Integrating prey dynamics, diet, and biophysical factors across an estuary seascape for four fish species

Estuary food webs support many fishes whose habitat preferences and population dynamics may be controlled by prey abundance and distribution. Yet the identity and dynamics of important estuarine prey of many species are either unknown or highly variable between regions. As anthropogenic development in estuaries increases, so does the need to understand how these environments may be supporting economically, culturally, and ecologically important fishes. Here, we examine how important estuary fishes integrate their prey across the seascape and what may influence prey dynamics. Specifically, we surveyed juvenile coho salmon Oncorhynchus kisutch, juvenile sockeye salmon O. nerka, Pacific herring Clupea pallasii, and surf smelt Hypomesus pretiosus diets along with zooplankton abundance in the estuary of the Skeena River (British Columbia, Canada) at a relatively fine scale. We found diets were highly variable, even within a species, but 1 or 2 prey composed most diet contents per species. Juvenile coho salmon primarily consumed terrestrial insects and larval fish, whereas sockeye salmon primarily consumed harpacticoid copepods. In contrast, small pelagic fish (Pacific herring and surf smelt) primarily consumed calanoid copepods, which were the most abundant prey in the environment. We found that certain prey groups were correlated with biophysical factors. For example, calanoid copepod abundance was positively correlated with salinity, whereas harpacticoid copepod abundance was highest over eelgrass sites. Identifying key prey species and how they distribute within the estuary seascape is an integral link in understanding the food-web foundation of fish habitat use in areas under pressure from anthropogenic development.