Between 20,000 and 30,000 hectares of one of the sources of Blue Nile are infested by this floating aquatic plant. An invasion that threatens the survival of this site of which three million people depend.
This spring morning, dozens of young people, cold water up to their knees, move what looks like thick green carpets invading the northeastern shore of Lake Tana, the largest in Ethiopia. "They work every day for five hours to get rid of the water hyacinth"says the head of the kebele, the smallest administrative subdivision in the country.
Despite its beautiful appearance, this floating aquatic plant with lilac flowers is a real scourge for one of the sources of the Blue Nile. "We used to fish, people drank water from the lake, but now it dries quickly", storm Amsalu Addis, 36, one of the paid workers 100 birr (3 euros) a day.
Nobody really knows how this extremely invasive plant from South America came to Ethiopia almost eight years ago. Ethiopian researchers make assumptions: Tana may have been contaminated with used fishing gear carrying water hyacinth fragments from Sudan or Egypt, or by birds from Lake Victoria, the largest in Africa bordered by Kenya, Uganda and Tanzania, also affected.
On average, between 20,000 and 30,000 hectares of Lake Tana are infested with water hyacinth, which would double its surface in one to three weeks. It can cover up to 50,000 hectares during the high season in September-October.
According to researcher Ayalew Wondie, a biologist at the University of Bahir Dar, capital of Amhara Regional State, its rapid spread is primarily the result of poor practices of local residents such as "Recession agriculture (Soil work after flooding), free grazing and the discharge of urban chemical and agro-chemical pollutants ". He deplores the recent use of fertilizers and pesticides by farmers, which promotes the proliferation of the aquatic plant in water that has become very rich in nutrients.
Manual lifting of mass
The presence of this plant is a threat to biodiversity: it prevents the penetration of light and reduces the rate of oxygen in the water at the risk of asphyxiating certain species of fish such as tilapia. It also causes considerable water loss by evapotranspiration. It is also a threat to the activity of hydroelectric dams and irrigation systems. It finally complicates navigation and fishing because nets and engines are taken in its roots; hundreds of fishermen would have abandoned their activities.