Unusual narwhal sea ice entrapments and delayed autumn freeze-up trends
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Sea ice entrapments of narwhals (Monodon monoceros) occur when rapid changes in weather and wind conditions create a formation of fast ice in bays or passages used by whales. Between 2008 and 2010, four entrapments of narwhals were reported in Canada and Greenland. In each case, large groups (40–600 individuals) succumbed in the sea ice at three separate summering localities, two of these where entrapments had never before been reported. We examined long-term trends in autumn freeze-up timing (date when sea ice concentration rises above some threshold) on the 6 largest narwhal summering areas using sea ice concentration from satellite passive microwave data (1979–2009). We found strongly positive and significant trends (P < 0.001) in progressively later dates of autumn freeze-up in all summering areas. Autumn freeze-up occurs between 0.5 and 1 day later per year, or roughly 2–4 weeks later, over the 31-year time series. This indicates that sea ice conditions on narwhal summering areas are changing rapidly. The question remains whether entrapment events on summering areas are random or whether narwhals are adapting to changes in sea ice freeze-up by prolonging their summer residence time.
KeywordsClimate change Narwhal Natural mortality Sassat Sea ice entrapment
This work was supported by a NASA ROSES grant (Grant no. NNX08AF71G) (KLL, HS), the Greenland Institute of Natural Resources (MHJ), and Fisheries and Oceans Canada (PR). Thanks to the hunters in Greenland and Canada, Blair Dunn, and Jack Orr for providing details on narwhal entrapments. Thanks to Steve Ferguson, Keith Reid, and Sue Moore for thoughtful reviews of the paper.
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