THE INFLUENCE OF DROUGHT ON THE FISH AND INVERTEBRATE POPULATIONS OF AN UPLAND STREAM IN WALES

The exceptionally dry summer of 1976 presented an opportunity to assess the impact of drought on the fauna of an unregulated stream in Wales. The total number of macroinvertebrates varied from 1808 m-2 in Sept. 1976 to 7412 m-2 in Aug. 1977. Two main effects of drought were observed: an initial reduction in abundance during the drought and a change in community structure in the following year, probably through impaired reproduction. Trout density ranged from 0.075 .+-. 0.028 to 0.402 .+-. 0.093 m-2 and salmon parr density varied between 0.014 .+-. 0.004 and 0.414 .+-. 0.119 m-2. The only detrimental effect of the drought on the fish fauna was the elimination of the 1976 year class of young salmon, probably due to prolonged high water temperature.