Native and Introduced Larval Fishes of Suisun Marsh, California: The Effects of Freshwater Flow

Abstract

We sampled ichthyoplankton weekly in Suisun Marsh in the San Francisco Estuary from February to June each year from 1994 to 1999. We collected approximately 227,900 fish, predominantly shimofuri goby Tridentiger bifasciatus (60%) and prickly sculpin Cottus asper (33%). Principal components analysis and canonical correspondence analysis were used to explore relationships among several environmental variables and the 13 species that made up 99.96% of the catch. A group of native fishes (prickly sculpin, Sacramento sucker Catostomus occidentalis, threespine stickleback Gasterosteus aculeatus, longfin smelt Spirinchus thaleichthys, and Sacramento splittail (also known simply as splittail) Pogonichthys macrolepidotus) were associated with the cool temperatures and higher outflows characterizing early-season conditions in Suisun Marsh. In contrast, a group of introduced species (shimofuri goby, inland silverside Menidia beryllina, striped bass Morone saxatilis, and threadfin shad Dorosoma petenense) were associated with the warm temperatures and lower outflows that characterize late-season marsh conditions. Catch composition was similar among marsh waterways (called sloughs), except for Cordelia Slough in the western marsh, which had lower total catch, greater diversity, and most of the threespine stickleback, northern anchovy Engraulis mordax, and longfin smelt. Longfin smelt were captured mostly in February and March, whereas delta smelt Hypomesus transpacificus catches were later and more evenly distributed throughout the marsh. Delta smelt catches overlapped temporally and spatially with catches of the introduced wakasagi H. nipponensis. Sacramento splittail catches were confined mostly to 1995, a year when high flows peaked during their spawning season in March and April. Our results suggest that temperature and interannual variations in freshwater flow are important for determining habitat quality for native and introduced larval fishes. We conclude that mimicking natural flow regimes in this highly regulated system is important for early life stages of native fishes.