Field and laboratory studies on the passage of adult and juvenile fish through hydroelectric turbines are reviewed, with special emphasis on tidal schemes in operation. Although the types of injury which fish incur and their frequency are well documented, little appears to be known about the specific hydraulic conditions within the turbine structure which actually cause the injury, despite the fact that the four main causes of fish loss (abrupt changes in pressure, water turbulence, shearing currents and mechanical contact with turbine blades) have been identified.

Factors causing fish mortality fall into two main categories: (a) hydraulic conditions of pressure change, cavitation, shearing and turbulence, producing direct, characteristic injuries; (b) conditions influencing the likelihood of actual fish collision with turbine components. These factors can be enhanced or reduced by alterations in turbine operation and, possibly, design.

The significance of fish behaviour and ecology with respect to prediction of numbers of fish passing through installations, plus criteria which govern actual mortality rates have yet to be elucidated.