Migration timing affects the foraging ecology of Fraser River sockeye salmon stocks in coastal waters of British Columbia, Canada

Coastal migrations of juvenile Pacific salmon Oncorhynchus spp. have evolved to take advantage of optimal ocean foraging conditions and maximize early marine growth and survival. The growth and survival of salmon during the early marine period is affected by both the diversity of encountered coastal habitats, with varying productivity and plankton phenology, and stock-specific migration timings that determine the match-mismatch with their prey. In 2015 and 2016, we investigated temporal and spatial patterns in environmental conditions and zooplankton prey as well as diets and stock composition of juvenile Fraser River sockeye O. nerka during their outmigration through the tidally mixed Discovery Islands and Johnstone Strait in British Columbia (Canada). Three groups of sockeye diet profiles reflected variations in environmental conditions and prey communities. First, in the Discovery Islands, earlier migrating stocks primarily encountered and foraged on small, energy-poor zooplankton prey (barnacles and cladocerans). Second, later migrating stocks foraged mainly on larger, more energy-rich copepod and larvacean prey. And third, in the highly mixed waters of Johnstone Strait, large energy-rich calanoid copepods dominated diets irrespective of migration timing and year. Foraging success was typically low throughout the areas sampled and across the migration period, which may amplify the importance of prey nutritional quality. Our findings highlight the importance of accounting for spatial and temporal differences in foraging environments for migrating species such as juvenile salmon. Furthermore, we demonstrate that the timing of stock migration affects the foraging conditions experienced.