Research Document - 2009/101

Chinook salmon predation by resident killer whales: seasonal and regional selectivity, stock identity of prey, and consumption rates

By J.K.B. Ford, B.M. Wright, G.M. Ellis, and J.R. Candy

Abstract

Resident killer whales (Orcinus orca) found in coastal waters of the cold-temperate northeastern Pacific are fish-feeding predators that specialize on Pacific salmon. Field studies have shown that although most available salmonids are consumed, Chinook salmon (Oncorhynchus tshawytscha) is the whales’ primary prey species, most likely because of its large size, high lipid content, and year-round occurrence in coastal waters. Chinook salmon availability appears to be important to the survival and recovery of resident killer whale populations. In this report we describe the results of recent field studies and analyses aimed at improving our understanding of the role played by Chinook salmon in the seasonal foraging ecology and energetics of resident killer whales. An additional 410 prey items identified from scale and tissue samples collected at the sites of resident feeding events provide further support for the importance of Chinook salmon in most seasons and coastal areas. Genetic stock identification of prey samples indicate that killer whales feed on Chinook salmon originating from a variety of regions between Southeast Alaska and Oregon, with stocks in the Fraser River system being of particular importance both coast-wide and in Critical Habitats. An updated analysis confirms the long-term correlation between survival of resident killer whales and range-wide Chinook abundance, though recent declines in Chinook abundance have not yet been associated with increased mortality rates. Estimates of Chinook salmon consumption based on daily prey energy requirements and diet composition suggest that resident killer whale populations at their current abundance may require over 1,000,000 Chinook per year, roughly equivalent to recent annual levels of harvests of this species in commercial and recreational marine fisheries. Estimates of Chinook salmon requirements for northern and southern resident killer whale populations in their Critical Habitats are also provided, as is an estimate of the Chinook abundance that would be required to support killer whale recovery over the next decade. Although the information in this report may be useful for future conservation and management of resident killer whales and their primary prey, further studies are needed to resolve existing uncertainties about year-round diet composition and feeding rates.

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