Supplementation of taurine and methionine to all-plant protein diets for rainbow trout (Oncorhynchus mykiss)

Taurine has been demonstrated to be a conditionally indispensable nutrient for several carnivorous fish species. Currently, trout feeds contain reduced levels of fishmeal which had been the primary source of dietary taurine. The reduction of fishmeal in the diet, and thus dietary taurine, may limit growth. A 12-week feeding trial was conducted using a factorial treatment design with taurine supplementation (3 levels) and methionine supplementation (3 levels) as the main effects. The diets were formulated to be fishmeal free, containing only plant proteins. Taurine was supplemented at 0, 5, and 10 g/kg dry diet and methionine was supplemented at 0, 5, and 10 g/kg dry diet in a 3 × 3 factorial design. All diets were formulated to contain 43.8% crude protein from intact protein and 20% lipid. Fifteen fish with a mean initial weight of approximately 18.4 g per fish were stocked in each of 27 tanks. The unsupplemented diet had 3.6 g of taurine/kg diet and 11.5 g total sulfur amino acids (met + cys)/kg diet. Supplementation with taurine improved growth (p = 0.096, g gain per fish), and with 5 g/kg diet an increase in feed consumption (p = 0.095) was observed. Methionine supplementation reduced growth (p = 0.010, g gain per fish) and feed consumption (p = 0.055). Neither taurine nor methionine supplementation had an effect on protein retention efficiency, energy retention efficiency or feed conversion ratio. Increased methionine intakes resulted in decreased (p = 0.009) intraperitoneal fat deposition, whereas taurine levels had no effect. Plasma GH levels were not affected by either methionine or taurine supplementation, but decreased plasma IGF-I concentrations (p = 0.009) were associated with methionine supplementation (5 g/kg diet). In conclusion, dietary taurine inclusion does appear beneficial for trout fed all-plant protein diets, and supplementation of methionine above the requirement does not appear to spare taurine.