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Abstract
The mid-reaches of the Great Fish River in the Eastern Cape Province of South Africa experience regular outbreaks of the pest blackfly Simulium chutteri as a direct consequence of increased flows caused by an inter-basin transfer scheme. There are opportunities to control these outbreaks through flow manipulation by using an upstream impoundment that has an annual shutdown period for maintenance. In this study, the stones-in-current biotope was surveyed at five sites prior to, and immediately succeeding, a flow shutdown. Changes in aquatic macroinvertebrate community composition and densities of larval and pupal blackfly were assessed and related to changes in hydraulic habitat due to the flow reductions. Although flows at those sites where blackfly densities were highest did not decrease to the levels desired for effective pest control, results from two sites indicated that flow reduction can lead to reductions in densities of larval blackfly without detrimental impacts on non-target macroinvertebrates. Simulations using a blackfly outbreak model indicate that flow reduction from an average of over 7 m3 s−1 to below 2 m3 s−1 for three to four weeks would effectively control blackfly outbreaks. Such an approach could be achieved through negotiations among affected Farmers' Associations, Irrigation Boards and the regional Department of Water Affairs and Forestry.