Current strategies for the management of water hyacinth on the Manyame River System in Zimbabwe.
The Manyame River System consists of the Manyame River and its tributaries, the Mukuvisi and Nyatsime rivers, which discharge water into manmade Chivero and Manyame lakes situated at 29 and 33 km, respectively, from Harare. Water hyacinth (Eichhornia crassipes) is the predominant floating aquatic weed in the system. About 15 years ago, the water was covered with various aquatic weeds, namely: water hyacinth (35%), Pistia stratiotes (36.6%) and Myriophyllum aquaticum (1.7%). Ten years later, the estimated weed coverage was water hyacinth (3%), P. stratiotes (0.3%), M. aquaticum (4%). Also, Azolla filiculoides had appeared in Lake Chivero, with a 1% coverage. In 2000, the weed coverage on the system was 2.4, 0.8, 6.8 and 3.5%, respectively, and 9.5% of the water surface of Lake Chivero had been invaded by Hydrocotyle ranunculoides. The control of water hyacinth was accomplished through chemical, biological and mechanical means, while the management of P. stratiotes was accomplished mainly through the classical biological control method. The appearance of M. aquaticum and H. ranunculoides may have been facilitated by the absence of natural enemies, and reduced competition for space and nutrients by the previously dominant water hyacinth and P. stratiotes. In 2000, the Zimbabwe Aquatic Weed Management Committee was formed to manage the aquatic weed problem in a holistic manner.