Effect of different levels of synthetic tryptophan on growth performance and carcass amino acids profile of rainbow trout (Oncorhynchus mykiss) fry

In this study, the effects of different dietary levels of synthetic tryptophan on some growth parameters and carcass quality of rainbow trout was investigated. For this purpose, 120 fish (mean weight 6.5±0.4 g) were randomly distributed in three treatments with four replicates and fed with diets supplemented with 0 (control diet), 0.2%, and 0.4% tryptophan for 8 weeks. Results showed that addition of tryptophan at 0.2 and 0.4% levels significantly improved the weight gain, specific growth rate, daily growth rate, and survival compared to the control group (p<0.05). The lowest feed conversion ratio (1.01±0.04) and highest protein yield ratio (2.48±0.02) were observed in 0.4% tryptophan treatment (p<0.05). There was no significant difference in length gain and condition factor between treatments (p>0.05). The highest crude protein (61.62±1.21%) was observed in 0.4% tryptophan treatment, which was significantly different from those in control and 0.2% tryptophan treatments (p <0.05). There was no significant difference in moisture content between treatments (p> 0.05). Regarding the carcass amino acids profile, the highest amounts of methionine (1.59±0.02%), methionine + cysteine (2.15±0.03%), threonine (2.35±0.01%), isoleucine (2.37±0.04%), leucine (4.20±0.06%)و and valine (2.81±0.03%) were measured in 0.4% tryptophan treatment (p<0.05). In general, findings of the current research illustrated that most growth parameters of rainbow trout were improved by adding tryptophan to the control diet, although the fry carcass had higher amino acid and crud protein values by supplementing diet with 0.4% tryptophan.