Lake Ossa, located in the Littoral region on the coast of Cameroon, has been plagued for more than five years by the invasion of the Salvinia molesta plant, commonly known as water fern. By 2021, almost 50% of the lake’s 4,000 hectares were covered by this plant. Since then, teams from the African Marine Mammal Conservation Organization (AMMCO) have been trying to implement measures to eradicate these invasive species.
The likely cause of this proliferation is a sudden increase in man-made nutrients from upstream in the Sanaga catchment area, such as nitrogen and phosphorus. The consequences of this phenomenon are both ecological and socio-economic : obstruction of shipping lanes, reduced catches by fishermen, destruction of nets and competition with the manatee’s main source of food. This observation prompted AMMCO to work to reduce or even eradicate this plant from Lake Ossa. With the support of Louisiana State University, the IUCN, Save Our Species and the PBNF, AMMCO has initiated biological control using a natural predator : the Salvinia weevil, Cyrtobagous salviniae.
Avec le soutien du gouvernement Camerounais, des charançons ont pu être importés des États-Unis. Ainsi, en juillet 2021, 1 000 charançons ont été introduits dans une parcelle pilote dans un coin du lac. Des sorties mensuelles sur le terrain ont permis de suivre l’évolution du charançon, son impact et les paramètres physico-chimiques de l’eau.
The first few months seemed fruitless because it was difficult to detect signs of weevil feeding or other signs of plant degradation. In February 2022, the presence of the very first weevils was confirmed and signs of feeding observed.
The green carpet of salvinia has turned brown, indicating a state of stress in the plant.
Today, the weevils are multiplying exponentially and can be seen in most parts of the lake. Their estimated density is 4 to 30 weevils per kg of Salvinia (April 2023). Satellite analysis also shows a 70% reduction in Salvinia cover across the whole lake. Another method of combating Salvinia is to remove it by hand and turn it into ecological charcoal, a way of adding value to this unattractive plant.
These combined actions seem to be having a positive impact, as manatees have been observed on several occasions in the lake this year, after more than two years without any direct sightings. As far as fishermen are concerned, fish catches have improved and the navigable areas of the lake are increasing. As for the potential negative impacts (human health, fish, etc.), the monitoring carried out by AMMCO and the observations of residents have reported none to date. One of the current challenges in the fight against the weevils remains their spread in inaccessible areas with a high density of Salvinia.
However, it is important to realise that biological control only makes it possible to react to the visible effects of Salvinia without acting at source. With the support of the PPI Programme, AMMCO has initiated a study to analyse and understand the sources of nutrients in Lake Ossa. In the short to medium term, this will enable us to take action and advocate for better Integrated Water Resource Management (IWRM) in the Sanaga catchment area.