The optimal rearing density enhances fish performance, but there is little information about the effect of rearing densities on triploids. This research aimed to estimate the impacts of rearing densities on the growth, immunity, and antioxidant activity of triploid rainbow trout. A total of 1071 fish (35.16±1.10 g) were randomly stocked into 18 fiberglass tanks and divided into 6 groups based on rearing density (RD), including 10, 20, 30, 40, 50, and 60 kg/m3. After 70 days, final weight, weight gain rate, specific growth rate, and protein efficiency ratio were significantly higher and feed conversion ratio was significantly lower in rainbow trout reared at 20, 30, and 40 kg/m3 in comparison with fish reared at 50 and 60 kg/m3 (P < 0.05). Serum cortisol level was highest in fish reared at 60 kg/m3, then RD50 group, and lower in other groups; cortisol level of RD20 and RD30 groups was also significantly lower than the RD10 fish (P < 0.05). Moreover, serum lactate level was significantly higher and serum total antioxidant capacity and lysozyme activity and liver superoxide dismutase activity were significantly decreased in fish reared at 50 and 60 kg/m3 compared to 20, 30, and 40 kg/m3 (P < 0.05). Serum osmolality and total immunoglobulin content and liver catalase and glutathione peroxidase activities were significantly decreased in fish reared at 10, 50, and 60 kg/m3 compared to other groups (P < 0.05). Serum alternative complement (ACH50) activity and globulin content were also significantly lower in RD50 and RD60 groups compared to the other groups (P < 0.05). In addition, liver malondialdehyde content was highest in RD60 group, then RD50, followed by RD10, and lowest in other groups. Overall, the optimal rearing densities for juvenile triploid rainbow trout can be suggested to the fish farmers at intermediate densities of 20-40 kg/m3.
