Coastal and Offshore Patterns of Rorqual and Porpoise Densities in Canadian Pacific Waters

Information on the density and distribution of wide-ranging cetaceans is difficult to obtain but essential for assessing population status, evaluating risk from anthropogenic threats, and informing spatial management. In Canadian Pacific waters, several cetacean populations are recovering from historical depletion, yet recent patterns of habitat use and range expansion remain poorly documented, particularly in offshore areas. In summer 2018, we conducted systematic ship-based surveys throughout the coastal and offshore waters of British Columbia. Density surface modelling was applied to four commonly encountered species: humpback whale, fin whale, Dall's porpoise, and Pacific harbour porpoise. Our results provide the first systematic coverage and spatially-explicit density estimation for cetaceans in Canada’s entire Pacific Exclusive Economic Zone, and highlight areas of apparent reoccupation by recovering populations. Densities of fin whales and Dall’s porpoises were high in offshore waters west of the continental shelf break, whereas humpback whales and harbour porpoises primarily occurred in shelf and nearshore habitats. We discuss patterns of spatial separation and overlap among ecologically similar species in relation to habitat preferences and potential niche partitioning.