Assessment of Downstream Fish Bypasses for Atlantic Salmon Smolts at Four Hydroelectric Facilities on the Ariège River (France)
Abstract.—A 2-year experiment was conducted on the Ariège River (southwest France) to assess effectiveness of downstream fish passage facilities for Atlantic salmon Salmo salar smolts at four consecutive small hydroelectric plants located on a diversion structure. The efficiency of these bypasses, evaluated using passive integrated transponder (PIT) telemetry, varied significantly between sites. The highest efficiency (greater than 65%) was obtained with two entirely new downstream passage facilities, whereas less than 40% of PIT-tagged smolts migrated through two other bypasses that were simple retrofits of existing trash sluices. Performances of two bypasses were significantly improved using either a submerged horizontal screen designed to limit upwelling or mercury lights at the bypass entrance. At two sites, a difference in fish size was also observed between smolts using the bypasses and those passing through the conventional intake trash racks, with larger fish using the bypass and smaller fish being entrained through the turbine intake. It was also demonstrated that the probability for spillway passage by tagged fish increased with increases in the proportion of total river discharge spilling and with decreases in fish size. The results of this study demonstrate how different parameters can influence downstream bypass efficiencies and provide information on how downstream passage for migratory fish can be improved at small hydropower projects.