In Uganda little is known of the giant Salvinia, the species is poorly documented and yet it ranks second close behind the water hyacinth on a list of the world's most noxious aquatic weeds. Salvinia molesta also known as the Kariba weed is an invasive free-floating aquatic water plant native to south-eastern Brazil. Over the years, the species has spread widely throughout the World and is invasive in a variety of aquatic habitats, including lakes, rivers and rice paddies. It is currently listed among the 100 most invasive species..
The species has been known to exist in Uganda although little is known of its introduction. Many infestations have been documented in the Lake Kyoga region the most recent being in 2014 (NewVision).
During the July Water Fowl Counts in Eastern Uganda, NatureUganda team noted that the Kibimba Rice Scheme Dam was heavily infested by the Kariba weed. Just last week (2nd August, 2018), the NewVision ran a story on how the weed had rendered the Uganda shs.4.7 billion Leye Dam in Kole District useless.
The dense vegetation mats formed by the Kariba weed reduce water flow, lower the light penetration and oxygen levels in the water. This stagnant dark environment negatively affects the biodiversity and abundance of freshwater species, including fish and submerged aquatic plants. S. molesta can alter wetland ecosystems and cause wetland loss and also poses a severe threat to socio-economic activities dependent on open, flowing and/or high quality water bodies, including hydro-electricity generation, fishing and boat transport.
S. molesta is spread within an aquatic system by the movement of plants by wind, water currents, floods and animals. Spread between aquatic systems is assumed to be mainly by humans moving plants intentionally (e.g. as ornamentals), unintentionally as a hitchhiker on boats, or in shipments of aquatic plants and fish. .
Proliferation of this weed and other aquatic weeds is often indicative of increased nutrient levels in watersheds and wetlands. However, sustainable management of the whole ecosystem, like decreasing the nutrient level and improving sewage drainage and effluent treatment; is likely to reduce the nutrient levels and thus the biomass of floating plants
On a positive note, many approaches have been put forward to combat this invasive species ranging from manual or mechanical removal to biological control using the Salvinia weevil; Cyrtobagous salviniae that feeds on the plant. This biological control method is being experimented in Lake Kyoga by NARO and it could be used to fight the species in other areas of the country. Chemical control using herbicides has been used in some instances although this should be the last resort as it is expensive and comes with other implications on the aquatic life. Because of the great difficulties associated with its manual or mechanical removal, chemical and biological control, regulatory prevention remains the most effective management strategy available.
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