Prey Availability, Consumption, and Quality Influence Growth of Subyearling Chinook Salmon in Riverine and Reservoir Habitats

We examined prey availability, prey consumed, and diet energy content as sources of variation in growth of natural fall Chinook Salmon subyearlings rearing in riverine and reservoir habitats in the Snake River.  Subyearlings in riverine habitat primarily consumed aquatic insects (e.g., Diptera, Trichoptera, Ephemeroptera) of which a high proportion was represented by adult, terrestrial forms.  In the reservoir, subyearlings also consumed aquatic insects but also preyed heavily at times on nonnative, lentic amphipods (Corophium spp.) and mysids (Neomysis mercedis), which were absent in riverine habitats.  The availability of prey was typically much higher in the reservoir due to N. mercedis often comprising over 90% of the biomass, but when this taxon was removed from consideration, biomass estimates were more often higher in riverine habitat.  Subyearling diets were generally 17-40% higher in energy in riverine habitat than in the reservoir.  Observed growth in both length and weight were significantly higher in riverine habitat than in the reservoir during 2009-2011.  Little is known about how temporal and spatial changes in the food web in large river landscapes influence populations of native anadromous fishes, but our results suggests there may be negative growth consequences.