Functional and Taxonomic Responses of Fish Assemblages to Low-Head Dam Removals

1. Low-head dams are widespread in river ecosystems around the globe, but due to their effects, removals have become a major mechanism for river restoration. Investigations of fish responses to dam removals have been mostly conducted in the short term and have shown mixed assemblage responses (e.g., increased or decreased species richness following removal). Here, we examined the effects of low-head dam removals on taxonomic composition and functional diversity at a relatively long term (6 years post dam removal). We hypothesised that differential responses of fish assemblages to dam removal are driven by suites of guild and morphological traits and that removal impacts functional complexity, with free-flowing sites having greater distributions of functional diversity than impounded sites.
2. We conducted our study in the West Fork River, West Virginia, where three low-head dams were removed in 2016. Boat electrofishing surveys were conducted 1 year before the removal (2015) focusing on species of recreational fisheries importance. Then, we conducted surveys in the same sites 6 years after the removal (between September and October 2022). These electrofishing surveys were conducted at sites that were previously impounded but are now free-flowing and those that currently remain impounded by low-head dams.
3. Results from our univariate and multivariate analyses indicated that current impounded sites and free-flowing sites have statistically distinct taxonomic and functional fish assemblages. Free-flowing sites were associated with overall higher taxonomic richness and greater biomass of some recreationally important species, including muskellunge, smallmouth bass and channel catfish. Conversely, currently impounded sites were associated with lower taxonomic richness and greater biomass of some recreationally important fishes, including green sunfish, bluegill and largemouth bass. Furthermore, functional diversity indices showed high functional richness (FRic) at free-flowing sites, while less evident changes were observed for functional evenness (FEve) and functional divergence (FDiv).
4. Our findings suggest that long-term changes in the fish assemblages following the dam removals led to greater taxonomic and functional diversity while maintaining fisheries of high recreational importance. While some previous short-term assessments suggested decreased functional complexity shortly after removals, our results suggest increases in functional complexity in the long term. Though dam removals may have high upfront socioeconomic costs, the long-term effects in this study appeared to lead to greater ecosystem function and enhanced recreational opportunities that may outweigh initial costs.