Recherche et élimination des facteurs inhibiteurs de croissance dans les élevages piscicoles en circuit fermé

Recirculation of the water of fish rearing systems requires a biological treatment based on the nitrifying activity of autotrophic bacteria. In parallel heterotrophic bacterial populations utilise the organic substrate, produced by the fish, to grow. Opposite to the autotrophic populations, the heterotrophic bacteria (1) have a quick growth and (2) might contain fish pathogenic strains. The main aim was to characterise the heterotrophic bacteria populations ?.nd to understand (1) which parameters do control the dynamics of their populations and (2) their influence on the quality of the recirculating water. In a rearing system, the heterotrophic bacterial populations are stable in number and in genus, for a given operating mode. The bacteria are either fixed (7-106 UFC per g of packing in the biological filter and to 107 UFC per cm2 of pipe wall) or free in the rearing water. Three levels of free bacteria are observed in the system: 103 UFC-ml"1 at the outlet of the UV reactor, 5 10J UFC-ml1 at the inlet of the biological filter, 104 to 105 UFC-ml"1 between the outlet of the biological filler and the inlet of the UV reactor. The free bacteria originate from the fixed bacteria, which are mainly located in the packing of the biological filter. The dominant heterotrophic bacteria strains are Gram-, strict aerobic or microaerophilic bacillus. Vibrionaceae (aerobic or facultative aerophilic) are also observed. Nitrates, phosphates and humic substances accumulate in recirculating water. Fixed bacteria utilise organic matters originating from fish faeces as carbon source, as humic substances are less biodegradable. Humic substances accumulation could result in a >0% decrease of the fish growth rate. Ozone efficiently eliminates them but the concentration of total residual oxidants has to be carefully controlled avoid harming the fish gills.