Non-Native Fish Predator Density and Molecular-Based Diet Estimates Suggest Differing Impacts of Predator Species on Juvenile Salmon in the San Joaquin River, California
The Sacramento-San Joaquin Delta is a major survival bottleneck for imperiled California salmonid populations, which is partially due to a multitude of non-native fish predators that have proliferated there throughout the 20th century. Understanding the diets of salmonid predators is critical to understanding their individual impacts, role in the food web, and the implications for potential management actions. We collected the stomach contents of Striped Bass Morone saxatilis, Largemouth Bass Micropterus salmoides, Channel Catfish Ictalurus punctatus and White Catfish Ameiurus catus sampled from three 1-km reaches in the lower San Joaquin River in 2014 and 2015 during the peak juvenile salmon outmigration period. We tested each stomach (n = 582) for the presence of juvenile Chinook Salmon Oncorhynchus tshawytscha and other prey items using a genetic barcoding technique. Channel Catfish had significantly higher frequency of Chinook Salmon in their stomachs (27.8% of tested Channel Catfish contained Chinook Salmon DNA), compared to the other three predators (2.8% to 4.8%). However, non-native fish species occurred at greater frequencies in the diets of all four predator species than salmon. Using depletion estimation from electrofishing, we were able to generate population densities for Striped Bass and Largemouth Bass in our reaches. Largemouth Bass were evenly distributed throughout all three reaches, at a mean density of approximately 333 (± 195 SE) per km of river. Striped Bass were patchily distributed, ranging from 21 to 1,227 per km. Extrapolating the frequency of salmon detected in stomachs to the predator abundance estimates, we estimate that the population of Largemouth Bass we sampled consumed between 3 and 5 Chinook Salmon per day per 1-km study reach (consumption rate of 0.011 salmon per predator per day), whereas the Striped Bass population consumed between 0 and 24 Chinook Salmon per day (0.019 salmon per predator per day).