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Aquatic ferns: The silent invader of Lake Ossa.


                                             Aquatic ferns: The silent invader of Lake Ossa.

Besides its natural beauty, the Lake Ossa Wildlife Reserve is home to a diverse aquatic fauna. But for some time, when one sees the lake from the top of a hill, one notices some strange vegetation on the surface of the water. At first sight it seems completely harmless, even attractive. Originally from Brazil, its recent appearance on the lake currently represents a real danger for biodiversity.

The reed leaves are surrounded by a thick green carpet like turf on the water. As we get closer, we see that this "turf" is in fact made up of a mass of floating aquatic ferns. Its scientific name? Salvinia molesta. Called "SanTiago" or also "Boko Haram" by the residents of the Wildlife Reserve, it proliferates at a very high speed, under favorable conditions, covering a large part of the water surface. The danger of its presence: "asphyxiation" of the ecosystem and of the surrounding plant and animal species.

Its presence was noticed by fishermen as early as 2016 in Dizangue in negligible quantity. That same year, its expansion accelerated sharply. AMMCO is conducting research to accurately determine the causes of this proliferation. The first results show a significant enrichment of water in nutrients such as nitrogen and phosphate which are essential elements for the development of plants.

The first step taken to stop its spread was the mechanical extraction of the plant from the lake. Other NGOs working in the RFLO, including the Zoological Society of London (ZSL), have thus supported the fight against this threat, notably by supporting fishermen in the extraction of that invasive plant; unfortunately this was not enough to stop the expansion of Salvinia.

This plant prevents sunlight from entering the water, thereby inhibiting the development of phytoplankton on which young fries and certain types of fully grown fishes feed; this logically contributes to the reduction of the fish stock. In addition, it seems to compete with Echinochloa, a plant that is highly consumed by the African manatee, an emblematic faunal species of Lake Ossa, thus making the ecosystem less and less favorable to the development of the mammal.

This invasive vegetation, in addition to modifying and destroying the living environment of the aquatic fauna, could, according to the villagers of the area, have some toxicity and contain a substance capable of killing fish, or even the manatee. This remains a hypothesis to verify, but it seems indeed conceivable that the proliferation of salvinia, followed by its massive decomposition during the dry season would consume a large amount of oxygen present in water, thus making the environment hypoxic (insufficient in oxygen) and causing the death of large fish, a phenomenon observed regularly in the lake since this proliferation.

The risk to the fisheries sector is not negligible. The plant obstructs the lake, complicates navigation and some areas become inaccessible by canoe.

As they are floating plants, they sometimes mix with fishing nets as they pass and it becomes impossible to recover them. Matthieu, one of the fishermen of the Beach district in Dizangue, says that "Salvinia slowly occupies all the fishing grounds. We have the impression that it hunts the fish, because if a zone was full of fish, ​​as soon as it settles there, the fish becomes rare. We even think that this herb changes the quality of the water, which is why the fish rots quickly there. Sometimes, when I set my fishing line in the evening and I come to remove it at dawn, I can not find any fish, nothing but Salvinia garbage", he explains.

This situation considerably influences the profitability of the activity and the situation described above is only one case among many others. The fishermen thus see their activity constrained by the proliferation, while being for the moment powerless against a threat of that magnitude. The effectiveness of the mechanical extraction by hand remains for the moment to be proven and the fishermen do not have for the moment the equipment geared to the fight against the invasion.

Faced with the disturbing expansion of this plant in Lake Ossa, AMMCO started a preliminary mapping survey with information of the quantification and distribution of Salvinia in an attempt to estimate the percentage of recovery of the lake as well as its speed of propagation, with a goal to implement adequate, efficient and sustainable eradication measures.


                                             Aquatic ferns: The silent invader of Lake Ossa.