When to sample: flow variation mediates low-head dam effects on fish assemblages

Abstract

Low-head dams are a primary source of anthropogenic disturbance to river systems. They have the ability to change rivers from lotic to lentic habitats, affect sediment transportation, connectivity, water quality, linkages with wetlands and the quality of in-stream and riparian habitats. These changes can effect fish community structures and have the potential to alter the success of organisms in the system. However, there is no standard sampling protocol to assess dam impacts under varying flow regimes. The Vermilion River and its tributary, the North Fork Vermilion, are both impacted by low-head dams creating lentic habitats within each system. In October 2012 and May 2013, fish assemblages were sampled using direct current (DC) electrofishing at a total of 12 sites. Community composition at base flow responded strongly to both dams and differed between the two drainages. In sharp contrast, there was no effect of dams or differences between rivers during high flow. To adequately assess dam impacts on fish assemblages, we recommend sampling after base flow has been established. Sampling at high flow will reflect the homogenization of habitats and fish assemblages.