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

Abstract

Killer whales (Orcinus orca), both ecotype-B and -C, are important to the Ross Sea, Antarctic ecosystem. The ecotype-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, 2002-2003 to 20082009 austral summers. These “resident” whales appear to feed principally on fish, including the large Antarctic toothfish (Dissostichus mawsoni). 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 RS killer whale numbers in the southern Ross Sea is related to an industrial fishery-driven, densitydependent 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 RS killer whales would force more direct competition with other predators for capture of the smaller-fish prey. Therefore, we propose, the RS 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.