Examining Retention-at-Length of Pelagic Fishes Caught in the Fall Midwater Trawl Survey

The Fall Midwater Trawl Survey has provided data on aquatic organisms in the San Francisco Estuary for over five decades. In 2014–2015, a study was conducted to investigate and quantify the efficiency of this trawl for catching the endangered fish species Delta Smelt (Hypomesus transpacificus). In an analysis based on that study, we calculated retention probability—the probability that a Delta Smelt is retained in the cod end of the trawl—as a function of fish length and fit a selectivity curve reflecting the relationship between size and retention. Here we return to the same gear efficiency study and further utilize the data set by (1) fitting selectivity curves for three additional pelagic fish species: Threadfin Shad (Dorosoma petenense), American Shad (Alosa sapidissima), and Mississippi Silverside (Menidia beryllina), (2) refitting the selectivity curve for Delta Smelt to incorporate between-haul variability, and (3) calculating the lengths of 50% and 95% retention in order to characterize and compare the resulting selectivity curves. We also present retention data on age-0 Striped Bass (Morone saxatilis), all of which were retained in the cod end. We found that Threadfin Shad, American Shad, and Delta Smelt are 95% retained at 45, 49, and 61 mm fork length, respectively. Because data were limited for Mississippi Silverside, American Shad, and age-0 Striped Bass, we used body shape, in conjunction with retention data, to develop hypotheses about selectivity based on whether each species’ body shape resembles that of Threadfin Shad, which are more deep-bodied and laterally compressed, or Delta Smelt, which are more fusiform. We also found that retention-at-length was more variable for Delta Smelt than for Threadfin Shad, potentially because length is a good predictor of retention in deep-bodied, laterally compressed fish whereas maximum girth is a better predictor of retention in fusiform fish.

Trophic ecology of two non-native hydrozoan medusae in the upper San Francisco Estuary

Blooms of some gelatinous zooplankton are increasing worldwide, often disrupting foodwebs. Invasions of non-native jellyfish are a growing problem in many estuaries, including the San Francisco Estuary, where at least two species of Ponto-Caspian hydrozoans, Maeotias marginata Modeer, 1791 and Moerisia sp., are abundant. The present study investigated their trophic ecology, testing the following hypotheses: (1) diets over the bloom and at the diel scale are comprised of a variety of prey items; (2) hydrozoans are generalist feeders; (3) hydrozoans feed on the larvae of declining fish species; and (4) the potential for prey competition exists between the hydrozoans and two declining planktivorous fishes, striped bass (Morone saxatilis) and threadfin shad (Dorosoma petenense). Both hydrozoans ate a variety of crustaceans, most notably calanoid copepods, which were found in greater proportion in the guts than in the environment. The only fish larvae consumed were gobies. Density of Moerisia sp., was negatively correlated with gut fullness for both fishes, and diet overlap was high between shad and hydrozoans, but low for bass. Because of strong spatial and temporal overlap between hydrozoans and shad, competition for zooplankton may be occurring. These hydrozoans have invaded other systems, and should be monitored to assess potential ecological interactions in these locations.