Fish size and assemblage patterns following dam failure in Black Rock Brook, New York

Man-made barriers, such as dams, alter the water flow dynamics of rivers, affecting fish communities and habitat connectivity. In 2011, flooding associated with Hurricane Irene caused the failure of the Cornwall Lower Reservoir dam, which subsequently altered the flow of Black Rock Brook in Black Rock Forest, New York. Fish diversity, abundance, and individual size were surveyed by electrofishing in 2010 before dam failure and again in 2017, providing a rare opportunity to compare fish assemblage and morphometrics following barrier loss and determine whether the observed patterns were localized above or below the former dam. Fish length and weight were compared between years using Welch’s two-sample t-tests, species diversity was evaluated using Shannon diversity indices, and richness and evenness were summarized using observed species counts, Pielou’s evenness, and rank-abundance curves. Blacknose dace captured in March 2017 were significantly shorter and lighter than those captured in April 2010 in both the downstream reach and the pooled upstream reaches. Observed species richness and Shannon diversity were also lower in 2017 at both locations, and rank–abundance patterns indicated greater dominance by a small number of taxa. The surveys differed in both year and seasonal timing, so the observed patterns cannot be attributed solely to dam failure and may also reflect broader temporal or environmental variation. Nevertheless, the findings highlight the value of pre-disturbance baseline data for documenting biological differences following barrier loss in small headwater streams. For stream managers, the results demonstrate the importance of long-term monitoring and repeated seasonal sampling for distinguishing dam-related responses from broader variation and informing management decisions.