Climate‐linked freshwater habitat change will have cost implications: Pest blackfly outbreaks in two linked South African rivers

Abstract Calls for implementation of environmental flows have been growing over the past 20 years, and their implementation is now being recognized. The need for such assessments to occur in conjunction with maintenance of environmental water temperatures has also been emphasized. When the costs of departures from natural thermal regimes are considered, this moves from being an academic exercise to a tangible management imperative. Pest outbreaks of aquatic macroinvertebrates typically occur when environmental conditions disproportionately and overwhelmingly favor a particular species. This has been the case for at least three major river systems of economic importance in South Africa, namely, the Orange River and its major tributary (Vaal River) as well as the Great Fish River. In these rivers, two species of pest blackfly, Simulium chutteri and S. damnosum (Diptera: Simuliidae), have become problematic following on from changes in natural flows through either impoundment or interbasin transfers. In this study, a statistically robust reference thermal condition was defined and exceedances assessed (based on a 2°C increase in water temperatures). The implications of these assessments were then applied to future possible pest blackfly outbreaks, factoring in changes in flows in response to global climate change. Results show that the current seasonal variation in the likelihood of pest outbreaks is replaced by high perpetual outbreak probabilities. Interactions between major environmental variables become synergistic, with major cost implications to regional economies. Lessons from this study can be generalized and used as a means of predicting similar synergistic effects in other aquatic macroinvertebrate disease vectors (such as bilharzia vector snails and mosquitoes) in response to global climate change.