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

<p class="p1">doi: <a href="http://dx.doi.org/10.15447/sfews.2016v14iss1art2" target="_blank">http://dx.doi.org/10.15447/sfews.2016v14iss1art2</a></p><p class="p1">Although many alien fish species have colonized the Sacramento–San Joaquin Delta (Delta), few are as pervasive and abundant as Mississippi Silversides (<em>Menidia audens</em>). Moreover, Mississippi Silversides are hypothesized to be an intra-guild predator of the endangered Delta Smelt (<em>Hypomesus transpacificus</em>). Because of their prevalence in the Delta and poten-tial 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 sev-eral 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 densi-ties increased in the western Delta region. We also found that the occurrence of this highly prolific alien species was associated with higher water tempera-ture, 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 deserv-ing additional attention in future studies involving Mississippi Silverside and suggests that the species favors habitat conditions that are likely to be detri-mental for pelagic species such as Delta Smelt.