The Influence of Environmental Variation on the Columbia River Estuarine Fish Community: Implications for Predation on Juvenile Salmonids

Predation is often assumed to be the ultimate cause of mortality for juvenile salmonids in marine waters, but the specific biological or physical factors that influence predation are poorly understood.  The Columbia River estuary is a useful model ecosystem to understand the relationship between avian predators, alternative prey, environmental variation, and predation on juvenile Pacific salmonids (Oncorhynchus spp.).  Here, we explore the influence of a suite of local and ocean basin-scale environmental variables on the composition and abundance of the estuarine fish assemblage, an important determinant of avian predation on juvenile salmon in the Columbia River estuary.  Multivariate analyses indicated that variables representing both freshwater (river flow) and marine (ocean temperature, upwelling, plume volume) conditions explained up to half of the variation in the fish assemblage.  Many of the same environmental variables were related to the abundances of individual fish species.  Our results also suggest that the estuarine fish assemblage in the future will be quite different from the current one, with likely repercussions for predator-prey interactions.  Our results from estuarine habitats provide a useful model for understanding the dynamics of predation in marine habitats, which are much more logistically difficult to study.