To what extent do type C killer whales (Orcinus orca) feed on Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea, Antarctica?

Information on type C killer whales in the Ross Sea region is reviewed, in particular on the trophic overlap between type C killer whales and Antarctic toothfish. Killer whale population ecology (high consumption rates, low abundances, low production rates, often specialised diets, unknown potential for foraging innovation) means that they are particularly vulnerable to changes in the ecosystem. It is also possible that changes in killer whale feeding can affect the structure and stability of whole ecosystems.



There is circumstantial evidence that suggests that toothfish are an important prey item for type C killer whales in the Ross Sea: (1) type C killer whales near McMurdo Sound have been commonly observed carrying toothfish in their mouths; (2) comparison of the relative nutrient density of toothfish with silverfish and other prey shows that toothfish represent a high-energy food resource of much higher quality than other potential prey in the Ross Sea region; (3) densities of other alternative potential prey (Antarctic silverfish, cryopelagic fish) seem too low to justify killer whales coming to the Ross Sea for feeding and the development of a fish-eating ecotype; (4) anecdotal observations of type C killer whales with toothfish in their mouths in McMurdo Sound have declined since 2000, consistent with reduced catch rates of toothfish by scientific fishing in McMurdo Sound, though the reasons for observed changes in this location are not known.



Other information reviewed here is inconclusive: (1) Habitat overlap information is inconclusive, because it is not known to what extent toothfish forage pelagically or how deep type C killer whales can dive. Recent and unpublished information shows that type C killer whales in the Ross Sea can routinely dive to 200-400 m, with a maximum of >700m. This is deep enough to reach demersal prey over much of the Ross Sea shelf. (2) Stable isotope values of killer whales and toothfish do not support or refute the hypothesis that toothfish are a major prey item in the Ross Sea in summer. Information on the isotope values of skin of type C killer whales during the full time period in which  they are in the Ross Sea, and in different locations inside and outside the Ross Sea,  as well as turnover times for stable isotopes in killer whale skin, are required to interpret the isotope data. (3) Comparison between the consumption rates of killer whales and maximum biomass of toothfish potentially eaten by predators at two scales (McMurdo Sound, Ross Sea shelf) suggested that it was possible that type C killer whales could feed substantially on toothfish in summer, but much depends on the number and distribution of killer whales in the Ross Sea region.



At present, the balance of evidence suggests that toothfish are likely to form a significant part of the diet of type C killer whales in McMurdo Sound in summer, but it is not possible to say whether toothfish are an important prey item to type C killer whales in other locations on the Ross Sea shelf (e.g. Terra Nova Bay, Bay of Whales, Sulzberger Bay) or at the scale of the whole Ross Sea shelf and slope.



Basic information necessary to evaluate reliably the risk to type C killer whales in the Ross Sea from the toothfish fishery is urgently needed, including: prey type, foraging behaviour, abundance (and trends) and demographics. Suggested methods are biopsy sampling (analysis for isotopes, fatty acids, genetic tagging), focal-follows (e.g. from boat, ice-edge, helicopter), photographic sightings, tagging (satellite, suction-cup tags), aerial and acoustic surveys.