Bioactive substances such as β-carotene are regarded as an effective dietary aquafeed supplement to boost the immunity of fish cultured in high densities. The present study aimed to evaluate the influence of the dietary incorporation of β-carotene derived from Spirulina platensis (0, 0.5, and 1.0 g kg−1 diets) on growth performance, feed efficiency, hemato-biochemical indices, immunological responses, hepatic antioxidant enzymes, and associated immune-antioxidant genes expression in Nile tilapia, Oreochromis niloticus fingerlings (initial weight of 5.85 ± 0.06 g) reared at two stocking densities (SD100, 100 fish m−3 and SD200, 200 fish m−3) for 70 days. Fish were randomly allocated into eighteen plastic tanks (0.5 m3 each plastic tank) in triplicate and fed three times per day at 9:00 a.m., 11:00 a.m., and 3:00 p.m. with three isonitrogenous and isolipidic diets. The results showed that there was no difference in feed intake among the experimental diets (P > 0.05). The diet containing 0.5 g kg−1 β-carotene increased weight gain, final body weight, specific growth rate, and feed conversion ratio of fish reared at SD200. Fish grown at SD100 or SD200 showed a significant increase in hematocrit and hemoglobin levels when fed a diet supplemented with 0.5 or 1.0 g kg−1 β-carotene compared to those on an unsupplemented diet (P < 0.05). Alanine amino transferase and aspartate amino transferase levels were lowest in fish group fed diet containing 1.0 g kg−1 β-carotene at SD100 as well as in fish fed diet supplemented with 0.5 and 1.0 g kg−1 β-carotene at SD200. Fish raised at SD200 and supplemented with 1.0 g kg−1β-carotene showed significantly improvements in total protein, albumin, and growth hormone levels compared to those fed on an un-supplemented diets (P > 0.05). Supplementing with 0.5 or 1.0 g kg−1 β-carotene diets significantly increased the concentrations of C3, C4, and IgM in fish reared at both SD100 and SD200 compared to those without supplementation (P < 0.05). The hepatic antioxidant activity of malondialdehyde was lowest in fish-fed diet supplemented with 1.0 g kg−1 β-carotene at SD100. At SD200, fish fed a diet supplemented with 1.0 g kg−1 β-carotene showed the highest levels of glutathione (GSH) and glutathione peroxidase (GPx) activity (P < 0.05). The highest activity of catalase (CAT) was in fish fed diet supplemented with 0.5 g kg−1 β-carotene at SD200 and SD100 whereas, the highest activity of superoxide dismutase (SOD) was in fish fed diet supplemented with 1.0 g kg−1 β-carotene at SD100. The same trend was seen in the associated hepatic genes expression of superoxide dismutase 2, CAT, and GPx. When compared to other diets, fish raised at SD100 and fed a diet supplemented with 0.5 g kg−1 β-carotene showed the highest levels of gene expression for interleukin (IL)−1β and interleukin-8 (IL-8) (P < 0.05). But those reared at SD200 and fed a diet without β-carotene supplementation had the lowest transcript expressions of IL-1β and IL-8. In conclusion, adding β-carotene up to 1.0 g kg−1 to the basal diet improves and boosts growth performance, feed efficiency, immunological responses, hepatic antioxidant enzyme, and associated immune-antioxidant gene expression in Nile tilapia reared at SD200.
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