AN ARTIFICIAL PRODUCTIVE ECOSYSTEM BASED ON A FISH BACTERIA PLANT-ASSOCIATION .1. DESIGN AND MANAGEMENT

An artificial ecosystem integrating three biological compartments (fis h, bacteria, plants) in a closed system was developed with the aim of associating fish production with a vegetable crop purifying the fish w ater. The nitrogenous compounds excreted in dissolved form by the fish , and transformed by the bacteria, provide nitrogen nutrition for the plants. This association has the double advantage of savings in water for fish culture and the recycling of fish excretion as the main sourc e of minerals for producing edible plants. A pilot system of 2 m3 was set up for intensive animal and plant production and installed in a gr eenhouse to enable continuous production throughout all seasons. The f ish chosen were tilapias (Oreochromis niloticus) and the plants were t omatoes (Lycopersicum esculentum) grown according to the nutrient film technique in recirculating hydroponics. A granular filter bearing the nitrifying bacteria was inserted between the fish tank and the plants . The system's design was aimed at optimizing the functioning of the e cosystem, by the size of the different elements (fish tank, bacterial filter, hydroponic troughs) as well as by the choice of the recirculat ing water flow rate. During the first production cycle, we followed th e evolution of the physico-chemical characteristics of the water and o f the plant tissues, especially the nitrogen (NH3, NO2, NO3) and miner al compounds (K, Ca, Mg, SO4, PO4), in order to evaluate the functioni ng of the three compartments and to progressively develop the manageme nt of the plant compartment. The latter determined the overall equilib rium of the ecosystem by its capacity to absorb NO3 and NH4 in the rec irculating water. The results were satisfactory as there was a stabili zation of the nitrogenous compounds, in particular NO3, at a low level and a large plant production; in this first trial no attempt was made to improve the animal production. This trial highlighted the main con ditions to ensure the equilibrium of the ecosystem: size relationship between the three interacting compartments, dynamic management of the plant compartment (staggered crops) and the application of a mineral c omplement to obtain optimum plant growth. The nature of the mineral co mplement will depend on the composition of the water available on the production site.