Interactive effects of rearing densities and dietary vitamin E levels on growth, stress, and immune responses, and antioxidant status in triploid rainbow trout ( O

Abstract

This study aimed to evaluate the interactive effects of rearing densities and supra‐nutritional levels of vitamin E on fish performance. Triploid rainbow trout juveniles (40.44 ± 1.27 g) were distributed into 18 tanks at medium (MD, 30 kg/m ³ , 45 fish/tank) and high (HD, 60 kg/m ³ , 90 fish/tank) densities. The trial was designed with two fish rearing densities (MD and HD), each fed 0, 200, or 400 mg/kg vitamin E‐supplemented diets (actual contents of 104.7, 303.5, and 496.8 mg/kg, respectively), a 2 × 3 factorial design. After 63 days, the final weight, body weight increase, SGR, FCR, and PER were significantly affected by both rearing density and vitamin E main effects ( p  < 0.05). Additionally, significant density × vitamin E interactions were observed for the final weight, FCR, and PER ( p  < 0.05), with HD400 fish performing comparable with MD groups. Serum cortisol levels were significantly lower in all MD groups and HD400 fish than in the other groups, whereas an inverse pattern was observed for serum osmolality ( p  < 0.05). Serum lysozyme activity and total Ig and globulin contents in MD groups were significantly higher than in HD groups (density effect, p  < 0.001), and supra‐nutritional vitamin E supplementation significantly increased these parameters (vitamin E effect, p  < 0.05). Serum alternative complement (ACH50) and liver glutathione peroxidase (GPx) activities in the MD and HD400 groups were significantly higher than in HD200, and the lowest in the HD0 group ( p  < 0.05). Liver superoxide dismutase (SOD) activity was lowest in the HD0 group. Liver malondialdehyde content was significantly lower in MD200, MD400, and HD400 than in other groups ( p  < 0.05). Among the tested levels, 400 mg/kg vitamin E supplementation ameliorated the negative impacts of high rearing density (60 kg/m ³ ) on triploid rainbow trout juveniles by improving growth, innate immunity, antioxidant capacity, and tolerance to high‐density stress.