The "fish reservoirs" of the Arcachon Basin (France) are artificial ponds created in lagoonal marshes. Opportunistic populations of microorganisms and of meiofauna increase the tendency to create dystrophic crisis. Fry stocking of soles (Solea vulgaris ) into the fish ponds have been made to estimate the influence of the predatory activity and bioturbation on the food web. For 10 soles/m super(2), the abundance of opportunistic populations of meiofauna decreases and this result indicates a modification in competitive interactions contributing to a better spatial heterogeneity in the ecosystem. These results could be used in aquaculture ponds to regulate opportunistic microorganisms and meiofauna.