Dietary specialization in two sympatric populations of killer whales (Orcinus orca) in coastal British Columbia and adjacent waters

Two forms of killer whale (Orcinus orca), resident and transient, occur sym patrically in coastal waters of British Columbia, Washington State, and sou theastern Alaska. The two forms do not mix, and differ in seasonal distribu tion, social structure, and behaviour. These distinctions have been attribu ted to apparent differences in diet, although no comprehensive comparative analysis of the diets of the two forms had been undertaken. Here we present such an analysis, based on field observations of predation and on the stom ach contents of stranded killer whales collected over a 20-year period. In total, 22 species of fish and 1 species of squid were documented in the die t of resident-type killer whales; 12 of these are previously unrecorded as prey of O. orca. Despite the diversity of fish species taken, resident whal es have a clear preference for salmon prey. In field observations of feedin g, 96% of fish taken were salmonids. Six species of salmonids were identifi ed from prey fragments, with chinook salmon (Oncorhynchus tshawytscha) bein g the most common. The stomach contents of stranded residents also indicate d a preference for chinook salmon. On rare occasions, resident whales were seen to harass marine mammals, but no kills were confirmed and no mammalian remains were found in the stomachs of stranded residents. Transient killer whales were observed to prey only on pinnipeds, cetaceans, and seabirds. S ix mammal species were taken, with over half of observed attacks involving harbour seals (Phoca vitulina). Seabirds do not appear to represent a signi ficant prey resource. This study thus reveals the existence of strikingly d ivergent prey preferences of resident and transient killer whales, which ar e reflected in distinctive foraging strategies and related sociobiological traits of these sympatric populations.