Effects of dams in river networks on fish assemblages in non-impoundment sections of rivers in Michigan and Wisconsin

Regional assessment of impacts of dams on riverine fish provides resource managers essential information for dam operation, potential dam removal, river health assessment, and overall ecosystem management.  Such an assessment is challenging because characteristics of fish assemblages are not only affected by dams, but also influenced by natural variation and human-induced modification in addition to dams.  This study evaluated the impacts of dams on river fish assemblages in river non-impoundment sections using multiple fish assemblage indicators and multiple approaches to distinguish influences of dams from other natural and human-induced factors.  We found that environmental factors in addition to dams should be incorporated when evaluating regional effects of dams on fish assemblages.  Without considering such co-influential factors, the evaluation is inadequate and potentially misleading.  The role of dams alone in determining fish assemblages at a regional scale is relatively small (< 20%) compared to other environmental factors.  However, our results clearly demonstrate that dams have significant impacts on fish assemblages after excluding river size and land-use influences. The influences of the upstream dams, downstream dams, and distance to dams, and dam density differ among the fish indicators, which have different implications for maintaining river biotic integrity, protecting biodiversity, and managing fisheries.