Research Document - 2005/042

Linking prey and population dynamics: did food limitation cause recent declines of 'resident' killer whales (Orcinus orca) in British Columbia?

By Ford, J.K.B., G.M. Ellis, P.F. Olesiuk

Abstract

Two populations of fish-eating killer whales (Orcinus orca) in British Columbia, known as residents, are listed under the Canadian Species-at-Risk Act due to their small population size and recent unexplained declines in abundance. Threats considered to potentially affect survival and recovery of these populations include environmental pollutants, physical and acoustic disturbance, and reductions in the availability or quality of salmonids, their primary prey. Recent studies have shown that chinook salmon and, to a lesser degree, chum salmon, are important prey for resident killer whales, but other smaller salmonid species are not. In this report, we assess whether food limitation was potentially a significant factor in recent declines of these whale populations. We examined the relationship between trends in killer whale population dynamics based on long-term photo-identification data, and abundance levels of chinook and chum salmon off the British Columbia coast over the past 25 years. Resident killer whale population productivity is regulated primarily by changes in survival. Periods of decline were primarily due to unusually high mortality rates that were experienced by all age- and sex-classes of whales and were synchronous in the socially-isolated two resident communities. Fluctuations in observed versus expected mortality rates showed a strong correlation with changes in chinook salmon abundance, but no relationship to chum salmon abundance. A sharp drop in coast-wide chinook abundance during the late 1990s was closely associated with a significant decline in resident whale survival. The whales’ preference for chinook salmon is likely due to this species’ relatively large size, high lipid content and, unlike other salmonids, its year-round presence in the whales’ range. Resident killer whales may be especially dependent on chinook during winter, when this species is the primary salmonid available in coastal waters, and the whales may be subject to nutritional stress leading to increased mortality if the quantity and/or quality of this prey resource declines. Chinook salmon is clearly of great importance to resident killer whales, but determining whether the species is the principal factor limiting whale productivity will require on-going monitoring of both salmon and whale population trends.

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