Is the Response of Estuarine Nekton to Freshwater Flow in the San Francisco Estuary Explained by Variation in Habitat Volume?

Abundance of estuarine biota can vary with freshwater inflow through several mechanisms. One proposed mechanism is that the extent of physical habitat for an estuarine species increases with flow. We estimated the contribution of variation in habitat volume to the responses of eight species of estuarine nekton to changes in freshwater flow in the San Francisco Estuary. Resource selection functions for salinity and depth were developed for each species (and for five additional species) using five monitoring data sets. The TRIM3D hydrodynamic model was run for five steady flow scenarios to determine volume by salinity and depth, and resource selection functions were used as a weighting factor to calculate an index of total habitat for each species at each flow. The slopes of these habitat indices vs. flow were consistent with slopes of abundance vs. flow for only two of the species examined. Therefore, other mechanisms must underlie responses of abundance to flow for most species.