Nearshore and offshore feeding of juvenile salmon and forage fish in the Salish Sea

The Salish Sea provides essential early marine growth environment for several pelagic fish species, including Pacific salmon (Oncorhynchus spp), Pacific herring (Clupea pallasi), and other forage fish (e.g., surf smelt, Hypomesus pretiosus; sand lance, Ammodytes hexapterus). Early marine feeding and growth can be important to survival to adulthood for these species. Early marine growth of Endangered Species Act-listed Puget Sound Chinook salmon (O. tshawytscha) is tightly correlated with smolt to adult returns. Variability in early growth is linked to variation in feeding rate. Therefore, factors which affect feeding success, such as competition, during this life stage could have important influences on marine survival. Recent work has shown spatial-temporal and dietary overlap among juvenile salmon and herring and suggested that potential for competitive interactions exists. However, the extent and strength of potential interactions remain poorly understood and little is known about the feeding of other pelagic planktivores. We describe feeding habits of juvenile salmon, herring, and surf smelt in the Salish Sea to quantify variation in diet composition and characterize dietary overlap. Monthly, yearly, and regional variation was evident, but consistent patterns were distinguishable. During the critical summer growing season, Pacific herring dominated the biomass of the shallow pelagic planktivorous fish community and exhibited extensive diet and spatial overlap with juvenile Chinook salmon and coho salmon (O. kisutch). Crab larvae and amphipods were important prey items. Diet compositions of chum salmon (O. keta) and surf smelt differed from those of the other species, including greater amounts of gelatinous organisms (ctenophores, larvaceans). Our results suggest that assessments of basin carrying capacity and potential species interactions need to account for the population and feeding dynamics of all major daylight planktivores, especially herring