The trout ( Salmo trutta L.) populations of the headwaters of the Rivers Severn and Wye, mid-Wales, UK

Brown trout ( Salmo trutta L.) populations were studied in two contrasting headstream systems in mid-Wales. An electrofishing survey in 1979 compared the long-term effects on trout populations in the upper Severn and upper Wye systems of mature conifer forest (Severn) and sheep pasture (Wye), respectively, and identified a previously unknown trout population in the upper Severn. In June 1979, brown trout were found in six out of seven stations in the Wye system. In the Severn system trout were found at only one station out of 10. Population densities and biomasses ranged from 0 to 0.32 fish m −2 and 0–5.6 g m −2 in the Wye and the age composition of the samples indicated good recruitment in most years. In the Severn station, population density was c.0.05 fish m −2 and biomass was c.3.1 g m −2. There was evidence of irregular recruitment. Growth in the Wye system could be described in terms of an asymptotic length of 21.5 cm, a von Bertalanffy [1] growth constant (K) of 0.34 and a I-group mean length in June of 7.9 ± 0.95 cm (95% C.L.). Growth in the Severn population appeared to be similar to but rather more rapid than, that in the Wye. The population of the upper Severn occupies a stream area of c.1700 m 2 and probably fluctuates between c.50 and c.400 individuals. Complete or partial isolation is likely to have occurred within the last 55 years. Acidification of upland streams often causes local extinction of trout. The occurrence of this small population in the Hafren may indicate that this is an acid tolerant strain, or that the water in the relevant portion of the Hafren is better buffered than previously supposed, or that the population is maintained by occasional incursions of spawners from further downstream.