CVPIA Predation Contact Point Study - 2022: The impact of submerged aquatic vegetation removal on fish predation in a tidal river channel

Proliferation of non-native submerged aquatic vegetation (SAV) has the potential to cause widespread ecosystem changes, and has been attributed to declines in native fish populations around the world. One pathway for these declines is non-native SAV may render ecosystems more hospitable to fish predator species by creating habitat structure, by altering lower trophic food webs, or by affecting predator-prey interactions. It is presumed that non-native vegetation removal will generally favor native fish, however, fish community responses to SAV removals are not well understood. Using a field-based Before-After-Control-Impact study design, we measured the impact of manual SAV removals on short-term changes in predator abundance, predation risk on juvenile Chinook salmon ( Oncorhynchus tshawytscha ; a native fish of management concern), and the aerobic scope of predator and prey in California’s Sacramento-San Joaquin Delta. We found that, while SAV removals decreased abundances of the most common SAV-associated predator, largemouth bass ( Micropterus salmoides ), they resulted in higher predation risk of tethered prey, likely due to the removal of refuge habitat and the immigration of an open-water predator, striped bass ( Morone saxatilis ). SAV removals also buffered against a seasonal decline in environmental oxygen supply, increasing the aerobic scope of juvenile Chinook salmon and largemouth bass; whether such gains for prey would outweigh the persistent aerobic advantage of predators is an open question. While limited in spatial and temporal scope, this study has put into question any short-term benefits of small-scale SAV removal efforts for native fish populations, especially in areas where open-water predator species are abundant.