Efficiency of a downstream bypass as estimated by the mark-recapture technique and behaviour of Atlantic salmon (Salmo salar L.) smolts at the Bedous water intake on the Aspe River (France) monitored by radiotelemetry

Two experiments were conducted at the Bedous water intake on the Aspe River in 1995 and 1998 to test the efficiency of a downstream bypass for salmon smolts (Salmo salar L.). The surface bypass was located on the right bank a little way upstream from the trashrack. During the experiment period, the discharge into the bypass varied from 0.4 m(3)/s to 1.2 m(3)/s, representing from 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 efficiency was very low (with a success rate 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 smaller proportion of smolts of less than 19 cm in length 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 near the bypass entrance appeared to have no effect on the movement patterns of the smolts with the hydraulic factor being preponderant. However, the light did affect the rhythm of capture in the trap as more fish were captured when it was turned off.