CHANGES IN FISH AND CRUSTACEAN COMMUNITIE S OF A TROPICAL LAGOON, LAKE TOGO, SUBMITTED TO ALTERNATE PHASES OF OPENING AND CLOSING BELT

The strong productivity of coastal lagoons is due to exchanges between sea water and freshwater. In Togo, when the belt is closed as it was in 1983-84, salinity is low (1 psu at Agbodrafo) and specific diversit y decreases (I(sh) = 2.13). On the opposite, when the lagoon is linked to the sea as it was in 1989, salinity rises everywhere (7 psu at Agb odrafo) and the fish communities are more numerous (I(sh) = 2.74). In the latter case, the fish Communities belong to marine and marine-estu arine species (Liza, Caranx, Penaeus, Gerres...) and nutrient transpor ts and fish migrations, from the sea, induce important yields (300-400 kg/ha). In 1984, after four years of closing belt, estuarine species dominate; especially Sarotherodon melanotheron which represents 50 % o f the total catches. In such a context, as fishing is intensive, conti nuous reproduction is certainly a key factor for the spreading of this species. Nevertheless, values for yields (160 kg/ha/year), maximal le ngth (250 mm), catch mean length (120 mm), length at first sexual matu rity (85 mm) are smaller than those observed in other lagoons of Niger ia and Ivory Coast. This indicates slower growth of Sarotherodon in To go. The lack of communication with the sea and the fishing pressure co uld contribute to proliferation of Sarotherodon melanotheron. The resu lting high abundance could be responsible for a diminution of growth.