Effects of dietary tryptophan levels on growth performance, plasma profile, intestinal antioxidant capacity and growth related genes in rainbow trout (Oncorhynchus mykiss) fingerlings

A 56-day feeding trial was carried out to evaluate the dietary tryptophan requirements for rainbow trout, Oncorhynchus mykiss using 360 fingerlings (1.75 ± 0.02 g/fish; 5.3 ± 0.27 cm/fish) randomly distributed into 6 tanks with six treatments and three replicates. Six isoenergetic (20.9 kj g−1 energy) and isonitrogenous (450 g kg−1 protein) diets with increasing level of tryptophan amino acids (0.40, 1.60, 2.80, 4.0, 5.20 and 6.40 g kg−1) were prepared. Fish were hand-fed twice daily at 09:00 and 17:00 h at a rate of 5% body weight per day. The results revealed that growth parameters were significantly improved with each incremental level up to 4.0 g kg−1 tryptophan containing diet. Similarly high body protein, intermediate body fat and low moisture were observed in fish offered 4.0 g kg−1 dietary tryptophan, whereas body ash content did not produce any significant (p˃0.05) difference, except at lowest fed groups. Significant (p < 0.05) improvement in hemoglobin (Hb) level, hematocrit (Hct) content, red blood cell (RBC) count, total protein, alkaline phosphatase (ALP), albumin and globulin were also noted at 4.0 g kg−1 tryptophan fed diet. Declining pattern in alanine aminotransaminase (ALT) and aspartate aminotransaminase (AST) enzymes were observed up to the optimum level. Intestinal enzymes and antioxidant variables showed a significant upward trend (p < 0.05) in fish fed diets up to 4.0 g kg−1, except amylase and malondialdehyde (MDA) which showed a declined pattern across the treatments. Relative gene expression level of TOR and 4E-BP1 in the muscle were significantly (p < 0.05) high with increasing of dietary tryptophan up to a certain level. Based on the quadratic regression analysis of growth parameters and activity of hemato-biochemical enzymes, the optimum tryptophan requirement of the fish was estimated to be 4.0 g kg−1 of dry diet, corresponding to 8.89 g kg−1 of dietary protein.