Feeding Ecology of Delta Smelt During a Seasonal Pulse of Turbidity

The delta smelt (Hypomesus transpacificus) is a small, pelagic fish endemic to the San Francisco Estuary (SFE) and protected under federal and state endangered species acts. This study examines the diet of adult delta smelt during their spawning migration in the winters of 2010 and 2012. Delta smelt and their zooplankton prey were sampled concurrently during a seasonal pulse of turbidity at sites along their migratory route from the low salinity zone in Suisun Bay to the fresher waters of the Sacramento/San Joaquin Delta. Gut contents were identified to the lowest possible taxon and counted, along with zooplankton from associated prey samples to compare prey availability to prey consumed at the time the fish was caught. Preliminary results have revealed amphipods to make up a large portion of the gut contents, which is unexpected for a supposed pelagic-feeding fish. Data are still being collected, and it remains unclear whether this is a temporary foraging habitat shift associated with turbidity and migration, or a long-term shift associated with major changes in the SFE food web. If the latter, it implies a need for a change in monitoring and analysis strategies to improve understanding and protection of this fis