Impacts of water abstraction upon migratory fish species in the rivers Wye and Usk

To assess the potential impact of increased abstraction on twaite shad (Alosa fallax (L.)) and Atlantic salmon (Salmo salar L.), the latter at two life stages (returning adults and smolts) - cameras were deployed in the river Wye from 3 May to 4 September 2013 and 22 April to 8 September 2013 in the river Usk. Visual images were continuously recorded and examined for the number, direction, and timing of fish movements and then compared against environmental factors (temperature, flow and tidal state) and the position of movements in relation to the camera array.Shad were observed in the Usk between 08 May and 11 July and between 20 May and 11 July in the Wye. Migration was primarily influenced by both tide and flow with movements observed at temperatures above 12°C in both rivers. Shad exhibited a crepuscular pattern with little movement overnight and actively avoided fast flows during their migration.Adult salmon were recorded in both rivers throughout the whole study and migration seemed to be primarily influenced by both tide and flow once in fresh water. Movements were recorded at temperatures above 10°C with migration predominantly at dawn. Salmon in the Usk showed no preference to position in the river whilst migrating, unlike the Wye where a clear preference to the outer camera position was possibly caused by a structure in the water causing a flow break.Smolt migration was only visible in the Usk because a camera malfunction occurred in the Wye array in the first few months of deployment. Their migration was between 23 April and 22 May and was influenced primarily by flow and tide. Moving during the descending limb of the overall spring hydrograph, smolt migration occurred at temperatures above 10°C with prominent movement occurring during daytime, contrary to the literature. Smolts showed a strong correlation to faster flows in their seaward migration being observed moving in the highest water velocity areas of the river detected on an Acoustic Doppler Current Profile (ADCP).Flow was a primary factor in migration of these species and thus protecting the natural flow regime is essential to maintain and improve the conservation status of both species. Although it is assumed that abstraction would have a minimal effect on either river unless increased abstraction takes place during dry years which can increase the risk of barriers to migration by lowing the river levels further. This is supported by the 2012 egg surveys where no barriers to migration in the Wye were observed as eggs were found beyond each potential barrier. This study was particularly relevant as it was conducted during a dry year when flows were below average. In the Usk, low flows are highly likely to be significant particularly in relation to passage over major barriers such as the footings of Usk Town Bridge and Crickhowell Town Bridge. Lower flows will potentially increase the barrier effect by restricting the movements upstream of both salmon and shad. There is therefore potential for abstraction in the lower Usk to affect the spawning range of shad, in particular if abstraction from Prioress Mill reduces flow at Usk Town Bridge sufficiently to prevent passage. Thus a precautionary approach to abstraction is needed to support migration by protecting spring flows in low flow years during these vulnerable life stages