Beyond barriers: Fish assemblage recovery following dam removal on the Cuyahoga River, a Lake Erie tributary

Dam removals are increasing globally, yet ecological outcomes vary widely because biological recovery depends on post-removal connectivity and access to source populations. We evaluated how multiple dam removals and remaining fragmentation influenced fish assemblage recovery in the Cuyahoga River (OH, United States), a historically polluted Great Lakes tributary now undergoing extensive remediation and restoration. Using fish assemblage data collected from 1984 to 2024 across reaches downstream, within, and upstream of the Gorge Plant Dam (GPD), an 18-m barrier slated for removal, we quantified spatial and temporal change with ordination, trajectory, and distance-based redundancy analyses. Downstream assemblages showed a clear shift through time and gained 15 newly detected taxa after the 2020 Brecksville Dam removal increased longitudinal connectivity to Lake Erie by 39 river kilometers, more than doubling free-flowing habitat below GPD. In contrast, assemblages upstream of GPD exhibited limited change and patterns consistent with redistribution of resident species rather than colonization-driven turnover. Across the full time series, site position relative to GPD and the rate of dam removal were the strongest predictors of assemblage variation, with habitat gradients (e.g. depth, velocity, turbidity, and canopy cover) further differentiating reaches. Together, these results show that connectivity and barrier position shape assemblage recovery following dam removal. Most newly detected taxa downstream were native, though three were nonnative, indicating trade-offs between recolonization and nonnative spread.