Three experiments were conducted from 1996 to 1998 at the Camon hydroelectric powerhouse water intake, on the Garonne River, to test the efficiency of a surface downstream bypass for Atlantic salmon smolts. This bypass was built into the trashrack itself at its left edge. The efficiency of the device was evaluated using the mark-recapture method. Smolt behaviour in the intake canal was studied using radiotelemetry technique. In 1996, the bypass efficiency was low (34%). Radio-tracking showed that the bypass location was not responsible for its low efficiency, fish being listened most of the time in the vicinity of the bypass. Nevertheless, an unstable upwelling hided the device entrance. After installing submerged horizontal screen and plates upstream bypass entrance gate, the average efficiency increased to 73%. Good hydraulic conditions in the intake canal and good local hydrodynamic in the vicinity of the bypass entrance are essential to obtain a satisfactory downstream bypass efficiency. Intermittent nocturnal lighting has an effect on smolt behaviour in the intake canal by maintaining fish in directly lighted areas and on the rhythm of fish entry in the bypass, more fish being captured during the first part of the lighting off period. The catching of 7,715 wild salmonids has permitted to study downstream migration rhythms at dam. Daily downstream migration peaks seems to be linked with high water discharge and/or an increase of water temperature. Moreover, downstream migration activity at a dam appears to be mainly nocturnal.
