Re-assessing abundance of Southern Hudson Bay polar bears by aerial survey: effects of climate change at the southern edge of the range

The Southern Hudson Bay polar bear (Ursus maritimus Phipps, 1774) subpopulation is considered stable, but conflicting evidence lends uncertainty to that designation. Capture-recapture studies conducted in 1984-1986 and 2003-2005 and an aerial survey conducted in 2011/2012 suggested that abundance was likely unchanged since the mid-1980s. However, body condition and body size declined since then, and duration of sea ice decreased by about 30 days. Due to the conflicting information on subpopulation status and ongoing changes in sea ice, we conducted another aerial survey in 2016 to determine whether abundance had changed. We collected data via mark-recapture distance sampling and double-observer protocols. Results suggest that abundance declined 17% from 943 bears (95% CI: 658-1350) in 2011/2012 to 780 (95% CI: 590-1029) in 2016. The proportion of yearlings declined from 12% of the population in 2011 to 5% in 2016, whereas the proportion of cubs remained similar (16% in 2011 vs. 19% in 2016) suggesting low survival of the 2015 cohort. In a warming Arctic, duration of sea ice is predicted to continue to decline in Hudson Bay affecting all ice-dependent wildlife; therefore, further monitoring of this subpopulation is warranted. We recommend a conservative approach to harvest management and repeating the aerial survey in 2021.