Effect of flow variability modification and hydraulics on invertebrate communities in the great fish river (Eastern Cape province, South Africa), with particular reference to critical hydraulic thresholds limiting larval densities of Simulium chutteri lewis (Diptera, Simuliidae)

Abstract

AbstractFlow patterns in the Great Fish River in the Eastern Cape province of South Africa have changed from being seasonal, with predictable no‐flow periods during winter, to perennial following the completion of an inter‐basin transfer scheme in 1977 to provide a regular supply of irrigation water. Simulium chutteri (Diptera: Simuliidae) consequently became a problem species of pestilential proportions, due to increased flow volumes and current velocities favouring this species. In this study, aquatic invertebrates were sampled from the stones‐in‐current biotope with a range of current velocities at three sites on the Great Fish River, with a particular focus on pool/rapid areas favouring S. chutteri. The main aim of this paper was to determine whether critical hydraulic thresholds, including current velocity, could be derived for this species. Knowledge of the hydraulic preferences of S. chutteri is a prerequisite for any integrated control programme which combines larvicidal control with flow manipulation. S. chutteri was found to favour the rapids biotope with current velocities in excess of 90 cms−1, with preferences for higher current velocities increasing with life cycle stage. Additional hydraulic variables, at the scale measured in this study, did not correlate with larval densities. There is potential for more effective long‐term control of problem populations of larval blackfly in the Great Fish River through further research on the potential for using constructed in‐stream vanes to reduce current velocities in rapids of the river at critical periods of the year (July–October), based on flow duration/current velocity relationships. Copyright © 2007 John Wiley & Sons, Ltd.