Background Lake Ossa
Lake Ossa is one of the largest lakes in Cameroon, with a surface area of about 4,000 ha. It is located approximately 50 km inland from the Gulf of Guinea and about 12 km west of the city Edea. The morphology of the lake is very complex, and can be subdivided into four main parts. The maximum width of the shallow lake is 7 km, while the maximum depth is 7 m during the rainy season. The lake is connected to the River Sanaga (the largest and longuest river of the country) by a 3 km longmeandering channel. The river discharge rate controls the water level and the water characteristics of the lake.
The apparent color and Secchi depth of the lake vary with location and season. In the southeast, water is opaque and has a brown, muddy color due to the influence of the Sanaga River during the rainy season. Water in the north and west has a better visibility with a dark apparent color caused by its black muddy bottom.
Lake Ossa is also a habitat for many wildlife species including manatees, freshwater turtles, crocodiles, monitor lizards, snakes and many aquatic birds species. The African manatee, the most charismatic species of the lake feed mainly on macrophyte beds. The availability of macrophytes is strongly dependent on water levels of the lake.
More than 50 species of emergent macrophytes dominate the aquatic vegetation. Antelope grass (Echinochloa pyramidalis) is the most dominant macrophyte followed by seashore paspalum (Paspalum vaginatum) and bia (Sclerosperma manii). The terrestrial and island forests aredominated by two species: Lophira alata (a tree also known as Azobé or red ironwood) and Saccoglottis gabonensis (bitter back tree). The natural forest was largely reduced by agro-industry operations that occupy the largest proportion of the reserve to grow rubber plants (Hevea sp.) and oil palm (Elaeis guneensis). These agricultural practices have adverse impacts on the drainage basin downstream from the lake, which is exposed to erosion and landslides. The rest of the land is used by the local population for subsistence agriculture through seasonally farmed plantain (Musa spp), cassava (Manihot esculenta), taro (Colocasia esculenta), etc.
Although the limnology of Lake Ossa is still largely unknown, it is clear that the lake is an important habitat for over 18 families of fish, dominated by Cichlidae and Mormyridae. Fish is the main protein source for the couple of thousand people who live around the lake (local population around the lake). About 400 fishermen are actively fishing in lake Ossa using a variety of fishing techniques including gillnets, purse seines and Chinese bamboo traps. Fishermen have noticed a drastic ‘’generational’’ decrease in fish stocks as many reported that their parents harvested a higher amount and larger sized fish with very little effort, compared to the present. Although no empirical study has been conducted so far, the reports from fishermen are an indication that the fishing industry of Lake Ossa has become unsustainable, as a result of increased fishing effort and/or decreased habitat quality.