Abstract:

Water hyacinth invaded Lake Victoria and Lake Kyoga in Uganda during the second half of the 1980,s and
was firmly established by the end of that decade. During the 1990's, the weed spread rapidly, multiplied
profusely and adversely impacted aquatic resources and services such as fisheries and fishing activities,
availability of potable water, hydropower generation, transportation by water, and recreational and tourist
facilities. The impacts of water hyacinth constrained efforts, which directly contributed to poverty reduction
in the country. This paper highlights the contribution of research in Uganda to the control of the noxious
weed. Research focused on understanding the processes and mechanisms of water hyacinth distribution,
establishment and proliferation; the dynamics of migratory movements of weed mats; the environmental
and socio-economic impacts of water hyacinth; the role of various control efforts, and the impacts of control
measures. Data was compiled through regular surveillance and from field experiments. Distribution and
establishment of water hyacinth was influenced by direction of regular winds and water currents; and by
availability of shelter and environments with suitable nutrient-rich water depth and bottom substrate along
the shore. The initial proliferation of water hyacinth gave rise to extensive stationary strips, which ultimately
fringed about 2200 ha of shoreline length of Lake Victoria in Uganda, 570 ha of the shore of Lake Kyoga,
and 500 ha along River Nile (in Uganda). Proliferation of mobile water hyacinth occurred in sheltered bays,
particularly in Murchison, which is enriched with nutrients notably phosphorus. Maximum cover of 1800
ha of the mobile weed was estimated in Lake Victoria - Uganda. Mobile water hyacinth cover was distributed
from the production bays to several storage bays through annual weed migrations across Lake Victoria.
Cont rol of the weed was achieved through the effects of biological control with Neochetina weevils, ecological
succession dominated by native macrophytes especially hippograss, and by physical biomass removal at
selected sites of socio-economic importance. The effects of biological control and environmental stress led to
unprecedented collapse and sinking of huge biomass of mobile water hyacinth resident in the collection
bays. Decomposition of the sunken water hyacinth caused prolonged depression of dissolved oxygen to
anoxic levels close to the lake bottom, led to elevation of soluble reactive phosphorus, and to increased
diversity and abundance of phytoplankton, macro-invertebrates and fishes. Water hyacinth was brought
under control by the end of 1998 in most infested water systems of Uganda except rivers Kagera and the
Nile. In the year 2000, persistent weed resurgence occurred in environments rich in phosphorus notably in
Murchison Bay. Control of water hyacinth in rivers and nutrient enriched environments is key to the
management of the weed in Uganda.