The performance at varied over-stocking of Oreochromis niloticus was evaluated in a hybrid biofloc-recirculating aquaculture system (HBR) viz., 150 fish/m3 (HBR-SD150), 200 fish/m3 (HBR-SD200), 250 fish/m3 (HBR- SD250), and 300 fish/m3 (HBR- SD300) along with 150 fish/m3 (BFT-SD150) and 150 fish/m3 (RAS-SD150) each maintained in triplicate. Tapioca flour was used to maintain the carbon to nitrogen ratio of 15:1 within the HBR and BFT units throughout the experimental period of 120 days. At the conclusion of the experiment, assessments were made of productivity, biological and haematological indices; and proximate composition of the fish and the floc. Evern though significantly higher productivity was observed in HBR-SD200, a quadratic regression model applied to the productivity as a function of stocking density {Stocking density (fish/m3) = −21.008 × 2 + 532.15× - 3090.2, R2 = 0.6929} and the optimum productivity for O. niloticus (Nile tilapia) found to be 13.2 Kg/m3 at 229.89 fish/m3. Notably, HBR-SD150 groups exhibited significantly higher levels of haematological indices including NBT. The Proximate composition of the floc revealed that a significantly higher quantity of protein and ash was found in HBR-SD150 and HBR-SD300, respectively. There was no significant difference in carcass composition of Nile tilapia across the treatments except for that of crude protein levels. Profitability analysis demonstrated that low stocking density groups achieved greater profits than high stocking density HBR and control. The findings of present study indicate that the maximum production that can be accomplished from the HBR culture system is 13.2 Kg/m3 at a density of 229.89 fish/m3 without affecting the growth, immunity, and survival of Nile tilapia.
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