Impact of acadja fisheries on the population dynamics of Sarotherodon melanotheron and Hemichromis fasciatus in a Lake Nokoue (Benin, West Africa)

In Lake Nokoue fishermen have developed the acadjas system which operates as an extensive aquaculture practice. Little is known about the population dynamics of fish fauna which colonizes those acadjas. Therefore, population parameters of two cichlids of Lake Nokoue, Sarotherodon melanotheron and Hemichromis fasciatus, sampled in areas within and without acadjas were investigated using length-frequency data collected between June 2003 and December 2004. For the two species, asymptotic length, L-infinity was higher within than without acadjas (26.8 cm and 24.1 cm respectively for S. melanotheron; 18.5 cm and 16.5 cm respectively for H. faciatus). K and Phi' values recorded outside acadjas were higher than inside acadjas for H. fasciatus whereas the same values were very slightly different without and within acadjas for S. melanotheron. H. fasciatus is a fish predator and branches or woody debris of acadjas are not favourable for its hunting activities. The total and natural mortality rates for the two species were higher outside than inside acadjas showing so the role of protection insured by acadjas systems. Acadjas have more impact on H. fasciatus than on S. melanotheron. A possibility of management is to reorganize the distribution of acadjas over Lake Nokoue in order to keep some areas in which no acadjas would be allowed for fish species that growth is better without acadjas.