Effects of dietary tryptophan supplementation and stocking density on growth performance and stress responses in rainbow trout (Oncorhynchus mykiss)

The aim of the present study was to investigate the effects of dietary tryptophan (Trp) supplementation on growth performance and stress responses of rainbow trout (Oncorhynchus mykiss) at low (LD) and high (HD) stocking density. For this, three diets were formulated by supplementing 0% (control), 0.5% (T0.5), and 1% (T1) Trp corresponding to 0.77, 1.85 and 3.14 g Trp per 100 g protein. In the LD treatment, the fish were initially stocked at 15 kg/m(3); whereas, in the HD treatment, the fish were initially stocked at 25 kg/m(3). The fish were reared under these densities and with the aforementioned diets for 70 days. After the feeding trial, the fish were sampled to determine carcass proximate composition, and blood hematological and biochemical parameters. The results showed that T0.5 had higher growth performance compared to the other treatments under the LD condition; however, there was no significant difference among the treatments under the HD condition. The increase in the fish stocking density significantly increased FCR, but the Trp-treated treatments had significantly lower FCR compared to the control treatment. Increase in the fish stocking density significantly increased carcass moisture, but decreased the carcass protein and lipid levels. The T0.5 treatment had significantly higher carcass lipid levels compared to the other treatments at the LD condition. The T0.5 treatment had significantly lower cortisol and glucose, and higher T3 levels compared to the other treatments under the LD condition; however, there was no significant difference among the treatments under the HD condition. Blood RBC and hemoglobin values significantly increased in the fish fed with 0.5% Trp supplementation; moreover, the increase in the fish stocking density significantly increased these parameters. The T0.5 treatment had significantly higher blood hematocrit levels compared to the other treatments at the LD condition. In conclusion, dietary Trp supplementation at 0.5% level is beneficial to augment growth performance in rainbow trout, when the fish initial biomass is 15 kg/m(3). The mechanism of growth promotion effects of 0.5% Trp was found to be related to stress suppression. Further studies would provide data about the potential effects of Trp on rainbow trout under higher stocking densities.