Migration pattern of silver eel (Anguilla anguilla, L.) in an obstructed river system

The downstream migration of European silver eels in the River Fremur was examined to determine the potential effects of the numerous migration barriers that exist on waterways in western France. The Fremur has a 14 m high dam which creates a 3 x 10(6) m(3) water supply reservoir, 6 km from tidal limit. Based on 8-year records (1996-2004) of migrant silver eels captured in a Wolf trap located about 1 km below the dam, the influence of this dam on the migration was examined. These records indicate that 91% of silver eels were captured when the dam was overtopped (i.e. when the water reservoir was full). The timings of the overflow periods varied greatly between years mainly because they are function of the duration of the filling period which in turn is dependent on the level of summer water abstraction and annual hydrological conditions. Consequently, migration periods occurred at variable dates (between November and April) which is late in comparison with nonobstructed European river systems (generally between August and December). During overflow periods the migration of silver eels increased markedly during rain events (increasing river flow). This indicates that flow flushes, are essential so that settled silver eels can orientate themselves and pass over the dam. The hypothesis of a 'dam effect' that could stop temporarily or permanently some of silver eels in the reservoir is discussed. Finally, the influence of the migration delays on the condition of silver eels is considered.