Fish populations in Plynlimon streams D. T. Crisp1 and W. R. C. Beaumont2 121A Main Street, Mochrum, Newton Stewart, Wigtownshire, Scotland, DG8 9LY. 2Institute of Freshwater Ecology, The River Laboratory, East Stoke, Wareham, Dorset, BH20 6BB.
Abstract. 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 -2 downstream of Blaenhafren Falls at densities of 0.03 to
0.24 fish -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.
Citation: Crisp, D. T. and Beaumont, W. R. C.: Fish populations in Plynlimon streams, Hydrol. Earth Syst. Sci., 1, 541-548, doi:10.5194/hess-1-541-1997, 1997.