Protecting fish from entering turbines: the efficiency of a low-sloping rack for downstream migration of Atlantic salmon smolts
Résumé
In April 2015, the efficiency of a system of low-sloping rack and bypass has been assessed for Atlantic salmon smolts at the small hydroelectric plant (HEP) of Auterrive on the Gave d’Oloron River (France ; turbine discharge: 7.8 m3.s-1 ; rack inclination: 26° relative to the horizontal ; bar spacing: 20 mm ; bypass discharge: 0.5 m3.s-1). 239 hatchery smolts (mean length 185 mm) were PIT-tagged and released 100 m upstream the HEP, in 5 groups at different times of the day. Their passages downstream the HEP by the bypass and as well by the fishpass for upstream migration were monitored with RFID antennae. On average 80.7% of smolts migrated through the HEP bypass and 3.8% of smolts descended through the fishpass. In total, 84.5% (min-max: 75.5–91.9%) of smolts migrated downstream the HEP via safe routes. 50% of them did it in less than 23 minutes since their release and 75% of them did it in less than 2 hours. Fish migration time was similar for afternoon, evening and night releases (median times between 19 and 21min), but was significantly longer when the fishes were released in the morning (median migration time: 3hours 17min). Our findings give credence to the recommended design criteria for low-sloping racks, which is the main solution implemented in France for small HEPs.
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