California Sea Grant College Program

Pelagic fish declines and increased abundance of gelatinous zooplankton are increasingly common in physically and ecologically disturbed estuaries. This dissertation investigates the ecology and trends of three alien hydromedusae (Maeotias marginata, Blackfordia virginica, and Moerisia sp.) and striped bass Morone saxatilis in Suisun Marsh in the upper San Francisco Estuary. Chapter one examines the ecology and long term (1981-2005) trends of M. marginata medusae using by-catch data from an otter trawl survey. Maeotias marginata increased in abundance with its greatest spatial occurrence and abundance after 1992. Salinity and temperature were significant predictors of medusae abundance with moderate salinity and high temperature resulting in their highest abundance. Water transparency was related to medusae abundance, but had a smaller effect. Medusae fed primarily on pelagic invertebrates, although benthic / epibenthic prey and larval fishes were also found in the gut contents. Chapter two reports the environmental relationships and abundance of three hydromedusae from a two year (February 2004 to January 2006) plankton study. Moerisia was the most abundant species, followed by M. marginata and B. virginica. Salinity and temperature had the largest effect on predicted occurrence and abundance although turbidity and dissolved oxygen were significant at times. Maeotias marginata was captured at the lowest salinity and temperature and was the first and last species captured each year. Moerisia was captured in samples with slightly higher salinity, but similar temperatures as M. marginata. Blackfordia virginica was captured in moderate to high salinity. Chapter three investigates the seasonal trends in striped bass young-of-year (YOY) abundance (1980 to 2005) and their environmental relationships, habitat use, and prey relationships. Striped bass YOY declined in deep water sites, but not in shallow tidal marsh sites. Environmental conditions were poorly correlated with striped bass abundance in all seasons. Prey species declined similarly in all depth groups. The declining abundance of mysids and adult copepods were correlated with the YOY decline in deep water sites only. Increased mean YOY length and seasonal recruitment shifts after the decline of mysids, suggests additional affects of changing prey abundance.