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The factors affecting fish yields in lagoons were studied through the stability, species composition and size structure of the yields at high and low fishing intensities, respectively, in Togo and the Ivory Coast. Heavy fishing was followed by the development of a plateau phase resulting from a reorganization of species assemblage. In such a state, exploitation produced only a few dominant species for which average catch lengths were small; which were generally herbivorous and exhibited a continuous reproduction, sometimes a lowering of the age at first sexual maturity, permitting a faster turnover. Even if total catches did not collapse, fish biomass and catch per unit effort decreased when fishing effort increased. One option for management could be to locate the level of fishing effort on the plateau such that fishermen can be at least financially independent and provide sufficient fish for local subsistence.