Evaluation of longan (Dimocarpus longan) peel powder as fruit by-product additive in Nile tilapia (Oreochromis niloticus) feed: Effects on growth, immunity, and immune-antioxidant gene expressions

The study, herein, investigated the effects of the inclusion of longan peel (LP) powder in the diet of Nile Tilapia (<i>Oreochromis niloticus</i>), focusing on comparative evaluations of growth performance, immunity, and immune-antioxidant related gene expressions. For this purpose, a total of 300 healthy fish (average initial weight: 13.70 ± 0.06 g) were randomly distributed into five experimental treatment groups: 0 g kg^-1 (LP0), 10 g kg^-1 (LP10), 20 g kg^-1 (LP20), 40 g kg^-1 (LP40), and 80 g kg^-1 (LP80), all in triplicate, for 60 days. The results indicated that dietary supplementation with LP exhibited a significant influence (<i>P < 0.05</i>) in weight gain (WG), specific growth rate (SGR) and provided a better feed conversion ratio (FCR) in contrast to the control group (0 g kg^-1 LP). Moreover, skin mucus and serum immune parameters (lysozyme and peroxidase activity) were significantly higher (<i>P < 0.05</i>) in fish fed with different LP concentrations at both 4 and 8 weeks. Similarly, analysis of mRNA transcripts of immune (<i>IL-1, IL-8,</i> and <i>LBP</i>) and antioxidant (<i>GSTa, GPX,</i> and <i>GSR</i>) gene expressions showed a significant upregulation (<i>P < 0.05</i>) in LP-fed diet groups compared to the control group. Based on the polynomial regression analysis the inclusion of LP at 46-49 g kg^-1 can be used effectively in Nile tilapia diets for improving the growth, immunity, and immune-antioxidant gene expressions. All in all, our results prove that LP is a very promising feed supplement for the Nile tilapia in the context of aquaculture.