Dietary threonine requirement of juvenile european sea bass (Dicentrarchus labrax)

Groups of 35 European sea bass juveniles, weighing 7.5 ± 0.15 g/fish (mean ± S.D.), were kept in each of 18 flow-through 200-1 tanks supplied with marine water (temperature 21.1°C; salinity 35‰). Triplicate tanks of fish were fed for 65 days six isonitrogenous (48.6% crude protein) and isolipidic (14.1% by ether extract) semipurified diets obtained from a basal mixture formulated to minimize the threonine level (0.76% by weight). The basal diet contained maize gluten (200 g/kg), herring meat (50 g/kg) and gelatin (150 g/kg) and pure L-amino acids to simulate the amino acid profile of sea bass muscle protein, excluding arginine, lysine and tryptophan, the concentrations of which were kept close to previously defined requirement levels. The six diets used to evaluate the threonine requirement were obtained by adding 0.0, 3.5, 7.0, 10.5, 14.0 and 17.5 g L-threonine/kg to the basal diet. Different regression models were used to analyse the dose-weight gain relationship and all models adequately fitted the data (RSQ > 0.92) leading to similar estimates of dietary threonine requirement (1.12-1.26 g/100 g diet; i.e., 2.3-2.6 g/100 g protein). A slightly higher requirement value was obtained when nitrogen gain was chosen as the response criteria (i.e., 1.45 g/100 g diet; 3.0 g/100 g protein). The pattern of post-absorptive plasma-free threonine concentrations of juvenile seabass was in response to threonine intake and was proven to be indicative of dietary requirement (i.e., 1.28 g/100 g diet; 2.7 g/100 g protein). These figures are consistent with the threonine requirement value of 2.6 g/100 g protein calculated by the 'ideal protein' method (e.g., whole-body threonine A/E ratio relative to the dose-response lysine requirement value previously estimated for this species). Apart from reduced growth performance, no depression in feed intake or outward pathological signs were observed in juvenile sea bass fed diets limiting in threonine. Similarly, excessive dietary threonine (up to 190% of the requirement level) had apparently no adverse effect on feed intake and growth response.