Deciphering the phyto-immunomodulatory potential of orange (Citrus sinensis) peel in goldfish (Carassius auratus) diet against Aeromonas hydrophila

Abstract

The increasing demand for sustainable and functional feed additives in aquaculture has driven interest in agro-industrial by-products such as orange (Citrus sinensis) peel, which are rich in bioactive compounds. This study investigated the effects of dietary orange peel supplementation on growth performance, digestibility, physiological responses, and disease resistance in goldfish Carassius auratus. Fish were fed diets containing 0% (control), 2.5%, 5%, and 7.5% orange peel for 60 days, followed by analyses of growth, digestive enzymes, haematological, biochemical, immunological parameters, and a pathogen challenge test. Results showed that the 2.5% inclusion level significantly enhanced growth, with approximately two-fold higher weight gain and improved feed conversion ratio (ANOVA: p < 0.05). Protein digestibility increased by ~20% in 2.5% and 5% groups, while digestive enzyme activities, particularly amylase (~120% increase) and protease (~25% increase), were markedly elevated. Haematological indices (RBC, HB, HT) improved significantly (p < 0.001), while glucose, triglycerides, and cholesterol were reduced in higher inclusion groups (p < 0.001). Antioxidant (SOD, CAT) and immune responses (NBT, MPO) were significantly enhanced (p < 0.001), indicating improved oxidative stress resistance and immune competence. Carotenoid deposition increased significantly in muscle (~24%) and skin (~23%). Following Aeromonas hydrophila challenge, survival improved up to 1.7-fold in supplemented groups (p < 0.05). Overall, the results demonstrate that orange peel, particularly at 2.5% inclusion, serves as an effective, sustainable functional feed additive that enhances growth, physiological health, and disease resistance, offering practical implications for improving aquaculture productivity and resilience.