Impacts of flow regulation on freshwater pearl mussel (Margaritifera margaritifera) habitat in a Scottish montane river

The River Moriston in NW Scotland is a cobble-gravel bedded river that has been dammed and regulated for hydroelectric power (HEP) since 1956. The river supports a functional population of the critically endangered freshwater pearl mussel (Margaritifera margaritifera) in the lower part. In contrast the population in the upper reach is sparse and shows no signs of juvenile recruitment, leading to speculation that hydrological and geomorphic changes associated with HEP have degraded the habitat they depend on. A combination of historical analysis, field mapping and geomorphic survey of channel and active bar sites was used to assess habitat changes and current quality. During the post-dam period, the naturally stability of much of the channel has increased, active bars have been stabilised through vegetation colonisation, riparian tree cover has increased and the active channel width has significantly reduced locally (>50%); adjustments that are indicative of a reduction in the incidence of competent flows caused by flow regulation. However area and stability of habitat for freshwater pearl mussels have not been reduced greatly. The channel sites examined are characterised by mixed cobble-gravel substrates (D(50) range=46-188 mm), predicted to be highly stable, that provide suitable habitat for adult freshwater pearl mussels. However a degree of bed compaction at one site was observed that could be limiting the recruitment of juvenile mussels. It is hypothesised that the sparse, non-functional status of the freshwater pearl mussel population reflects significant historical pearl fishing and the limitation of recovery due to HEP related pressures of fish migration obstruction and bed compaction. The implications of these factors for conservation of the species are discussed.