Potential role of dietary red onion peel polyphenol-rich extract in enhancing growth, feed utilization, and antioxidant and immune responses in Nile tilapia (Oreochromis niloticus) cultured at high stocking density.

This study evaluated the impact of dietary inclusion of red onion peel extract (ROPE) on body composition, growth, feed utilization, immune response, antioxidant status, and economic efficiency of Oreochromis niloticus reared under high stocking density. A total of 630 mono-sex Nile tilapia (6.09 ± 0.11 g) were randomly distributed into four groups (n = 3 outdoor concrete tanks per treatment; 1 m3 water/tank). The experimental groups were: G1, low stocking density (30 fish/tank, basal diet); while G2, G3, and G4, high stocking density (60 fish/tank) supplemented with 0, 250, and 500 mg/kg ROPE, respectively. Fish were fed to apparent satiation three times daily for 14 weeks. Results showed that fish subjected to high stocking density (G2) exhibited reduced weight gain, final body weight and protein efficiency ratio, along with elevated feed conversion ratio compared to low-density group (G1). However, supplementation with ROPE significantly improved these growth and feed utilization indices (P < 0.01). Whole-body composition (moisture, protein, lipid, ash) did not differ significantly among treatments (P > 0.05). Additionally, G3 and G4 showed enhanced blood total protein and albumin levels, reduced alanine aminotransferase activity, and lower serum lipid peroxidation (P < 0.05). Antioxidant enzyme activities (catalase and reduced glutathione) and immune parameters (lysozyme activity and immunoglobulin M) were significantly elevated in ROPE-supplemented groups (P < 0.05). Economically, G4 yielded the highest final and relative profit margins among all high-density groups. In conclusion, dietary supplementation with 500 mg/kg ROPE effectively mitigated the adverse impacts of high stocking density by enhancing growth metrics, physiological health, and economic returns in Nile tilapia.