DISTRIBUTION AND ABUNDANCE OF BLACKFLIES (DIPTERA, SIMULIIDAE) IN RELATION TO IMPOUNDMENTS IN THE BUFFALO RIVER, EASTERN CAPE, SOUTH-AFRICA

1. The abundance of blackfly species along the length (140 km) of the multiply impounded Buffalo River, eastern Cape, South Africa, was stud ied between 1986 and 1988. 2. Twenty-three species of blackflies were recorded, most of which (95%) were usually found 5-10 km from the sour ce. 3. Impoundments favoured five lower-reach (pest) blackfly species at the expense of two upper-reach (non-pest) species. Impoundments hav e therefore shifted the distribution of blackfly species 'upstream', a nd have increased the distribution of pest blackflies. 4. The smallest impoundment, situated in the Upper Foothill Stony Run Zone, had the g reatest effect on blackfly distribution. 5. There was no typical filte r-feeding guild below impoundments. Possible reasons for this included the discharge of toxic algae from impoundments, the lack of compensat ory releases, the sudden drying of impoundment tailwaters and the pauc ity of natural lakes in southern Africa which precludes a pre-adapted 'outlet' fauna.