the Arctic more rapidly than anywhere else on the planet. And instead of walking on a consolidated sea ice surface, polar bears are forced to undertake long-distance swims to complete their migratory route. While they’re adept swimmers, energetically-costly swims of 50 kilometers (31 miles) or more are very risky.
To study the swimming behavior of the Arctic predators in our changing world, a team led by Nicholas Pilfold from the University of Alberta studied data on over a hundred polar bears (Ursus maritimus) that were monitored with GPS satellite-linked telemetry during their seasonal migrations from 2007 to 2012. These included 58 adult females and 18 subadults from the Beaufort Sea and 59 adult females from Hudson Bay.
The team identified a total of 115 swims in the records of 135 collared bears – nearly half of which completed at least one long-distance swim. The duration of these swims ranged between 1.3 and 9.3 days, and 51 to 404 kilometers (32 to 250 miles) in distance. In the Beaufort Sea, polar bears without offspring were more likely to swim long distances. Meanwhile, youngsters swam as frequently as lone adult females, but more frequently than females with young cubs.
In the Beaufort Sea in 2004, 25 percent of the adult females made at least one long-distance swim, but in 2012 – as the rate of open water gain increased – that proportion jumped up to 69 percent. "Given the continued trend of sea ice loss, we recognize that an increased frequency in the need to engage in this behavior may have serious implications for populations of polar bears living around the Arctic Basin," Pilfold said in a statement.