Tidal restoration of a managed wetland in California favors non‐native fishes

Abstract

Tidal wetland restoration is commonly used to recover ecosystem functions and services that were lost when wetlands were diked for reclamation or management. Less research has been conducted on the response of invertebrate and fish assemblages to tidal restoration than on plants and physical attributes. Blacklock Marsh, a wetland in Suisun Marsh, USA, was once managed for waterfowl hunting and cattle grazing until its dike was breached, restoring full tidal action. We sampled water quality, zooplankton, macroinvertebrates, and fishes in Blacklock Marsh, and compared these metrics to adjacent naturalistic waterways and to a managed wetland focused on waterfowl. Our goal was to compare food production and community assemblage on a restoring marsh to other types of nearby waterways. Blacklock Marsh had less chlorophyll-a and dissolved organic carbon than nearby dead-end sloughs and the managed wetland, less zooplankton biomass than the managed wetland, and lower fish diversity-with a fish assemblage dominated by non-native species-compared to all other waterways. The most abundant fish species in the restoring site, Mississippi silverside (Menidia audens), is a non-native fish and known predator of Delta smelt larvae (Hypomesus transpacificus), for which the restoration was targeted. Our research suggests that restoring tidal action to managed wetlands alone may worsen rather than improve conditions for at-risk and native fishes.

Ecology and Ecosystem Impacts of Submerged and Floating Aquatic Vegetation in the Sacramento–San Joaquin Delta

“…spread rapidly and covered hundreds of acres in the restoration site (Ustin et al 2015). SAV has already colonized tidal marsh restoration sites throughout the Delta in varying severity (Barker Slough, Little Holland Tract, Liberty Island Conservation Bank, Decker Island, Blacklock Marsh;Farrugia et al 2019;Young et al 2021b;Williamshen et al 2021). Despite SAV's success in many shallow-water habitats, planned restoration of areas already heavily colonized by non-native SAV (e.g., Franks Tract) may prove effective in reducing overall cover of SAV and FAV by creating physical habitat conditions that reduce niche space for SAV and FAV (e.g., deeper channels broken up by higher-elevation marsh areas) in combination with post-construction control efforts (Conrad et al, this issue).…”