The Impact of Submerged Aquatic Vegetation Removal on Fish Predation in a Tidal River Channel

Abstract

Proliferation of non-native submerged aquatic vegetation (SAV) can trigger widespread ecosystem changes, and has caused declines in native fish populations around the world. One pathway for these declines is non-native SAV may attract fish predator species by creating habitat structure, altering lower trophic food webs, or affecting predator–prey interactions. It is presumed that non-native vegetation removal will favor native fish, however, fish community responses to SAV removals are not well understood. Using a Before-After-Control-Impact study design in California’s Sacramento-San Joaquin Delta, we found that manual SAV removals decreased abundance of the most common SAV-associated piscivore, largemouth bass ( Micropterus salmoides ) by 30.2%, yet predation rate on tethered juvenile Chinook salmon ( Oncorhynchus tshawytscha ; a native fish of management concern) increased by 274.3%. This was likely due to removal of refuge habitat and immigration of an open-water predator, striped bass ( Morone saxatilis ) into cleared areas, corroborated by video of predation events. We also evaluated predator–prey dynamics using aerobic scope, quantified by the metabolic index (ɸ), or temperature-dependent ratio of environmental oxygen supply and organismal oxygen demand. SAV removals buffered against a 4–6% seasonal decline in ɸ for juvenile salmon (i.e., ɸ declined 78.5–115% less without SAV); whether such gains for prey outweigh the persistent aerobic advantage of predators is unclear. Multispectral imagery indicated that SAV recolonized the removal area within 1 year. While limited in scale, this study puts into question some short-term benefits of small-scale SAV removal efforts for native fish populations, especially where open-water predators are abundant.

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