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    An Apparent Decrease in the Prevalence of “Ross Sea Killer Whales” in the Southern Ross Sea

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    Document Number:
    WG-EMM-09/P01
    Author(s):
    D.G. Ainley, G. Ballard and S. Olmastroni
    Publication:
    (Aquat. Mamm., in press)
    Abstract

    Killer whales (Orcinus orca), both ecotype-B and -C, are important to the Ross Sea, Antarctic ecosystem; the type-C is referred to as “Ross Sea [RS] killer whale”. Herein, we review data on occurrence patterns and diet of RS killer whales, and present new information on numbers observed in the southwestern Ross Sea, 2002 - 2008. These “resident” whales appear to feed principally on fish, including the large Antarctic toothfish (Dissostichus mawsoni), as the review herein demonstrates. On the basis of sea watches from Cape Crozier, Ross Island, sighting frequency and average group size appears to have decreased; prevalence as indicated by casual observations from helicopter pilots flying over the area on a daily basis has also decreased in nearby McMurdo Sound. Consistent with a decrease in the catch-per-unit-effort of scientific fishing for toothfish in McMurdo Sound, we suggest and review evidence that the change in Ross Sea killer whale numbers in the southern Ross Sea is related to a industrial fishery-driven, density-dependent northward contraction of the toothfish stock, and not to changes in the physical (and in turn biological) environment. We surmise that in this closely-coupled foodweb, composed of very abundant penguin, seal and whale components, loss of the toothfish option for Ross Sea killer whales would force more direct competition with other predators for capture of the smaller-fish prey. Therefore, we propose, the Ross Sea killer whales have opted to move elsewhere in a scenario consistent with that of the Pacific coast of Canada, where numbers of resident killer whales have decreased following the loss of large fish as a prey choice.

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