Dietary tryptophan requirements and deficiency effects of olive flounder Paralichthys olivaceus at low water temperatures.

The olive flounder (Paralichthys olivaceus) is an economically important species in South Korea. However, the specific dietary requirement for tryptophan (Trp) has not been determined for this species at any growth stage. Thus, this study aimed to determine the dietary Trp requirement for juvenile olive flounder under low water temperature conditions. Two consecutive feeding trials were conducted, each using diets with different concentrations of dietary Trp: Exp-Ⅰ (0.39-14.46 g kg^-1) and Exp-Ⅱ (0.93-6.96 g kg^-1). The initial mean body weights of the olive flounder were 4.38 ± 0.03 g (Exp-Ⅰ) and 18.4 ± 0.1 g (Exp-Ⅱ). In Exp-Ⅰ and Exp-Ⅱ, fish were randomly distributed into 18 tanks (120 L), corresponding to six dietary groups with three replicates per group (30 fish per tank). Fish were fed the experimental diets for 13 weeks (Exp-Ⅰ) and 8 weeks (Exp-Ⅱ). The water temperatures during the trials were 18.2 ± 2.4 ℃ in Exp-Ⅰ and 17.5 ± 2.8 ℃ in Exp-Ⅱ, representing suboptimal thermal conditions. The results indicate dietary Trp concentration significantly affected the growth and survival of the fish. In particular, the lowest growth and survival were observed in fish fed the diet with the lowest concentration of Trp in both feeding trials. Additionally, the fish fed the Trp-deficient diet (0.39 g kg^-1) exhibited lower levels of hematological parameters, immunity, antioxidant capacity and whole-body crude protein compared to Trp supplemented groups (2.33-14.46 g kg^-1). In fish fed a Trp-deficient diet (0.39 g kg^-1), expression of tight junction proteins was down-regulated, abnormal development of mucosal folds was observed, and intestinal inflammatory cytokines were upregulated compared to Trp supplemented groups (2.33-14.46 g kg^-1). Fish fed Trp-supplemented diets showed significantly higher plasma levels of Trp, serotonin, melatonin and cortisol compared to those fed the Trp-deficient diet. The dietary Trp concentration required for fish growth was estimated to be 3.08 g kg^-1 (6.06 g kg^-1 of protein) in the juvenile stage (Exp-Ⅰ) and 2.17 g kg^-1 (4.27 g kg^-1 of protein) in the post-juvenile stage (Exp-Ⅱ), respectively, as determined by broken-line regression based on final body weight.