Geographic Variation in Feeding Success, Prey Availability, and Prey Selectivity of Age-0 Longfin Smelt in the San Francisco Estuary

Food availability strongly influences the quality of nursery habitats and subsequent population dynamics of fishes. Classical fish ecology suggests that larval growth, survival and recruitment are correlated and are largely dependent on feeding success and prey availability. This is likely true for San Francisco's threatened population of Longfin Smelt (Spirinchus thaleichthys), which has collapsed over the last 50 years, along with its prey, throughout historic nursery habitats. Recent studies by the UC Davis Otolith Geochemistry and Fish Ecology Laboratory have shown that larval and juvenile Longfin Smelt utilize shallow open bays, tidal sloughs and marshes as rearing habitats throughout the entire San Francisco Estuary (SFE) from winter through spring. We examined feeding success of larval and juvenile Longfin Smelt in previously unstudied wetland nursery habitats of the SFE, including northern and southern marsh and bay habitats. Correlations among fish size, feeding success, and diet versus ambient prey composition were investigated and compared among regions and habitats. Feeding success was strongly correlated with fish length, indicating a correlation between size and fitness. All larval Longfin Smelt fed selectively on the abundant copepod Eurytemora affinis, with juveniles > 25 mm supplementing their diets with larger mysid shrimp. Prey abundance was an order of magnitude higher in sloughs of the southern SFE (Alviso Marsh) than in all other regions and nursery habitats.These increased concentrations of food correlated with higher feeding success for larval Longfin Smelt. These results, in conjunction with classic fisheries theory, suggest that there is variability in the effects of different SFE nursery habitats on survival, growth, and recruitment of Longfin Smelt in the region. Understanding the factors that cause this variability in the effectiveness of nursery habitat is integral to making informed management decisions for this imperiled species.