A Mosaic of Estuarine Habitat Types with Prey Resources from Multiple Environmental Strata Supports a Diversified Foraging Portfolio for Juvenile Chinook Salmon

Abstract

Estuaries provide a mosaic of vital nursery habitat types for threatened Chinook salmon (Oncorhynchus tshawytscha) by promoting an ecological portfolio effect, whereby multiple habitat types and networked environmental strata maximize foraging opportunities for out-migrating Chinook salmon by varying the abundance and composition of prey through space and time. To study this portfolio effect, the foraging capacity of five estuarine habitat types was evaluated within the Nisqually River Delta (Puget Sound, Washington, USA). Within each habitat type, invertebrate prey resources were sampled from the terrestrial, aquatic, benthic, and epifaunal environmental strata and compared with juvenile Chinook salmon diets from corresponding sampling events. The estuarine emergent salt marsh supplied twice as much aquatic prey biomass as any other habitat type (720–5523 mg/m3), followed by the mudflat (246–2543 mg/m3) and eelgrass (Zostera marina; 141–2694 mg/m3). Despite some evidence for selectivity, juvenile Chinook salmon diets exhibited substantial compositional overlap, especially when compared with among-habitat differences in available prey resources. Fish that were captured in the emergent salt marsh, mudflat, and eelgrass habitat types consumed aquatic crustaceans such as mysids, while fish captured upriver in freshwater tidal forest and transitional emergent marsh habitat types ate a higher proportion of adult and larval insects. The availability and consumption of greater quantities of energy-poor crustaceans in the salt marsh and lower quantities of energy-rich insects upriver highlights a quantity-for-quality trade-off among estuarine habitat types. Overall results indicate that the timing, productivity, and diversity of prey across multiple habitat types and environmental strata determine an estuary’s capacity to support foraging for multiple life history strategies, size classes, and cohorts of juvenile Chinook salmon.