Biologists have warned that disappearing summer and fall sea ice will force polar bears to swim greater distances -- and spend more energy doing so -- as they try to find enough food to survive.
Now they've gathered direct evidence that it's already happening.
Using satellite data sent from radio collars worn by 135 polar bears in the Beaufort Sea and Hudson Bay from 2007 to 2012 to track where and how often the bears made long-distance swims (defined by the researchers as more than 50 kilometers or about 31 miles), researchers found a correlation between low sea ice and long-distance swimming. The results of the study, which was conducted by scientists from the University of Alberta and Environment and Climate Change Canada and incorporated additional data stretching back to 2004, were published online in the journal Ecography.
Such long swims were particularly frequent in the Beaufort Sea in 2012, the year when summer sea ice reached the smallest extent in the satellite record, the data showed. That year, 69 percent of the tracked bears made a long-distance swim.
We can expect to see more such swims in the future, the authors said, which bodes ill for the bears' future.
Long swims are probably "energetically costly to polar bears," lead author Nicholas Pilford, formerly with the University of Alberta but now a post-doctoral fellow at San Diego Zoo Global, said in a statement. "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."
In all, there were 115 long-distance swims in both populations. The median swim duration was 3.4 days and median distance was 92 kilometers (about 52 miles), the study found. Beaufort Sea bears were far more likely to set out on long swims, and lone subadults swam long distances as frequently as lone adult females. But females with cubs were much less likely to swim long distances, the study said, perhaps because long swims are risky for very young bears. (The necks of adult male polar bears are too wide to hold radio collars, so they are generally not tracked in this way and weren't included in this study.)
The longest swim the researchers recorded was a trek of 404 kilometers (about 251 miles) that a subadult female made over the course of nine days in the Beaufort Sea in 2009.
While long-distance-swimming Beaufort bears were generally heading out to sea to the edge of the pack ice, the Hudson Bay bears were usually headed from the pack ice to land. Most of the swims there were recorded in July, the month when ice usually melts out.