Tryptophan supplementation helps totoaba (Totoaba macdonaldi) juveniles to regain homeostasis in high-density culture conditions.

High-density culture brings with it chronic stress situations that affect fish welfare. In order to evaluate the effect of tryptophan (Trp) levels on the response to stress, Totoaba macdonaldi juveniles were stocked at low (13.5 kg m-3) and high (27.0 kg m-3) densities (32.5 and 56.4 kg m-3, respectively, at the end of the experiment) in 100-L tanks and fed for 63 days with experimental diets containing different Trp levels: control diet CD0.42 (0.42%) and three supplemented diets with 0.99, 1.55 and 2.19% (0.99Trp, 1.55Trp and 2.19Trp, respectively) (three tanks × density × diet). The high-density stocking fed with CD0.42 diets showed significantly increased blood parameters. Trp decreased catalase (CAT) activity in low- and high-density stocking, while the superoxide dismutase (SOD) activity showed no difference. Serotonin (5-hydroxytryptamine, 5-HT) content decreased, and the serotonin turnover ratio (5-HIAA:5-HT) increased in the brains of fish fed with the CD0.42 diet. Indeed, Trp-supplemented diets helped to restore homeostasis in high-density growth conditions as evaluated by the hematological and plasma parameters as well as the serotonergic activity. When the fish were provided a diet containing moderate Trp levels, plasma cortisol increased under high-density conditions. However, no differences were observed among stock densities when totoaba were fed with the 2.19Trp diet. Notably, survival was unaffected by both Trp or densities, but weight gain (WG) decreased with the dietary Trp levels in the high density culture. In sum, Trp supplementation decreased the parameter values linked to stress response on totoaba juveniles cultured at high stock densities.