Effects of a zero exchange biofloc system on the growth performance and health of Nile tilapia at different stocking densities

Biofloc technology (BFT) has recently gained attention as an environmentally sustainable production system. In this system, bacteria convert fish waste into microbial biomass (biofloc), improving the water quality in the tank and subsequently resulting in less water use. Information on the stocking density of Nile tilapia using this technology remains scarce. This work evaluated the growth performance, survival, gill lesions, body composition, nutritional value of the floc and clinical biochemistry variables of Nile tilapia (Oreochromis niloticus) during the grow-out phase (initial body weight approximately 100 g) with different stocking densities (20, 40, 60 and 80 fish m(-3)) in zero exchange BFT. The growth performance at stocking densities of 20 and 40 individuals m(-3) was better than that of 60 and 80 individuals m(-3), especially for daily weight gain and the survival rate. The best growth performance values were 1.69 g day(1) for daily weight gain and 1.70 for the feed conversion ratio for the 20 and 40 fish m(-3) treatments, respectively. The quadratic regression model to stocking density (y = - 0.0074 x(2) + 0.7665 x - 6.8512) revealed 51.79 fish m(-3) (13 Kg m(-3)) as the maximum for a zero exchange biofloc. During the grow-out phase of Nile tilapia, there were high levels of settleable solids (>= 74 mL L-1) and nitrate (>= 1.09 g L-1) that could explain the low growth performance and changes in the clinical biochemistry variables. Reduction of crude protein in carcasses was observed at the highest densities. However, all of the stocking densities evaluated showed degenerative effects to the gill structure. In conclusion, cultivation of tilapia in zero exchange BFT is optimum at stocking densities close to 13 Kg m(-3).