Effects of molluscivorous fish, water quality and pond management on the development of schistosomiasis vector snails in aquaculture ponds

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One of the possible negative side-effects of aquaculture in tropical areas is the spread of water-related diseases, in particular schistosomiasis. Fish ponds can provide breeding sites for freshwater snails transmitting schistosomiasis (Berrie 1966). Scientific attention has long been concentrated on the use of molluscicides as the most effective means of snail and schistosomiasis control. However, molluscicides have limited use in aquaculture as they are expensive, hazardous to many other forms of aquatic life including fish, and have to be applied regularly because of the rapid recolonization by snails. This paper describes a study made on snail populations in an aquaculture station in northern Cameroon and their possible relation with water quality, pond management and cultured fish species. Together with production experiments, the development of snail populations in the fish ponds has been monitored during 2 years, and the biological control potentiality of two fish species has been tested. The experiments are part of a larger programme on the integrated control of schistosomiasis in the upper Benue Valley of northern Cameroon (Slootweg 1991). Three species of fish were used in the pond experiments. The tilapia, Oreochromis niloticus (L.), was used for production experiments under different nutritional regimes. To control the excessive reproduction by O. niloticus, the African catfish, Clarias gariepinus Burchell, was introduced in several experiments. C. gariepinus is also known to eat snails; based on stomach contents analyses of fish caught in an irrigation scheme in Sudan, Coates (1984) suggested using C. gariepinus in the control of schistosomiasis vector snails. As a third species, the East African cichlid fish Astatoreochromis alluaudi Pellegrin was introduced solely to test its capability to control molluscs (Slootweg 1989). In the literature A. alluaudi has often been recommended as a means of biological snail control, but the number of actual field trials is very limited. One experiment in Xenya showed a significant reduction in snail populations over a long period (McMahon 1960; McMahon, Highton & Marshall 1977), although contradictory evidence is given by recent research (Kat SL Kibberenge 1990). One successful experiment has been described from an aquaculture station near Yaounde in Cameroon (Bard & Mvogo 1963; Mvogo & Bard 1964), but has never been repeated afterwards.