| Summary: | Two experiments were conducted in 1995 and 1998 at the Bedous water intake on the Aspe River to test the efficiency of a downstream bypass for a salmon smolts (Salmo salar L.). The surface bypass was located on the right bank a little upstream from the trashrack. During the experiment period, the discharge into the bypass varied from 0.4 m3/s to 1.2 m3/s, representing 1.6 % to 4.3 % of the turbine discharge. The efficiency of the device was evaluated using the mark recapture technique. The behaviour of smolts in the intake canal was studied using radiotelemetry. In 1995, the bypass was not very efficient (mean efficiciency of 17 %), due mainly to hydraulic conditions. A training wall was built in 1997 to reverse the flow pattern in the canal and to better guide the fish to the water intake of the bypass. The mean efficiency of the bypass was thus improved with 55 % of the smolts being recaptured in the trap in 1998. The efficiency of the device and the smolt behaviour were directly affected by the turbine operation and the hydraulic conditions in the intake channel. It appeared that a smalter proportion of smolts less than 19 cm in lenght were recaptured in the trap and that in spite of the tangential component of the velocity which created a louver effect, size selection was due to the spacing between the bars of the trashracks (30 mm). A light placed just above the water intake of the bypass appeared to have no effect on the movement patterns of the smolts with the hydraulic factor being preponderant. However, it modified the rhythm of capture in the traps as more fish were captured when the light was turned off. Une première expérimentation menée en 1995 au niveau de la prise d'eau de Bedous avait mis en évidence une faible efficacité du dispositif de dévalaison (17 % en moyenne), en grande partie due à la mauvaise position de l'exutoire. La mise en place d'un mur déflecteur d'environ 5 m de long à une trentaine de mètres des grilles en 1997 a entrainé des modifications de l'écoulement dans le canal ...
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