Objective This study aimed to investigate the effects of different stocking densities on growth performance and physiological and biochemical indexes of tilapia in an In-Pond Raceway System (IPRS), and to determine the appropriate stocking density for this culture system.

Method Three density gradients were established in separate raceways of the IPRS: Group A (164 fish/m²), Group B (136 fish/m²), and Group C. A traditional pond aquaculture (TPA) system stocked at 3 fish/m² served as the control (Group D). The growth performance, serum biochemical parameters, and antioxidant stress levels of tilapia in each group were measured.

Result After 138 days of culture, growth performance in the IPRS groups showed a clear declining trend with increasing density. Group C performed best, with final body weight (FBW), specific growth rate (SGR), and absolute growth rate (AGR) not significantly different from those of the control (P > 0.05), and superior to those of Groups A and B. Feed utilization rate was markedly improved under IPRS, as evidenced by lower feed conversion ratios (FCR) across all IPRS groups compared to the control. Serum biochemical parameters varied with density: acid phosphatase (ACP) and aspartate aminotransferase (AST) activities, as well as glucose (GLU) content generally increased from C to B and then decreased toward A, whereas the total cholesterol (T-CHO) content showed a decreasing trend. Alkaline phosphatase (AKP) and T-CHO activities were significantly higher in all IPRS groups than in the control (P < 0.05), with Group C maintaining relatively balanced activity levels. Alanine aminotransferase (ALT) and creatinine (CRE) concentrations rose steadily with increasing density and were significantly elevated in all IPRS groups relative to the control (P < 0.05). Antioxidant responses differed between tissues. In brain tissue, superoxide dismutase (SOD) and catalase (CAT) activities tended to decrease with rising density, while glutathione peroxidase (GSH-Px) activity first declined and then increased; SOD, CAT and GSH-Px in Group C was 147.84%, 143.72%, and 132.81% higher than in Group A, respectively. In liver tissue, SOD and GSH-Px activities increased with density, whereas CAT activity rose initially and then fell. Malondialdehyde (MDA) contents in both brain and liver were significantly lower in all IPRS groups than in the control (P < 0.05), indicating that the IPRS environment did not induce oxidative damage.

Conclusion Under the conditions of this experiment, a stocking density of 118 fish/m² is recommended for tilapia reared in IPRS. At this density, although serum ALT and AST levels were higher than in traditional pond culture, they remained within physiologically tolerable ranges. Fish exhibited good adaptation to the recirculating raceway environment, maintained strong growth performance, and achieved a favorable balance between production efficiency and health status.