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.

A suite of submersed aquatic vegetation (SAV) species, and especially Brazilian Waterweed (Egeria densa), has proliferated rapidly in California’s Sacramento–San Joaquin Delta. This expansion is concurrent with population declines in native fish species and increases in many non-native fish species, including Largemouth Bass Micropterus salmoides. In this study, we investigated the effect of SAV species composition and E. densa specifically on macroinvertebrate communities and juvenile Largemouth Bass diets. Invertebrate communities differed across sites in the Delta, driven primarily by changes in abundance of the amphipod Hyalella sp., oligochaetes, ostracods, and insect larvae of the family Chironomidae. Juvenile Largemouth Bass consistently consumed SAV-associated invertebrates, and preferentially consumed larger taxa, when available. Gut fullness of juvenile Largemouth Bass was lowest in sites dominated by E. densa, although there was no clear mechanism for this difference. However, SAV species composition had little effect on abundance of Hyalella sp., chironomid larvae, or damselfly naiads, prey items commonly consumed by juvenile Largemouth Bass. Our results suggest that E. densa does not provide a qualitative increase in macroinvertebrate food for fishes compared to other SAV species.

This paper reviews and highlights recent research findings on food web processes since an earlier review by Kimmerer et al. (2008). We conduct this review within a conceptual framework of the Delta–Suisun food web, which includes both temporal and spatial components. The temporal component of our framework is based on knowledge that the landscape has changed markedly from historical conditions. The spatial component of our framework acknowledges that the food web is not spatially static; it varies regionally and across habitat types within regions. The review highlights the idea of a changing baseline with respect to food web function. New research also indicates that interactions between habitat-specific food webs vary across the current landscape. For example, based on early work in the south Delta, the food web associated with submerged aquatic vegetation was thought to provide little support to species of concern; however, data from other regions of the estuary suggest that this conceptual model may not apply across the entire region. Habitat restoration has been proposed as a method of re-establishing historic food web processes to support species of concern. Benefits are likely for species that directly access such restored habitats, but are less clear for pelagic species. Several topics require attention to further improve the knowledge of food webs needed to support effective management, including: (1) synthesis of factors responsible for low pelagic biomass; (2) monitoring and research on effects of harmful algal blooms; (3) broadening the scope of long-term monitoring; (4) determining benefits of tidal wetland restoration to species of concern, including evaluations of interactions of habitat-specific food webs; and (5) interdisciplinary analysis and synthesis. The only certainty is that food webs will continue to change in response to the changes in the physical environment and new species invasions.

Largemouth Bass (Micropterus salmoides) were introduced into the Sacramento-San Joaquin Delta (the Delta) over 100 years ago. In the last 2 decades, the abundance of centrarchids (including Largemouth Bass) in the littoral zone has increased, while some native fish and fish that were previously abundant in the pelagic zone have declined. Largemouth Bass are now one of the most abundant piscivores in the Delta. Understanding the ecology of this top predator — including a comprehensive understanding of what prey are important in Largemouth Bass diets — is important to understanding how this species may affect the Delta fish community. To address this need, we conducted electrofishing surveys of Largemouth Bass at 33 sites every 2 months from 2008 to 2010, measuring fish fork lengths and collecting stomachs contents at each site. We characterized diets using Percent Index of Relative Importance for 3,004 Largemouth Bass, with samples that spanned all seasons. Amphipods dominated the diets of Largemouth Bass ≤175 mm FL year-round, with dipterans, odonates, and copepods and cladocerans representing other important diet items. Non-native red swamp crayfish (Procambarus clarkii) were the most important prey for Largemouth Bass >175 mm FL. Non-native centrarchids (including Largemouth Bass) and amphipods were important prey items as well. Prickly Sculpin (Cottus asper) were the most frequently consumed native fish. Other native fish and pelagic fish species rarely occurred in Largemouth Bass diets, and we discuss trends in how the frequency of co-occurrence of these fishes with Largemouth Bass in the electrofishing surveys was associated with their frequency in Largemouth Bass diets. The Largemouth Bass in the Delta appear to be sustained largely on a diet of other non-natives that reside in the littoral zone.