The influence of sluice gate operation on the migratory behaviour of Atlantic salmon Salmo salar (L.) smolts.

Numerous studies have examined the effects of high-head dams on salmonid smolt migration, but few have examined smolt behaviour at sluices managed to regulate water levels. We quantified passage success and time to passage of wild and hatchery origin juvenile (smolt) Atlantic salmon (Salmo salar L.) migration in relation to sluice operation (for water level and flood risk management) at Bala sluices on the upper River Dee, North Wales, including the occurrence of flow-reversal conditions from the Dee to Llyn Tegid (Lake Bala) immediately upstream. Ninety four smolts (67 wild, 27 hatchery origin) were caught, acoustic tagged, released at the capture site and tracked with a fixed receiver array in April to May 2015. Of the tagged fish, 91.5% successfully reached and passed the sluice gates. Of the tracked smolts that reached the sluices, 6.6% failed to successfully migrate downstream through the sluices and 3.2% took over 4 days to pass. There was no significant difference in travel speed, nor the proportion successfully passing, between wild and hatchery reared fish. Smolts migrated quicker at higher flows. Smolts travelled at lower speeds through the sluices than the preceding section from which they originated, suggesting that migration may have been delayed by the sluice gates. Smolts passed through the sluice reach more quickly with higher discharge. Specifically, smolts moved quicker through the sluice reach when the aperture of the gate nearest the outside bend and thalweg increased, but the heights of the other gates did not significantly affect smolt speed past the structure. Flow reversal episodes occurred only by day, whereas more smolt activity occurred by night (66.5% between dusk and dawn); under these conditions we found little evidence for loss of fish to the lake (three fish [3.2%] visited the lake and did not pass the sluices, but were last detected above the sluices), or contribution to delayed passage, as a result of flow reversal. It is concluded that at sites operating undershot sluices, where smolts migrate, upper water column orientated migration routes should be maximized where the thalweg hits the structure as this is likely to be the main route of transit adopted by smolts