Presence and de-smolting of juvenile anadromous brown trout Salmo trutta in an off-channel diversion used in small-scale hydropower

Abstract Rivers are frequently modified to help generate provisioning ecosystem services, but these modifications frequently affect fish movements. Small-scale hydropower schemes frequently incorporate off-channel diversions into which migrating fishes can be entrained, potentially delaying or preventing their migration. The aim here was to evaluate the presence of brown trout Salmo trutta (“trout”) in an off-channel diversion used for hydropower (480 m length) on an upland river in southwest England between September 2024 and August 2025. The focus was on trout smolts tagged with passive integrated transponder tags, but with tagged river-resident trout also considered. Across all sampling events, 696 individual brown trout were captured in the channel. During the smolt migration period (mid-March to late May), 160 trout were identified as a smolt. The number of days between smolt capture and recapture varied between 11 and 132 days; smolts captured early in the migration period remained in the diversion for significantly longer than those captured later. At the migration period conclusion, 3% of tagged smolts remained in the diversion versus 32% of tagged river-resident trout. These remaining smolts had reverted to river-resident trout, indicating that trout smolts remaining within diversion channels will de-smoltify if they fail to emigrate.