Research Document - 2005/041

Prey selection and food sharing by fish-eating ‘resident’ killer whales (Orcinus orca) in British-Columbia

By Ford, J.K.B., G.M. Ellis

Abstract

Three distinct, socially-isolated forms, or ecotypes, of killer whales (Orcinus orca), inhabit coastal waters of British-Columbia, Washington State, and southeastern Alaska. The so-called transient ecotype feeds primarily on marine mammal prey, the resident ecotype feeds primarily on fish, and the diet of the offshore ecotype is not known. A previous study of the diet of the resident and transient ecotypes using opportunistic collection of prey remains from kill sites as a primary measure of prey selection found that resident killer whales feed predominantly on salmonids, particularly on chinook salmon (Oncorhynchus tshawytscha). To address uncertainties concerning potential biases in the prey fragment sampling technique and questions regarding seasonal and geographic variability in diet, we conducted field studies of foraging behaviour during 1997-2004. Foraging by resident killer whales often involves cooperation among kin-related group members, and prey items are frequently shared by two or more whales. Adult males share prey less often than do females and subadults. Prey sharing does not appear to be related to prey size. Prey fragments left at kill sites result mostly from prey handling and sharing, and are reliable indicators of selection for different salmonid species by resident killer whales. Chinook is the predominant prey species taken by both northern and southern resident communities during May-August, but chum salmon (O. keta) is more prevalent in September-October, at least in northern residents. Coho salmon (O. kisutch) are taken in low numbers in June-October, but sockeye (O. nerka) and pink (O. gorbuscha) salmon are not significant prey species despite their high seasonal abundance. Non-salmonid fishes do not appear to represent an important component of resident whale diet during May-October. Their strong preference for chinook salmon may influence the year-round distribution patterns of resident killer whales in coastal British-Columbia and adjacent waters.

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