Research Document - 2007/088

Recovery Potential Assessment for West Coast Transient Killer Whales Using Coastal Waters of British Columbia

By J.K.B. Ford, G.M. Ellis, and J.W. Durban

Abstract

Mammal-eating ‘transient’ killer whales off Canada’s Pacific coast are listed as Threatened under the Species-at-Risk Act. A draft Recovery Strategy for transient killer whales was prepared by DFO in 2007, but insufficient information was available to set quantitative recovery goals in that document. Here, we present a Recovery Potential Assessment for West Coast Transient (WCT) killer whales to provide a basis for on-going recovery planning for this population. For this assessment, we used an archive of photo-identifications of individual WCT whales collected during 1479 sightings between 1974 and 2006. We applied a ‘capture-recapture’ approach to the analysis of this dataset and used Bayesian statistical techniques to estimate population abundance and dynamics. These analyses indicate that the WCT population grew rapidly from the mid-1970s to mid-1990s as a result of high survival and recruitment, the latter including immigration of animals into the nearshore study area. Population growth began slowing in the mid-1990s and has continued to slow in recent years as the population approaches an equilibrium point of around 262 whales. The rapid growth of the WCT population in the mid-1970s to mid-1990s coincided with a dramatic increase in the abundance of the whales’ primary prey, harbour seals, in nearshore waters. The recent slowing of WCT population growth suggests a carrying capacity in the mid to high 200s, given the current abundance of marine mammal prey in coastal waters. A Potential Biological Removal (PBR) of 1.60 animals/year suggests that the population could sustain very little human-induced mortality without declining.

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