Effects of impoundment on populations of bullhead Cottus gobio L. and minnow, Phoxinus phoxinus (L.), in the basin of Cow Green Reservoir

The distribution, growth and reproduction of bullhead, Cottus gobio , L., and minnow, Phoxinus phoxinus (L.), were studied at Cow Green Reservoir, Upper Teesdale, for 10 years following impoundment in 1970. Comparisons were made with pre‐impoundment results. The length‐for‐age of bullheads increased after impoundment. The length‐for‐age of minnows decreased. In winter both species were confined mainly to deep water or to the stone facing of the earth dam. In summer they were found throughout the reservoir though bullheads were scarce in the littoral zone. Female bullheads became sexually mature at an earlier age (84% at age‐group I) than before impoundment (33% at age‐group I) but no change was apparent amongst males (67–62% in age‐group I). The post‐impoundment sex ratio did not differ from 1:1 and individual fecundity did not change after impoundment. There was a significant reduction in individual fecundity of the minnow. The mean instantaneous rate of mortality ( Z ) for bullhead of age‐groups II to IV was 0.96 ± 0.34 year −1 , compared with 0.85 ± 0.29 year in the Tees before impoundment.