Abundance Trends, Distribution, and Habitat Associations of the Invasive Mississippi Silverside (Menidia audens) in the Sacramento–San Joaquin Delta, California USA

Abstract

Although many alien fish species have colonized the Sacramento-San Joaquin Delta (Delta), few are as pervasive and abundant as Mississippi Silversides (Menidia audens). Moreover, Mississippi Silversides are hypothesized to be an intra-guild predator of the endangered Delta Smelt (Hypomesus transpacificus). Because of their prevalence in the Delta and potential predation on Delta Smelt, Mississippi Silversides may have far-reaching effects on both the aquatic ecosystem and conservation management policies of the region. Yet little is known about how Mississippi Silverside abundance and distribution have changed within the Delta, or how they respond to various habitat attributes such as temperature, turbidity, and flow. We examined 19 years of beach seine survey data to evaluate how the abundance and distribution of Mississippi Silversides has changed over the years, characterize their habitat associations, and determine the environmental factors that predict their annual cohort strength. Concurrent with the decline of several pelagic fish species in the San Francisco Estuary in the early 2000s, we observed a significant increase in Mississippi Silverside catch that was accompanied by a moderate distributional shift in which densities increased in the western Delta region. We also found that the occurrence of this highly prolific alien species was associated with higher water temperature, higher turbidity, relatively low conductivity, and moderate to high levels of dissolved oxygen. Lastly, we demonstrated that freshwater input to the Sacramento-San Joaquin Delta during the summer and water exports during the spring months were both negatively correlated with the annual cohort size of Mississippi Silversides in the region. Our study identified the environmental variables deserving additional attention in future studies involving Mississippi Silverside and suggests that the species favors habitat conditions that are likely to be detrimental for pelagic species such as Delta Smelt. KEY WORDSMississippi Silverside, invasive species biology, Menidia audens, littoral fish, life history RESEARCH

Evidence of a Shift in the Littoral Fish Community of the Sacramento-San Joaquin Delta

“…This association indicates that the recent proliferation of invasive macrophytes in the Delta [22,23] directly facilitated the increase of Centrarchid species. By contrast, increase in Mississippi Silversides may be driven more by the high frequency of drought years since the late 2000s based on their close association with low flow periods [25]. …”

“…We also found that interannual variation in the littoral fish community composition largely followed a pattern of simple progression through time over the study period, though year-to-year variability in freshwater inflow seem to play a substantial role as well. Notably, biomass per volume has also doubled over the study period, largely driven by a handful of species from the Mississippi drainage that are now widespread in the Delta [25,33]. With sea level rise and land subsidence continuing to be an issue in many Delta islands, the Delta is likely to change from the current system of tidal freshwater channels to a more diverse area consisting of flooded islands and shallow open water [37].…”

“…Since then, the survey has been expanded several times but has remained focused on the Delta region and nearby locations. Over the years, it has collected substantial information on a number of littoral fish species [25,35,40,41]. Although the DJFMP was initiated in the 1970s, consistent sampling in spring and summer months within the Delta region did not begin until 1995.…”

“…Over a similar timeframe as the POD, there was a notable shift in zooplankton species composition throughout the upper San Francisco Estuary with little change to the overall zooplankton biomass [21]. These changes were accompanied by further expansion of introduced submerged aquatic macrophytes [22,23] and fish species [24,25] in the Sacramento-San Joaquin Delta (Delta), which is the upstream, tidal freshwater portion of the San Francisco Estuary (Fig 1). …”

“…Previous studies describing the POD shift in the upper San Francisco Estuary have generally focused on the declining pelagic fish species [16,18,20,29,30,31] that occupy open water habitat and exhibit relatively unique life histories [32]. The few analyses on Delta littoral fish species indicated that major shifts have also occurred in the littoral habitat [24,25,33]. However, such studies are scarce and lack a continuous time series dataset [24] or are more focused on the spatial variation in fish species composition [34,35].…”