The Frémur is a small catchment in Brittany (France) where many hydraulic works (dams, etc.) have reduced the recruitment by elvers and glass eels of the European eel population (Anguilla anguilla L.) for at least the last 50 years. A long-term study (from 1997 to 2003) of the two most important eel passes has shown that pass characteristics and location in the catchment have a major influence. A total of 105 000 eels were caught, and from 2000 to 25 000 eels colonised the river per year. Moreover, the size classes of eels caught in the two passes were significantly different. Eels were larger (mean length 135 mm) in the downstream pass than in the upstream pass (mean 118 mm). Management of hydraulic works by specific eel passes helps maintain eel colonisation upstream of a major obstacle in many catchments. This management can explain the maintenance of eel populations in many sectors close to the sea. However, the location of these works on the migration paths is essential for population parameters upstream
