Quantitative methionine requirement of juvenile sea-bass ( Dicentrarchus labrax)

Abstract

The methionine (MET) requirement of sea-bass was studied by the nutrient-response relationship using growth rate data and free MET levels in serum and whole blood after feeding. Two series of diets were employed, sharing a common basal diet. This basal diet contained a mixture of soybean, yeast and fish meals as protein source, and was formulated to be deficient in MET whereas the other essential amino acids levels corresponded to the pattern found in sea-bass muscle proteins. Graded levels of MET were obtained by supplementing the basal diet with crystalline MET (S series) or by substituting soybean and yeast meals by fish meal (F series). Fish were fed for 85 days on an optimal ration (1.5%/day). The MET requirement could be determined only with the F series diets using sigmoidal mathematical models. Inflexion points from curves obtained with the data for weight gain and for free MET in the blood were very similar. The MET requirement was estimated at about 1% of dry diet or 2% of dietary protein or 15 mg/day per 100 g live weight. The S series diets allowed us to determine, from growth data, the optimal level of MET supplementation: optimal growth rate occurred with 1.3% total MET content in the diet, with 0.6% in the crystalline form. Further supplementation depressed growth rate. Choices of types of diet, advantages of the biochemical method and use of sigmoidal models for computer fitting of experimental data are discussed.