Euphausiids in the diet of a North Pacific seabird: annual and seasonal variation and the role of ocean climate

Across the California Current System (CCS), euphausiid crustaceans are important prey for many vertebrate predators, including the seabird Cassin’s auklet Ptychoramphus aleuticus. However, the effects of ocean climate on euphausiid biology, and their consequences for predators, remain poorly understood. Over a 13 yr period (1996 to 2008), Euphausia pacifica, Thysanoessa spinifera and T. inspinata cumulatively averaged about 35% of the annual biomass in the nestling diets of auklets at Triangle Island, but there was marked inter-annual variation. Whereas ocean climate had little influence on the amounts of E. pacifica or T. inspinata delivered, diets included more T. spinifera if spring sea-surface temperatures in the previous year had been lower. Cold conditions might facilitate the production of a strong annual cohort, thus increasing adult biomass in the following year. Within seasons, the amount of E. pacifica in diets declined with date, and the decline was consistent across both warmand cold-water years. In contrast, diets were especially rich in both Thysanoessa spp. late in the provisioning period in warm-water years, when the harvest of Neocalanus cristatus declined dramatically due to a temporal mismatch between the auklet predator and this copepod prey. Both the annual nestling survival rate and the mean fledging masses of Cassin’s auklets were tightly correlated to the amount of N. cristatus in their diets, and for fledging mass there was a further small, additive effect of increased amounts of T. inspinata. Results of the present study add new insight into effects of ocean climate on euphausiid biology in the northern CCS, and their potential consequences for population processes of an important euphausiid predator.