Fish populations in Plynlimon streams

In Plynlimon streams, brown trout (Salmo trutta L.) are widespread in the upper Wye at population densities of 0.03 to 0.32 fish m(-2) and show evidence of successful recruitment in most years. In the upper Severn, brown trout are found only in an area of c. 1670 m(-2) downstream of Blaenhafren Falls at densities of 0.03 to 0.24 fish m(-2) and the evidence suggests very variable year to year success in recruitment (Crisp & Beaumont, 1996). Analyses of the data show that temperature differences between afforested and unafforested streams may affect the rates of trout incubation and growth but are not likely to influence species survival. Simple analyses of stream discharge data suggest, but do not prove, that good years for recruitment in the Hafren population were years of low stream discharge. This may be linked to groundwater inputs detected in other studies in this stream. More research is needed to explain the survival of the apparently isolated trout population in the Hafren.