Optimal dietary lysine levels for growth and protein utilisation of fingerling sea bass (Dicentrarchus labrax L.) fed semipurified diets

Duplicate groups of 60 fingerling sea bass (Dicentrarchus labrax) weighing 0.85 +/- 0.03 g were given five isoproteic and isolipidic semipurified diets (50% N x 6.25 and 14% ether extract on dry matter basis) containing graded levels of lysine and a practical (control) diet for 10 weeks. Fish were kept in twelve 60-1 tanks supplied with 2.51/min of sea water at a temperature of 25.5 +/- 1.2-degrees-C and salinity of 34.6 +/- 1.6 parts per thousand. A basal diet was formulated so that the amino acid content, excluding lysine, simulated that of sea bass muscle protein. The diet contained maize gluten 400 g/kg, herring meal 100 g/kg and gelatin 50 g/kg and mixtures of essential and non-essential pure L-amino acids which accounted for one-third of the total amino nitrogen. The five semipurified diets were obtained by adding 0, 4, 8, 12 and 16 g/kg of L-lysine HCl to the basal diet. By analysis of the dose-response relationship based on the growth data, the dietary requirement of lysine was found to be 21.7 +/- 0.10 g/kg diet (equivalent to 48.2 g/kg dietary N x 6.25). A similar value was obtained when the gross protein retention was regressed against the dietary lysine levels (22.2 +/- 0.25 g/kg diet). Growth, feed and protein utilisation of fish given the semipurified diets containing adequate levels of lysine were similar to those of fingerlings fed on the practical diet (P > 0.05). The results indicated that sea bass are able to utilise successfully large amounts of alternative protein sources such as maize gluten, providing that adequate levels of lysine and other essential amino acids are present in the diet.