Abstract
Sea ice is believed to be a major factor shaping gene flow for polar marine organisms, but it remains unclear to what extent it represents a true barrier to dispersal for arctic cetaceans. Bowhead whales are highly adapted to polar sea ice and were targeted by commercial whalers throughout Arctic and subarctic seas for at least four centuries, resulting in severe reductions in most areas. Both changing ice conditions and reductions due to whaling may have affected geographic distribution and genetic diversity throughout their range, but little is known about range-wide genetic structure or whether it differed in the past. This study represents the first examination of genetic diversity and differentiation across all five putative stocks, including Baffin Bay-Davis Strait, Hudson Bay-Foxe Basin, Bering-Beaufort-Chukchi, Okhotsk, and Spitsbergen. We also utilized ancient specimens from Prince Regent Inlet (PRI) in the Canadian Arctic and compared them with modern stocks. Results from analysis of molecular variance and demographic simulations are consistent with recent and high gene flow between Atlantic and Pacific stocks in the recent past. Significant genetic differences between ancient and modern populations suggest PRI harbored unique maternal lineages in the past that have been recently lost, possibly due to loss of habitat during the Little Ice Age and/or whaling. Unexpectedly, samples from this location show a closer genetic relationship with modern Pacific stocks than Atlantic, supporting high gene flow between the central Canadian Arctic and Beaufort Sea over the past millennium despite extremely heavy ice cover over much of this period.
Highlights
Sea ice is a dominant feature of the polar environment and is thought to shape patterns of genetic isolation in both marine and terrestrial island species (e.g., Geffen et al 2007)
We obtained sequence data for 38 ancient samples from Prince Regent Inlet (PRI) and 265 modern samples from Hudson Bay-Foxe Basin (HBFB) and Baffin Bay-Davis Strait (BBDS) (Genbank Accession numbers are provided in the Appendix, Table A2)
The results indicate that Arctic sea ice has not acted as a strong barrier to migration between the Atlantic and Pacific over the late Holocene as previously assumed, and that genetic diversity has been lost from eastern Canada in the period between ~500 ybp and the present
Summary
Sea ice is a dominant feature of the polar environment and is thought to shape patterns of genetic isolation in both marine and terrestrial island species (e.g., Geffen et al 2007). Large-scale removals over the last three centuries may have altered pre-whaling genetic differences between populations by disrupting patterns of migration to breeding areas (e.g., Alter et al 2009) or by eliminating distinct populations from areas and allowing colonization by another stock. Despite these uncertainties, genetic differentiation and stock identity remain important issues for managers and policymakers. Understanding the factors that govern stock structure and gene flow, including the interplay between changing sea-ice conditions and the legacy of whaling, is important for Arctic species that are likely to be affected by climate change, increasing oil and gas development, and shipping, such as the bowhead whale (Balaena mysticetus). Ongoing dramatic declines in sea-ice extent will likely affect genetic exchange rates in bowhead whales as well as other arctic marine mammals such as beluga and walruses (Laidre et al 2008; O’Corry-Crowe 2008), but evaluating these changes requires the characterization of genetic patterns prior to significant ice loss
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