California and Steller sea lion use of a major winter haulout in the Salish Sea over 45 years

Populations of California sea lions Zalophus californianus and Steller sea lions Eumetopias jubatus in much of the eastern North Pacific Ocean have experienced significant growth since being protected in the early 1970s (by the US Marine Mammal Protection Act and Canada's Fisheries Act) from commercial harvests and fisheries-related culls.However, there remains substantial and unexplained variance in the annual number of sea lions using a major winter haulout at the entrance to the Salish Sea.We used linear regression on principal components to show maximum annual sea lion counts, adjusted for population growth, varied as a function of herring biomass and not local sea surface temperatures or precipitation.Results suggest that Race Rocks, British Columbia, Canada, is used as a stopover for an increasing number of sea lions approaching southern Vancouver Island to feed during the nonbreeding season.Reports of resource-driven movements of sea lions are not new, but this is the first study to use a multidecade data set to show resource-driven movements can underlie long-term patterns of population growth.Finally, we found an unexplained change in the seasonal use of Race Rocks affecting both species.Arrival at Race Rocks has occurred in late summer since 1965, but up to 1979 departure had occurred the following spring; since as late as 1997, departure has occurred mid winter.This study highlights the complexities of enacting conservation plans for species with latitudinal distributions and undergoing long-term population change.