Research Document - 2014/088

Offshore Killer Whales in Canadian Pacific Waters: Distribution, Seasonality, Foraging Ecology, Population Status and Potential for Recovery

By John K.B. Ford, Eva H. Stredulinsky, Graeme M. Ellis, John W. Durban, and James F. Pilkington

Abstract

In 2011, Offshore Killer Whales off Canada’s Pacific coast were uplisted to Threatened from Special Concern under the Species at Risk Act. With this uplisting, it is required that a Recovery Strategy be prepared by DFO to facilitate recovery. Here, we present an assessment of the distribution, seasonality, foraging ecology and population status of Offshore Killer Whales (OKWs), as well as an assessment of the recovery potential of the population. This document is intended to support the development of goals and objectives in the future Recovery Strategy. For this assessment, we used an archive of observations and photo-identifications of individual OKWs collected during 137 encounters between 1988 and 2012, as well as detections of OKWs from a network of underwater acoustic stations during 2006–2012. The OKW population ranges widely in continental shelf waters from southern California to the eastern Aleutian Islands, and may occur in Canadian Pacific waters in any month of the year. Recent evidence suggests that this population feeds primarily on sharks, although some teleost fishes such as Chinook Salmon and Pacific Halibut are also consumed. To assess abundance and trends, we applied a ‘mark-recapture’ approach to the analysis of the photo-identification dataset using a Bayesian modeling framework. These analyses indicate that the OKW population is small, with an average annual abundance estimate of 300 (95% Highest Posterior Density Interval (HPDI) = 257–373). The population appears stable, with average annual survival rates of 0.98 (95% HPDI = 0.92–0.99) balanced by annual recruitment rates of 0.02 (95% HPDI = 0–0.07). A recovery potential assessment is provided that identifies threats to OKWs and their habitat, and measures to mitigate these threats. A Potential Biological Removal (PBR) of 0.55 animals per year suggests that the population could sustain very little human-caused mortality without declining.

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