Tidal Wetlands Associated with Foraging Success of Delta Smelt

Abstract

Delta smelt (Hypomesus transpacificus), an annual fish endemic to the San Francisco Estuary (SFE), is imperiled. One recovery strategy is to restore tidal wetlands, thereby increasing productivity and prey abundance. However, the link between tidal wetlands and foraging of delta smelt is not yet established. Using GIS, we quantified the area of tidal wetlands (km2) within a 2-km radius around sampling stations from which 1380 delta smelt were collected over 4 years (2011–2015). We quantified stomach fullness, a metric of foraging success, for each fish and regressed it against tidal wetland area, turbidity, water temperature, and other factors known to influence foraging success of delta smelt. Stomach fullness increased with both increasing tidal wetland area and increasing water temperature and was reduced at turbidities > 80 NTU. Model estimates show that stomach fullness increased twofold from the minimum (0 km2) to the maximum (4.89 km2) tidal wetland area. Of this increase, 60% was due to increased predation on larval fish, while 40% was due to increased predation on zooplankton. Delta smelt collected from areas with the highest tidal wetland area were six times more likely to have a larval fish in their guts than those collected from areas with the lowest. Thus, tidal wetland appears to confer substantial benefits to the foraging success of delta smelt, mainly via increased predation on larval fish.