Stocking density optimization and its impact on growth and physiological responses of Nile tilapia ( Oreochromis niloticus ) reared in hybrid biofloc-RAS culture system

A 120 days' experiment was conducted in a hybrid biofloc-RAS culture system with varied stocking densities of Nile tilapia (Oreochromis niloticus) viz., 150 fish/m3 (HBR-150), 200 fish/m3 (HBR-200), 250 fish/m3 (HBR-250) and 300 fish/m3 (HBR-300) in triplicates. The average initial weight of Nile tilapia was 4.79 +/- 0.05 g. Tapioca flour was employed to maintain the C: N ratio at 15:1 in HBR units. At the end of the experiment; the growth, digestive enzyme, and serum biochemical enzyme were analysed from the experimental fish. After 120 days of culture period, the fish were challenged with Aeromonas hydrophila. As compared to two controls with RAS-150 (150 fish/m3) and BFT-150 (150 fish/m3), all HBR treatments showed significant improvement in growth and feed utilization. The HBR-150 and HBR-200 had significantly higher digestive enzymes, albumin, globulin, and total protein; least in RAS-150. The liver and muscle tissue of Nile tilapia were analysed for LDH, MDH, AST, and ALT; and these values were found to be significantly higher at increased stocking density (HBR-300) as compared to the controls. The quadratic regression model fitted to stocking density of HBR groups {Biomass gain (y) = -0.1854 x 2 + 84.731x - 4793.3} revealed that a stocking density of 229 fish/m3 is the maximum for HBR culture system. After the challenge against Aeromonas hydrophila, significantly better survivals were observed in HBR-150 followed by HBR-200. Enhanced zootechnical performance, serum biochemical parameters, and metabolic enzyme responses were found in HBR-150 and HBR-200, and lower in the HBR-300 and RAS-150 experimental units. Profitability assessment of HBR and control groups describes low profit in high density comprared that of low stocking density. Thus, it may be concluded that a stocking density of Nile tilapia at 229 fish/m3 is ideal for a hybrid biofloc-RAS culture system (HBR) with better growth, immunity, physiological responses, and survival.