Fish behaviour and passage efficiency: lessons from North America.

The guidance efficiencies associated with many mechanical structures designed to divert fish are often much lower than expected. Existing models of fish passage often ignore fish behaviour or are based on assumptions that may be false. A series of experiments were 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 elicit strong avoidance behaviour when they encounter areas where hydraulic conditions change rapidly or are covered. Our findings have implications relevant to fish pass design and culvert restoration in Europe. The behavioural component of fish migration must be considered in order to improve current fish pass design and to develop alternative mechanisms that block access and divert fish