Response of downstream migrant juvenile Pacific salmonids to accelerating flow and overhead cover

Mechanical structures designed to divert or guide juvenile migrant salmon at dams often work less effectively than expected. This likely results from a lack of understanding of fish behaviour. In order to address this, a series of experiments was conducted at McNary dam, USA, to assess the influence of hydraulic transition and overhead cover on the behaviour of seaward migrating juvenile Pacific salmon. Fish passing through a flume encountered a choice of route that varied based on hydraulic factors and overhead cover. Direct observation revealed that individuals elicited strong avoidance behaviour when they encountered areas where hydraulic conditions changed rapidly or were covered. Our findings have implications relevant to fish pass design and culvert restoration in Europe. Critically, improvement of current fish pass design and development of alternative mechanisms that effectively block fish access to, and divert fish away from poor passage routes, requires consideration of the behavioural component of fish migration. Guest editors: R. L. Welcomme & G. Marmulla Hydropower, Flood Control and Water Abstraction: Implications for Fish and Fisheries