Diets of Native and Non-Native Piscivores in the Stanislaus River, California, Under Contrasting Hydrologic Conditions

The fish communities of the Sacramento–San Joaquin Delta and its tributaries in California’s Central Valley have been irreparably altered through introductions of numerous fish species, including Striped Bass (Morone saxatilis), black bass (Micropterus spp.), and catfishes (Ameiurus spp. and Ictalurus spp.). Research into how predation by non-native piscivores affects native anadromous species has focused on the Sacramento and San Joaquin river mainstems and Delta habitats, through which all anadromous species must pass. Yet, the ranges of non-native fishes extend into upstream tributaries. We collected diets from native and non-native piscivores in the Stanislaus River, a tributary to the San Joaquin River and a remaining stronghold for native fishes. Piscivorous fishes primarily consumed invertebrates and the native species fall-run Chinook Salmon (Oncorhynchus tshawytscha) and Pacific Lamprey (Entosphenus tridentatus). Juvenile Chinook Salmon and Pacific Lamprey were consumed at higher frequencies than any other potential fish prey species, particularly by Striped Bass and black bass. The frequency of native fishes in predator diets was similar across years, despite contrasting hydrologic conditions; 2019 (wet year), 2020 (dry year), and 2021 (critically dry year). Our results show that Pacific Lamprey were frequently consumed by native and non-native piscivores, and that juvenile Chinook Salmon experience substantial predation early in their migration, regardless of hydrologic conditions.