Effects of Small Dams on Cold Water Stream Fish Communities

—Dams provide many benefits to society, yet they have profound impacts on aquatic ecosystems. In addition to blocking fish migration, small surface-release dams on coldwater streams generally increase water temperature below the dam. The goal of this study was to evaluate how small, surface-release dams affect fish communities, separating the total effect into components related to fragmentation and habitat alteration. The approach taken in this study was to synthesize results of two large studies conducted in the Laurentian Great Lakes basin. The first study focused on low-head barriers (<0.5 m in height) built to prevent the upstream migration of sea lamprey Petromyzon marinus. In this study, 24 streams with sea lamprey barriers and 23 nearby reference streams without barriers were sampled. The second study evaluated 10 small hydroelectric dams. Dams that do not substantially warm downstream water temperature cause a moderate shift in fish community composition, whereas dams that warm downstream water temperature by more than 2°C cause substantial shifts in fish community composition. The abundance of stream resident coldwater species such as brook Salvelinus fontinalis and brown trout Salmo trutta does not appear to be affected by the presence of a barrier. Dams that increase water temperature result in substantial reductions in abundance, however. These results suggest that the effect of habitat alteration caused by dams on coldwater streams should receive as much, if not greater attention, as the effect of habitat and population fragmentation.