Cooperative Fish and Wildlife Research Units Program: Vermont
Education, Research and Technical Assistance for Managing Our Natural Resources

The Sacramento-San Joaquin Delta (the Delta) is one of the most invaded estuaries in the world, and non-native predators may be a major factor in the observed decline of Central Valley Chinook salmon over recent decades. In order for managers to take actions that might reduce predation related mortality for these ecologically, culturally, and economically valuable salmon, it’s important to understand the factors influencing the distribution and abundance of piscivores in the Delta. In this study, we use a multibeam imaging sonar (i.e., DIDSON) to conduct mobile surveys and quantify the abundances of piscivores in the Delta. We then used these data to identify the habitat features that are correlated with the abundance of piscivores. Prior to conducting the surveys, we used DIDSON data from captured fish to develop an algorithm to distinguish piscivores from non-piscivores with high confidence (98% accuracy). Results from the surveys indicated that predator abundances were most associated with areas of increased submerged aquatic vegetation (SAV) patches, sites that are straighter, and sites with increased bathymetric complexity. When applied to the entire survey area, this model was successfully able to predict known areas of high predator densities. These results indicate that one approach to reduce predator densities along these migration corridors, and improve juvenile salmon outmigration survival, is to reduce the extent of invasive SAV. Because experimental predator removals have been largely ineffective in the Delta, efforts to manipulate habitat to discourage non-native predator recruitment and favor native species recruitment may be more effective.