Over the last two decades, France has developed a methodology to estimate the mortality rate of the eels migrating downstream past a hydropower plant. This method has enabled the impact to be calculated on a scale of certain facilities and certain migration routes, prior to the implementation to measures to offset the impacts of the hydropower facilities. This document describes that method, which is based on knowledge of

• The migratory activity,
• The distribution of the fish over the different passage routes past the structures,
• The effectiveness of any possible downstream systems and
• The injury to the fish when passing through the spillways and turbines.

The formalisation of this method will allow to produce a mortality diagnosis on the scale of a hydropower facility, or even of a migration route if they have all the data needed for the different calculations. This method can be applied to any other species provided that the data needed are available. In France, for example, a similar approach is applied to estimate the mortality of Atlantic salmon smolts (Salmo salar).