Optimizing salinity and stocking density for red tilapia in zero-water-exchange biofloc system: integrated performance, physiological, and economic assessment

This study investigated the interactive effects of salinity levels (0‰, 18‰, and 36‰) and stocking densities (50, 100, 150, and 200 fish/m³) on water quality, growth performance, physiological responses, and economic returns of red tilapia (Oreochromis spp., initial weight of 12.33 ± 2.51 g/fish) reared in a biofloc technology (BFT) system using saline groundwater. A 3 × 4 factorial design with 36 fiberglass tanks (1 m³ each) was employed for 6 months. Key water quality indicators, fish growth indices, hematological and biochemical markers, antioxidant enzymes, immune parameters, and economic performance metrics were assessed. Results showed that increasing salinity and density significantly reduced dissolved oxygen (DO) levels and increased total ammonia nitrogen (TAN), NH₃, NO₂, and NO₃ concentrations (p < 0.001). Biofloc volume (BFV) increased with stocking density across salinities, peaking at 44.4 ± 1.06 mL/L at 0‰ and 200 fish/m³, while higher salinity (36‰) generally reduced BFV. Variations in biofloc composition (protein 22-33%) and fish muscle composition (protein and lipid reduction at 36‰ and 200 fish/m³) indicated metabolic adjustments under stress. The highest final weight (261 ± 1.69 g/fish) was observed at 36‰ salinity with low stocking density (50 fish/m³), whereas the most favorable combination of growth rate, feed conversion ratio, and protein efficiency ratio occurred at 18‰ salinity and moderate stocking densities (100-150 fish/m³). Growth performance and feed utilization declined markedly at 36‰ with high density (200 fish/m³). Hematological indicators (RBC, Hb, Hct) and immune biomarkers (lysozyme, IgM, complement C3) were suppressed at extreme salinity-density combinations, while oxidative stress (high MDA) and hepatic dysfunction (elevated AST and ALT) were evident. Economic analysis confirmed that 18‰ salinity with 200 fish/m³ yielded the highest profit (1000 ± 54.8 EGP/treatment) and lowest operating ratio, while 150 fish/m³ at the same salinity provided slightly lower profit but better fish welfare indicators and immune responses, whereas high-density and hypersaline conditions reduced profitability due to poor growth and increased feed costs. In conclusion, 18‰ salinity combined with 100-150 fish/m³ provides the optimal balance between biological performance, fish welfare, and economic viability in red tilapia BFT systems. These findings offer evidence-based guidelines for sustainable inland saline aquaculture, supporting enhanced production efficiency and profitability in arid and saline-prone regions.