Sea ice cycle in western Hudson Bay, Canada, from a polar bear perspective

Remote sensing has allowed insights into changing Arctic sea ice, but seasonal underestimation of ice presence may lead to misinterpretation of species behavior. We use the dependence of polar bears Ursus maritimus on sea ice to assess the utility of satellite-linked radio collar locations to indicate underestimation in sea ice on 2 ice data sets derived from satellites. We then define the ice-free period in western Hudson Bay, Canada, from a polar bear perspective using the correlation between the ice concentration and polar bear migration onshore and offshore. We found that the ice-free period in this region lengthened by 3 ± 0.8 wk over the period 1979-2015. Polar bears migrated onshore 2 wk earlier and offshore 1 wk later in the period 2005-2015 than in 1980-1989. Understanding the trends in polar bear migration and the onshore period is critical to understanding population status.