Protecting efficiently sea-migrating salmon smolts from entering hydropower plant turbines with inclined or oriented low bar spacing racks

Restoring the longitudinal connectivity of rivers is becoming a conservation priority in countries with high hydroelectric plant (HEP) development. Newly designed downstream passage solutions for fish are being installed in small and medium-sized HEPs in France, and an accurate evaluation of their functionality is needed. Here we addressed the efficiency of protection systems for the downstream migration of Atlantic salmon smolts at four HEPs (three 26° horizontally inclined racks and one 15° oriented to the flow rack in the bank alignment, all with 20 mm spaced bars). Between 239 and 300 hatchery-reared salmon smolts were PIT-tagged and released in 5-6 groups 100 m upstream of each studied HEP. Their passages through the HEPs were detected with radio frequency identification (RFID) antenna in the bypasses for downstream migration and the fish passes for upstream migration. On average between 82.8% and 92.3% of released smolts successfully passed the HEP through one of the two non-turbine routes. Resulting mean bypass passage efficiency ranged from 80.9 to 87.5% and all fish groups reached over 70% passage efficiency. Excepting one site, 50% of smolts passed through the bypass in less than 23 min after release, and 75% of them in less than 2 h 15 min. Combining our findings with previously estimated fish entrainment rates into the intake channel and turbine-related mortality rates, we assessed the overall fish survivals at the studied dam/HEPs which are between 98.24% and near 100%. Our results confirm recommended design criteria for inclined and oriented racks and the interest of the tested devices for the protection of downstream migrating salmon smolts.