Riverine Migration Success of Salmonid Smolts Following Their Entrainment in a Hydropower Off‐Channel Diversion in an Upland River

ABSTRACT Salmonid fishes typically express anadromy. During their juvenile riverine emigration, their downstream movements can be inhibited by hydropower schemes that entrain fish in their intakes. Here, the riverine migration success of smolts of brown trout Salmo trutta (“trout smolt”) and Atlantic salmon Salmo salar smolts (“salmon smolt”) was measured in a moorland‐fed spate river in southwest England. This river has some weirs and run‐of‐river hydropower schemes throughout its length that potentially inhibit smolt emigration through entrainment (hydropower) and passage delay (low‐head weirs). The capture of entrained fish from an off‐channel diversion for a run‐of‐river hydropower scheme in the upper river late in the smolt migration window enabled their implantation with an acoustic transmitter. These smolts were then released into the main river channel and their downstream migration tracked. Of 58 tagged smolts released, 39 were detected at least once in the river, with 27 detected as migrating successfully to the lower river/estuary. The trout smolts that reached the estuary were of similar size to those that were unsuccessful, whereas salmon reaching the estuary were significantly larger than unsuccessful individuals. While downstream smolt movements were not inhibited by the weirs or hydropower schemes, the migration period was characterised by high river levels, with some smolts reaching the estuary in less than 3 days. These results reveal that migration success rates of these smolts were relatively high following their release back into the main river channel, suggesting that smolt capture and release following entrainment can represent a successful mitigation strategy.