National Agricultural Research Organisation (NARO), Fisheries Resources Research Institute (FIRRI), P.O. Box 343, Jinja, Uganda. E-mail: firi@infocom.co.ug

Distribution of water hyacinth in Uganda, its control and impacts of control measures

Water hyacinth invaded Lake Victoria from Rwanda via River Kagera during the second half of the 1980's. Prevailing winds and associated currents facilitated weed distribution along the northwestern shores of the lake while sheltered bays and inlets favoured establishment. Infestation of water hyacinth in Lake Kyoga, Uganda occurred at about the same time, from an unknown source. Weed distribution was completed by 1990, facilitated mainly by the general east- west water flow regime. Distribution and establishment of water hyacinth along lower Victoria Nile, the northern shores of Lake Albert and the Albert Nile were completed by 1991, aided by down-river transport. Weed establishment occurred in zones sheltered from direct river currents and offshore winds. Fringes of papyrus, bulrush (Typha sp) and hippograss (Vossia cuspidata) were identified as indicator macrophytes of suitable environments for water hyacinth establishment. In Uganda, striking weed redistribution was achieved on Lake Victoria through annual cross-lake migrations. Similarly, proliferation and constant translocation of water hyacinth biomass down rivers, Kagera and the Nile, are important channels of weed redistribution in Lake Victoria and the Nile system.

Water hyacinth control was effected mainly by biological control with Neochetina weevils, environmental stress, and natural ecological succession with native macrophytes dominated by hippograss. Physical harvesting was used to alleviate weed burden at the small landing beaches and at key economic installations. Adverse ecological impacts of water hyacinth were mostly due to smothering and displacement of biodiversity and various inshore biological processes. Major impacts of control measures included prolonged depression of dissolved oxygen at the lake bottom, suspected loading of sediments with phosphorus, and a temporary boost in fishery production, in bays where massive sinking of the weed occurred due to stress by biological control and environmental factors.