Rivers have been used as a source of energy throughout history, but the construction of dams for a secure and even availability of water can disrupt the dispersal and migration opportunity for aquatic organisms, causing a tradeoff between biological and socio-economic values. Contemporary hydropower plants are no exception, and hydropower companies strive to optimize energy utilization while minimizing deteriorating effects on environmental values. The European eel and the Atlantic salmon are threatened and iconic fish species that have life cycles that are obstructed by dams in rivers. European hydropower companies traditionally compensate this negative effect by constructing fish passage solutions to allow two-way migration, albeit such solutions have highly variable success. Downstream passage solution design is particularly poorly known and hence targetted in many recent rehabilitation and research projects.
Downstream migrating silver eels and salmon smolts tend to experience high rack- and turbine-induced mortality at hydroelectric plants (HEPs). Recent attempts using low-sloping racks guiding eels and salmon smolts to bypasses have reached passage efficiencies of >90% and median delays of 24 h at small to medium sized HEPs, but detailed knowledge on behaviour by racks and bypasses are still largely lacking. It is neither known if such solutions are applicable to large HEPs, and one major concern relates to the bar spacing of evaluated low-sloping racks (15-18 mm). A recirculating flume with two 25 m long experimental arenas, 4 m wide and 2 m deep, has now been built at the Vattenfall Ecohydraulic Centre in Sweden, where we are currently studying the importance of bar spacing for behaviour and performance of downstream migrating silver eels and salmon smolts. The flume facility, the study design and preliminary results will be presented to incite discussion.