Sadam Ekwalla is tired, but happy. With a big smile on his face, the 33-year-old fisherman moors his small wooden canoe on the banks of Lake Ossa, in Dizangué, in western Cameroon. Leaving at 4 a.m., he returned after more than six hours of navigation with a beautiful harvest of pink carp, part of which will be intended for sale and the rest for his family. ” I am really happy. We had difficult years when we couldn’t fish, he remembers as he untangles his net. Salvinia had invaded everything. The lake was like the pitch of a football stadium, you could no longer see the water and there was nothing you could do. Today, part of this cursed plant has disappeared. »

According to Sadam Ekwalla and eight other fishermen met by Le Monde, it all started between 2016 and 2017. The sailors saw the appearance of Salvinia molesta, a very invasive aquatic fern which doubles in size every eight to ten days, on this lake. fresh water covering an area of ??4,507 hectares where African manatees live, marine mammals classified as “vulnerable” in the list of endangered species of the International Union for Conservation of Nature (IUCN).

“We’d pass by one place and the next day everything was full of Salvinia. We cast our nets and a few hours later it had swallowed everything, relates Ernest Djocky, 53 years old. I lost two nets, almost 200,000 CFA francs [300 euros]. We didn’t even see the manatees anymore. » The situation is such that fishing becomes impossible, the canoes no longer being able to make their way through. Some fishermen leave the area, looking for new sites. Others, like Sadam Ekwalla, are reorienting themselves towards the profession of motorcycle taxi or in agriculture.

The villages bordering the lake organize cleaning days to get rid of this plant that they have nicknamed “Boko Haram”, “Satan 2” or “Devil”… But very quickly, Salvinia covers up to 50% of the lake. “Even the banks were under attack. It was unheard of, still surprises Ernest Djocky. Fortunately, thanks to Dr. Aristide’s weevils, there is an improvement. »

Nitrogen and phosphorus

Indeed, faced with the proliferation of the fern, Aristide Takoukam Kamla, marine biologist and founder in 2012 of the African Marine Mammal Conservation Organization (Ammco), an NGO specializing in the protection of marine species in Cameroon, decided to act from 2019. At the time, he was completing his veterinary doctorate with a specialization in the African manatee at the University of Florida, in the United States. This scientist, who “fell madly in love” with the marine mammals of Lake Ossa during his studies in ecology at the University of Dschang (west), noticed that Salvinia proliferated due to the enrichment of the lake with nitrogen and phosphorus.

“These are elements that plants need for nourishment,” he explains. Since Salvinia is a floating plant, it depends on nutrients found in the water. As long as the water is enriched, it develops, but if the water loses its enrichment, it dies… I did not want to give up and let the lake die. I would never have forgiven myself. »

After obtaining his doctorate in Florida, in 2019, he flew to the University of Louisiana to train with partners specializing in breeding weevils that feed on ferns. “I was convinced that biological control was the method that had to be used,” he insists. It had already had results elsewhere, including in 22 African countries including Senegal, Benin and South Africa. The Cameroonian government authorizes it to import these insects. In his small laboratory in Dizangué, he experiments by taking Salvinia from Lake Ossa, which he puts into contact with weevils in artificial ponds. The results are positive. In 2021, convinced, the State issued him the permit to release the weevils into the lake.

But the results are slow in coming and the population is growing impatient. Aristide Takoukam Kamla and his team are worried: they have the impression that the weevils they release are disappearing. They ask themselves questions, discuss with their partners in Louisiana and change their method. The insects are now released into suitable enclosures. “They stayed together long enough to reproduce well and acclimatize to the lake environment,” explains the biologist. At the end of 2022, the results appear. Salvinia changes color and turns brown. The clumps are perforated by swarms of weevils and disappear from the water surface.

The fight is not over

“The communities understood that biological control was the solution,” says Aristide Takoukam Kamla. We went from 50% Salvinia coverage in 2021 to almost 15% today. Fishing has resumed and manatees are increasingly visible in the lake. » But the fight is not over. Because Salvinia has created an environment conducive to the proliferation of another invasive plant: Rhynchospora racemosa. A resilient duo that requires significant financial and material resources. In the meantime, the most urgent thing, according to the biologist, remains to find the causes of the enrichment in nitrogen and phosphorus of the water from the Sanaga, the longest river in Cameroon.

For their part, fishermen and residents “pray that Salvinia disappears forever,” says Noé Bessinga, chief of Songueland, a village located on the edge of the lake: “This plant has caused us all the harm. Fishermen abandoned their camps. Farmers abandoned their fields on the other side of the lake because they no longer had access to them. » But with the relaunch of fishing activities, environmental defenders now fear another scourge, of which man is the only culprit: the resumption of manatee poaching.