Improving bypass performance and passage success of Atlantic salmon smolts at an old fish-hostile hydroelectric power station: a challenging task

Old hydroelectric power stations often provide unsafe migration routes to fish, apart from passing through
mobile gates during high flow. The installation of retrofitted bypass is considered to be a potential solution to
improve fish passage at such old structures, but their performances are often insufficient. In the Ambl`eve River
(Belgium), a mobile 3.3 m high dam feeds two principal Francis turbines (12 + 14 m3 s-1) and is equipped with
three mobile gates, a modern vertical slot upstream fish pass and a retrofitted downstream bypass functioning at
a very low flow (1% of maximum turbined flow). A bypassed river section (length 8.4 km) downstream of the
dam was set at 3 m3 s-1 and supplies a Francis microturbine. From 2015 to 2016, we used 1150 hatchery Atlantic
salmon smolts (Salmo salar), and we placed antennae with automatic radio-frequency identification (RFID)
stations to analyse the migration routes used (n = 5). We tested the attraction efficiency and the performance of
the bypass with and without the placement of a guidance system at the entrance and examined the proportion of
smolt passage at the different migration routes under three functioning configurations. The placement of the
guidance system markedly improved the attraction efficiency and the overall passage efficiency. The median
time spent at the entrance of the bypass was from 7 to 12 min, and the time spent near the entrance of the bypass was less than 1 h for 67.1% (release 1), 88.2% (release 2) and 63.7% (release 3) of the smolts. During the threerelease events, the smolts arrived near the entrance of the bypass mostly during the dusk and dark periods (87.5%, 96.0% and 95.5%, for releases 1, 2 and 3, respectively). In a configuration without opening a mobile gate, the bypass was the most used migration route, followed by the microturbine and the main turbine. Stopping the microturbine and opening a mobile gate has consequences of making it the first choice of passage followed by the main turbine and the bypass. The re-establishment of safe and quick downstream migration has to be considered with a holistic view of the functioning of the hydroelectric powerstation.