Weir-Induced Serial Discontinuity and Mitigation Strategies

Most of the world’s rivers are affected by dams and weirs, which results in modifications of serial continuity and declines of riverine fish species. Information on the quantitative and qualitative effects of weirs as well as on the efficiency of restoration measures is crucial for successful management of stream ecosystems. We developed and tested an evaluation system for the quantification of weir-introduced serial discontinuity and for an assessment of the ecological functionality of fish passes. Weir-induced effects exceeded the effects from variation of geographic location, geology, and drainage system. The effects were quantified concerning abiotic habitat properties as well as in the community structures of fishes, macroinvertebrates, macrophytes and periphyton. In addition to their roles as migration corridors, fish passes provided important key habitats for juvenile and small rheophilic fishes. Consequently, fish passes can play an important role as compensatory habitats and this function should be better considered in river restoration. The evaluation system presented herein, by including several taxonomic groups and physicochemical habitat variables, provides a universally applicable tool for the ecological assessment of serial discontinuity and its mitigation.