Shallow-Water Piscivore-Prey Dynamics in California's Sacramento-San Joaquin Delta

Abstract

Predation is one mechanism that could lead to low native fish abundance in macrophyte dominated shallow-water habitats in the Sacramento-San Joaquin Delta. We used beach seine and gill net sampling to identify and compare the distribution and feeding ecology of three piscivores (striped bass, Morone saxatilis, largemouth bass, Micropterus salmoides, and Sacramento pikeminnow, Ptycho- cheilus grandis) at five nearshore sites in the Sacramento-San Joaquin Delta. Sampling was con- ducted March-October 2001 and 2003. We addressed the following questions. What are the spatial and temporal distributions of age-1 and older striped bass, largemouth bass, and Sacramento pikeminnow? What prey are eaten by these predators? What is the relative importance of predator size versus seasonal prey availability on incidence of piscivory for these predators? What is the likely per capita impact of each piscivore on prey fishes, particularly native fishes? All 76 of our individual station visits yielded at least one of the three species, suggesting that pis- civorous fishes frequently occur in Delta shallow- water habitats. All three piscivores had diverse diets comprised of numerous invertebrate and fish taxa. There were noticeable seasonal shifts in prey fish for each of the three piscivores. In general, most native fish were consumed during spring (March-May) and the highest prey species richness occurred during summer (June-August). Largemouth bass likely have the highest per capita impact on nearshore fishes, including native fishes. Largemouth bass preyed on a greater diversity of native fishes than the other two piscivores and consumed native fishes farther into the season (July versus May). Based on binomi- al generalized additive models, incidence of pis- civory was predominantly a function of size for largemouth bass and Sacramento pikeminnow. Largemouth bass became predominantly piscivorous at smaller sizes than Sacramento pikeminnow; about 115 mm versus about 190 mm respectively. In con- trast, incidence of piscivory was predominantly a function of season for striped bass. Striped bass were typically most piscivorous during summer and fall regardless of size. We conclude that shallow- water piscivores are widespread in the Delta and generally respond in a density-dependent manner to seasonal changes in prey availability.