An artificial productive ecosystem based on a fish/bacteria/plant association. 1. Design and management

An artificial ecosystem integrating three biological compartments (fish, bacteria, plants) in a closed system was developed with the aim of associating fish production with a vegetable crop purifying the fish water. 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 source 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 greenhouse to enable continuous production throughout all seasons. The fish chosen were tilapias (Oreochromis niloticus) and the plants were tomatoes (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 ecosystem, by the size of the different elements (fish tank, bacterial filter, hydroponic troughs) as well as by the choice of the recirculating water now rate. During the first production cycle, we followed the evolution of the physico-chemical characteristics of the water and of the plant tissues, especially the nitrogen (NH3, NO2, NO3) and mineral compounds (K, Ca, Mg, SO4, PO4), in order to evaluate the functioning of the three compartments and to progressively develop the management of the plant compartment. The latter determined the overall equilibrium of the ecosystem by its capacity to absorb NO3 and NH4 in the recirculating water. The results were satisfactory as there was a stabilization 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 conditions 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 complement to obtain optimum plant growth. The nature of the mineral complement will depend on the composition of the water available on the production site.