Lowhead Dam Removal in an Urban River System Leads to Shifts in Fish Assemblages in the Short-Term

Lowhead or run-of-river dams, which can have significant impacts on river ecosystems, are common on rivers around the world. Although lowhead dam removal is becoming an increasingly popular restoration method, the quantitative effects of lowhead dam removal on river ecosystems remain difficult to predict. In this study, we investigated the short-term (<2 years) effects of two lowhead dam removals on fish assemblage assemblages in the Scioto and Olentangy Rivers of Columbus, Ohio. Non-metric Multidimensional Scaling (NMS) and analysis of similarities (ANOSIM) revealed that upstream assemblage composition shifted significantly from before to after dam removal (ANOSIM; R = 0.714, p = 0.001). Similarly, assemblage composition was different between years 1 and 2 at Olentangy River reaches both upstream and downstream of dam removal (ANOSIM; R = 0.136, p = 0.019). Shifts in fish diversity metrics were accompanied by changes in relative abundances of taxa within feeding guilds. Overall, our results indicate that dam removal can act as a pulse disturbance with quantitative short-term effects on fish assemblages. Understanding the short-term responses of fish assemblages will be critical in shaping longer-term responses.