Limitations of Active Removal to Manage Predatory Fish Populations

Abstract

AbstractPrograms to control predatory fishes have successfully increased the survival of imperiled prey fishes in some cases, but efficacy depends on the population dynamics and ecological interactions between the predators, prey, and the rest of the community. In California's Sacramento–San Joaquin Delta, extremely low survival of downstream‐migrating juvenile salmonids has been ascribed in part to predation by abundant nonnative fishes that have become naturalized in the system. This study aimed to determine the effectiveness of removing some of these predatory fish to increase the survival of juvenile salmonids in the San Joaquin River. In 2014 and 2015, we used predation event recorders (PERs) to estimate predation rates before and after localized, experimental predatory fish removals and additions. We also estimated survival of juvenile Chinook Salmon Oncorhynchus tshawytscha before and after predator manipulations in 2014 using acoustic tags. We found no statistically significant evidence for an effect of predator removals or additions on survival rates of Chinook Salmon (as measured by acoustic tags) or on predation rates (as measured by PERs), despite a one‐time reduction of approximately 40–70% of all predators from “removal” sites and relocation to “addition” sites. We review examples of successful predator control programs elsewhere, highlighting key differences between those programs and our study in the Sacramento–San Joaquin Delta.