Aquatic weeds : to control or not to control. the case of the Midmar Dam, KwaZulu-Natal, South Africa

Abstract

The Midmar Dam within the uMngeni Catchment, KwaZulu-Natal is important for water provisioning and recreational use. In 2014, an estimated 60 ha of the dam was infested with Egeria densa, which can spread at a rate of 50% per annum under optimal conditions. E. densa limits access to, and the recreational use of, the dam. We use the travel cost method to estimate the recreational value of the Midmar Dam, informing the maximum desirable cost of control. The model estimates that the most likely extent of the invasion would be between 233 ha and 771 ha. The estimated cumulative NPV is R684 million for the best-case scenario, which allows for clearing operations and a spread rate of 15%. If no clearing is done, the cumulative NPV is reduced to an estimated -R20 million. This study therefore suggests that management of the problem is imperative, but that control efforts should not exceed R687.8 million over 30 years. Doing so will constitute a net loss to society.