Identifying indicators and quantifying large-scale effects of dams on fishes

Although localized effects of individual dams on stream fish assemblages have been relatively well-studied, less is known about the effects of multiple dams within a stream network on fishes and the patterns that emerge when the combined effects of individual and multiple dams are viewed across entire river basins, ecoregions, and states. This study evaluated multiple stream network fragmentation metrics representing localized (e.g., distance-to-dams) and cumulative (e.g., total upstream reservoir storage) dam influences on streams in Michigan, Wisconsin, and Minnesota, developing an approach for identifying suitable fish indicators of dam effects. We used change point and correlation analyses to determine associations of stream fish catch per unit effort and various stream network fragmentation metrics with data from more than 2000 fish survey sites stratified by stream size, thermal regime, and ecoregion. Of the identified indicator species, predominantly warmwater, large river, and/or lentic species were positively associated with stream network fragmentation, whereas cold and coolwater lotic species were negatively associated with fragmentation. These results suggest a combination of downstream thermal effects and upstream influences from impoundments generated by dams. Variance partitioning analyses based on identified indicator species revealed greater upstream-dominated dam influences in headwaters than mid-sized streams, and a greater relative influence of dams vs. other non-dam anthropogenic influences in cold streams than warm streams. Overall, a combination of localized and cumulative fragmentation metrics, as well as upstream and downstream-oriented measures, were influential in indicator species responses, emphasizing the importance of selecting a diversity of fragmentation metrics when assessing effects of dams on stream fishes. Understanding multiple dam influences on stream fishes, including localized effects from individual dams and cumulative effects from all dams within a river basin, would provide useful information for a variety of management activities, including dam operation and dam removal prioritization. Dams significantly affect conservation and management options for stream fishes, with identification of multi-scale dam influences on fishes being critical to restoration and maintenance of aquatic biodiversity throughout the world.