Behavioural Guidance Systems for Downstream Migrating Fish: A Mini-review

The lack of free-flowing rivers worldwide is one of the factors driving the decline of freshwater migratory fish populations, as dams obstruct migratory pathways. To restore river connectivity, downstream solutions are necessary, in addition to upstream efforts. Fish tend to follow bulk flow as they migrate downstream, and unless enough flow is redirected, they need to be guided towards the bypass. Guidance systems can be classified as mechanical, behavioural or hybrid. Some mechanical barriers have been proven effective for guiding out-migrants, but often there are high costs associated with scaling up these barriers for large hydropower plants. Although higher approach velocities often limit behavioural barriers, they are continuously developed and implemented, due to the lower costs associated with their operation. Furthermore, the fish are less likely to get injured or impinged with behavioural guidance, due to the lack of mechanical components. I discuss some of the currently existing behavioural guidance systems, their potential and drawbacks.