Recent trends among aquatic biota in the catchment of the River Wye (Wales) and the effects of Riparian management

Doctoral thesis English OPEN
Clews, Esther.
  • Subject: Q1

Rod catches of Atlantic salmon (Salmo salar) from the River Wye were previously the greatest in England and Wales. However, a 30-year decline in catches of salmon and brown trout (S. trutta ) prompted management action. Since 1996, the Wye and Usk Foundation have excluded livestock, managed riparian trees, protected banks, cleared migratory barriers and limed selected tributaries. The aim was to enhance salmon habitat and extend spawning opportunities. The outcomes of such activities in Britain are still poorly understood. This thesis i) identified variations in the water quality, aquatic invertebrates and salmonids of the Wye catchment ii) evaluated the impact of recent management on habitats and aquatic organisms iii) assessed whether any larger-scale factors could explain management effects. Because no suitable project-specific data were collected, routine monitoring data and surveys were applied in the most applicable post-hoc experimental designs. Ecological quality varied widely among the Wye's tributaries. Combined biotic indices supported the need to mitigate acidification in some upland streams and reduce diffuse nutrients in the lower catchment. Riparian management appeared to reduce bank poaching and increase algae by comparison with reference streams. Post-treatment invertebrate communities were richer in recently managed streams than in controls. However, there was no evidence that management reversed the decline in salmonid populations. The typical life-cycle of salmonids in the Wye might delay response to management, but this effect cannot be evaluated with only six years' post-treatment data. Alternatively, local effects could be masked by larger-scale trends. In particular, salmonid abundance in the Wye declines significantly with increasing summer temperatures, decreasing summer rainfall and discharge. I conclude that riparian management has had some of the desired outcomes at the reach or tributary scale. However, salmonid numbers in the Wye potentially reflect climatic effects, implying a need to consider climate-change in future management action.
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