Response of European sea bass (Dicentrarchus labrax) to graded levels of methionine (total sulfur amino acids) in soya protein-based semi-purified diets.

The dietary methionine (Met) and total sulfur amino acid (TSAA) requirements of European sea bass (Dicentrarchus labrax) (initial body weight 13.4 (SD 0.2) g) were estimated in a 12-week dose-response experiment. Seven isonitrogenous (7.6 % DM) and isoenergetic (gross energy, 21.2 MJ/kg DM) diets, based on soya protein and crystalline L-amino acids containing graded levels of L-Met (1.6-16.2 g/kg) at a constant cysteine (4 g/kg) level and a fish meal-based diet, were fed each to triplicate groups of fifty fish kept in 250 litre tanks in a thermoregulated (23 +/- 0.5 degrees C) seawater system. The Met and TSAA-deficient diet resulted in higher mortality, impaired feed intake and growth relative to the other treatments (P < 0.01). No signs of lens opacity due to limiting Met intake were observed and no feed intake or growth depression occurred at the highest level of dietary TSAA. Met and TSAA requirements for optimal N deposition or weight gain as fitted with the broken-line model resulted in estimated values of 8.0 and 12.0 g/kg diet (for example, 1.8 and 2.7 % dietary protein) and 9.1 and 13.1 g/kg diet (for example, 2.0 and 3.0 % dietary protein), respectively. Plasma levels of Met, homocysteine and cysteine increased in response to excess dietary TSAA, corroborating requirement estimates from growth data. N gain resulted in a linear function of TSAA consumption at marginal Met (TSAA) intake. The TSAA intake needed to maintain N balance resulted in a value of 20.0 mg TSAA/kg average body weight0.75 per d, which represents 23 % of the total (maintenance+accretion) requirement.