Long-term effects of stocking density on biofloc quality, growth performance, and physiological responses of red tilapia in a hypersaline groundwater biofloc system

This study investigated the long-term effects of stocking density on biofloc quality (water quality and proximate composition), growth performance, feed utilization, and physiological responses of red tilapia (Oreochromis sp.) reared in a biofloc technology (BFT) system at hypersaline conditions (36 ppt). A total of 2,325 fingerlings (initial body weight: 10.90 ± 0.14 g and initial total length: 7.2 ± 0.3 cm) were randomly assigned to four stocking densities (75, 150, 250, and 300 fish m^–3) in triplicate and reared for 180 days in 12 concrete tanks (1 m³ each). Increasing stocking density significantly impaired water quality, reducing dissolved oxygen while elevating total ammonia nitrogen, nitrite, and total suspended solids. Biofloc crude protein content increased significantly with stocking density, from 25.3% at 75 fish m^–3 to 30.2% at 300 fish m^–3, accompanied by a significant rise in crude lipid from 4.75% to 5.79%. Growth performance and feed utilization efficiency (final weight, weight gain, specific growth rate, feed conversion ratio, protein efficiency ratio) were significantly higher at lower densities (75–150 fish m^–3), whereas total biomass yield increased with density, reaching 61.35 kg m^–3 at 300 fish m^–3. High-density groups (250–300 fish m^–3) exhibited physiological stress and oxidative damage, evidenced by elevated cortisol, liver enzymes (aspartate aminotransferase, alanine aminotransferase), and malondialdehyde, along with reduced innate immune parameters (IgM, lysozyme) and antioxidant enzyme activities (superoxide dismutase, catalase, and glutathione peroxidase). These findings indicate that although higher densities maximize total yield, they compromise fish health and immune competence. Based on integrated welfare and productivity metrics, stocking densities of 75–150 fish m^–3 are recommended for sustainable production of red tilapia in hypersaline BFT systems.