Antioxidant‐related parameters,digestive enzyme activity and intestinal morphology in rainbow trout (Oncorhynchus mykiss) fry fed graded levels of red seaweed,Gracilaria pygmaea

This study was conducted to investigate the effects of dietary red seaweed, Gracilaria pygmaea level on growth, antioxidant‐related parameters, digestive enzyme activity and intestinal morphology of rainbow trout (Oncorhynchus mykiss) fry. Five isocaloric and isonitrogenous diets were formulated to contain four G. pygmaea levels (30 (GL‐30), 60 (GL‐60), 90 (GL‐90) and 120 g/kg (GL‐120)), and a control diet was used, without inclusion of seaweed. Results of the feeding trial indicated that final body (FBW: 2.8–3.5 g) and specific growth rate (SGR: 4.8–5.2) were significantly improved when 60 g/kg G. pygmaea was supplemented in the diets. However, there was a significant reduction (p < .05) of final body weight (FBW) and specific growth rate (SGR) in fish fed 120 g/kg G. pygmaea. Protease activity was significantly lower in fish fed GL‐120 diet when compared to GL‐30 or control (p < .05). Lipase was significantly affected by dietary seaweed regardless its level of supplementation (p < .05). The inclusion of the Gracilaria in the diet led to evident changes in the fish antioxidant status with significant reduction of superoxide dismutase, glutathione peroxidase activity and lipid peroxidation products. The histological study demonstrated that all groups of fish displayed normal morphology of anterior intestine and pyloric caeca. Villi absorptive area in the anterior intestine of treatment groups GL‐90 and GL‐120 significantly decreased (p < .05). Overall, the results obtained in this study indicate that dietary G. pygmaea supplementation up to 90 g/kg improves growth performance in rainbow trout fry without compromising antioxidant responses and digestive function.