San Francisco Estuary and Watershed Science

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.