Interdecadal variability in predator–prey interactions of juvenile North Pacific albacore in the California Current System

Abstract

Predator-prey interactions are critical components of multispecies models, and most attempts to quantify these interactions in pelagic marine environments rely on observational diet studies. In conjunction with 3 historical studies, a new diet study quantifies decadal patterns of predator-prey interactions for juvenile North Pacific albacore Thunnus alalunga in the California Current System (CCS). Juvenile albacore in the CCS eat young-of-year prey and derive 68 to 89% of nutrition from fishes, 9 to 30% from cephalopods, and the remainder from crustaceans. Despite resur- gence in abundance of Pacific sardine Sardinops sagax in the CCS, only Northern anchovy Engraulis mordax, Pacific saury Cololabis saira, and cephalopods have consistently been important to albacore diet. These results support theoretical predictions of optimal foraging models that albacore will pre- fer hunting in cold, near-shore waters containing anchovy and saury while minimizing foraging in warmer, offshore waters of sardine habitat. A bioenergetics model is used to calculate daily and annual consumption rates. The importance of calculating an ensemble prey energy density (EDn) value for steady-state consumption models is demonstrated, and the relationship between EDn and consumption rates is generalized to other marine predators. Low variability in EDn consumed by juve- nile albacore translates into equally stable consumption to biomass (Q:B) ratios over decades.