Use of chemical profiles in assessing the feeding ecology of eastern North Pacific killer whales

Blubber biopsy samples from adult male North Pacific killer whales (Orcinus orca), were analyzed for fatty acids, carbon and nitrogen stable isotopes and organochlorine contaminants. Fatty acid profiles were sufficiently distinct among the three reported ecotypes (“resident,” “transient” or “offshore”) to correctly classify the whales in this study by ecotype using a previously developed discriminant function model. In addition, a new discriminant function model was developed using data from whales from both the new and the previous studies. PCB profiles in blubber also allowed unambiguous classification of all three killer whale ecotypes (also using both the old and new models). OC concentrations and ratios were used to provide additional insight on the dietary preferences of killer whales biopsied in Alaska, particularly for the offshores about which little dietary information is available. Surprisingly, mean ∑DDT concentrations in the offshores exceeded those of the Alaska transients and were 20 times higher than those of the residents. In addition, mean ∑PCB concentrations of offshores were very similar to those of the transients and were 10 times higher than those of the residents. If the offshores are fish-eaters, concentrations of ∑PCBs and ∑DDTs should be more similar to those in the fish-eating residents, rather than to those of the marine mammal-eating transients—however, the reverse was true. Thus, it appears that offshores feed at a high trophic level or consume species containing high levels of ∑PCB and ∑DDT, perhaps shark or tuna species. Ratios of certain contaminants have been used to define regions from which prey may originate. Offshore contaminant ratios (e.g., ∑DDTs/∑PCBs and p,p’-DDT/∑DDTs) generally fell between those of the West Coast transients and those of the Alaska residents and transients. Because the offshores are known to have a range that extends from Alaska to California, their contaminant ratios and other chemical profiles may represent those from a mix of prey species acquired from California to the Arctic. However, this study demonstrates that offshore killer whales consume prey species that are distinctly different from those of sympatric resident and transient killer whales. To identify the particular species that comprise the diets of offshore killer whales, as well as for the other ecotypes, contaminant profiles and ratios, as well as fatty acid and stable isotope profiles must be measured in many more putative prey species collected from the killer whales’ foraging areas.