Estimating the Density of Prey Available to Juvenile Salmonids in a Large River: Applications of an Aquatic Ecosystem Model

Degradation of juvenile salmon freshwater habitat has been implicated as a contributing factor to the California salmon fishery collapse in 2008 and 2009.  Previous studies have shown correlations between food abundance in freshwater habitats and salmonid growth and survival, suggesting that food resources could influence juvenile salmon cohort size.  In rivers, the primary food source for juvenile salmon are drifting invertebrates, whose density can vary based on environmental conditions such as water temperature and flow.  In this study, we used an aquatic ecosystem model (AQUATOX) to estimate the density of drifting invertebrates available to juvenile salmon under different environmental conditions.  AQUATOX uses a series of coupled differential equations to simulate the transfer of nutrients, biomass, and energy between different compartments (e.g., detritus, plants, animals) of the river ecosystem.   We linked AQUATOX with a hydrologic model that predicts water temperature and flow based on environmental conditions.  After calibrating the aquatic ecosystem model we were able to recreate the observed seasonal trends in nutrient dynamics and provide reasonable estimates for the drift invertebrate biomass.  The results from this study will be incorporated into a bioenergetics based life-cycle model that explores how salmon respond to freshwater dynamics throughout their life history.