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 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.
Highlights
The introduction of non-native species has commonly been identified as one of the major causes of global biodiversity loss (Vitousek et al 1996; Sala et al 2000)
Based on the minimal catch per unit effort (CPUE) in the spring months (Figure 2), and the appearance of a new cohort in the June fork length density histogram, as well as the scarcity of adults in the following July fork length density histogram (Appendix A, Figure A-2), it appears that most Mississippi Silverside adults in the Delta do not survive to a second year
Mississippi Silversides rarely exceed 110 mm fork length (FL) in the estuary; only 21 fish larger than 110 mm FL have been captured by the Delta Juvenile Fish Monitoring Program (DJFMP) since 1995
Summary
The introduction of non-native species has commonly been identified as one of the major causes of global biodiversity loss (Vitousek et al 1996; Sala et al 2000) This is especially apparent in the highly invaded San Francisco Estuary (estuary), where large numbers of alien species have successfully colonized (Cohen and Carlton 1998; Winder et al 2011) while populations of native species have either declined or become extirpated (Bennett and Moyle 1996; Moyle 2002). The Delta is a complex network of interconnected channels, which forms the inland portion of the estuary (Whipple et al 2012) This apparent increase of Mississippi Silverside abundance was concurrent with the severe declines of several pelagic fish species (Sommer et al 2007) in the estuary. Multiple interacting stressors are implicated in this species' precipitous decline (Baxter et al 2010; MacNally et al 2010), and predation by non-native species has been identified as a potential contributing factor (Sommer et al 2007)
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