Nutrient cycle and sludge production during different stages of red tilapia (Oreochromis sp.) growth in a recirculating aquaculture system

The nutrient cycle of input feed and sludge production was evaluated for five stages of red tilapia growth in a recirculating aquaculture system. Five weight groups of red tilapia, 20±0.00 (20), 39.70±0.44 (40), 80.38±0.41(80), 113.62±1.92 (120), and 177.67±1.81 (180) g in triplicates were selected as treatments and randomly introduced to the experimental units (75 fishes/unit) and cultured for a 3-week period. The body weight and biomass of the fish were correlated with the assimilation rates of some minerals supplied by the input feed. It was estimated that red tilapia could capture on average, 11.46% Fe, 13.43% Zn, 6.81% Mn, 3.55% Cu, 26.81% Ca, 20.29% Mg, 32.53% N, 7.16% K, and 15.98% P of input feed during a culture period (from 20-200 g). The sludge settled over the hydroponic troughs could capture average rate of 23.93% Fe, 86.05% Mn, 46.17% ZN, 21.49% Cu, 15.71% Ca, 88.87% Mg, 5.55% N, 5.85% K, and 17.90% P of input feed in each experimental unit. The dry matter of sludge showed significant differences (P<0.05) among treatments and ranged from 5.00% to 10.00% of dried input feed. The concentrations of total nitrogen, phosphorus, and magnesium in water were not significantly different (P>0.05) among experimental fish groups at the end of experimental period and continuously increased during the 21-day experimental period. The electroconductivity (EC) of water continuously increased during the experimental period. The pH of water decreased in all treatments at the end of experiment. The results of this study predicted the fate of input feed nutrients in a representative recirculating system where the particular diet was used. It was also demonstrated that the aquaculture effluent carries out a large amount of nutrients, including solids form which can be accumulated in the hydroponic(s) troughs.