Effect of the dietary essential to non-essential amino acid ratio on growth, feed utilization and nitrogen metabolism of European sea bass (Dicentrarchus labrax)

A study was undertaken to evaluate if dietary essential to non-essential amino acid ratio (EAA/NEAA) affects growth, feed utilization and nitrogen metabolism of European sea bass. Three semi-purified diets including fish meal and mixtures of crystalline amino acids as the main nitrogen (N) sources were formulated to have a dietary EAA/NEAA ratio of 40/60, 50/50 and 60/40; another diet, fish meal based was used as a control. All diets were formulated to be isolipidic (15% DM) and isonitrogenous (8% DM). Triplicate groups of juvenile sea bass (initial body weight of 7.3 g) were fed each diet, twice daily to apparent satiation, for 42 days.Weight gain and specific growth rate of fish fed the control, 60/40 and 50/50 diets were similar and significantly higher than those of fish fed the 40/60 diet. Voluntary feed intake of fish fed the 60/40 diet was significantly lower than that of fish fed the 50/50 and 40/60 diets. Feed efficiency and protein efficiency ratio significantly improved with the increase of dietary EAA/NEAA ratio.Whole-body composition, visceral and hepatosomatic indices were not significantly affected by diet composition, except whole-body lipid content that was significantly lower in fish fed the 40/60 diet than in the other groups.Daily N retention (g kg ABW− 1) was significantly lower with the 40/60 diet than with the other diets. As percentage of N intake, N retention was significantly higher with the 60/40 diet than with the other diets. Energy retention (kJ kg ABW− 1 day− 1 and % of E intake) significantly increased as dietary EAA/NEAA increased. Ammonia-N and urea-N excretion (mg N kg ABW− 1 day− 1) did not differ significantly among treatments. Ammonia-N excretion (% N intake) was significantly higher with diet 40/60 than with the other diets.Specific activity of glutamate dehydrogenase was significantly lower in fish fed the control and 60/40 diets than in the other groups. Specific activity of alanine and aspartate aminotransferases and glucose-6-phosphate dehydrogenase did not significantly differ among groups. Fatty acid synthetase activity was significantly higher with the 40/60 diet than with the other diets.Results of this study suggest that a dietary EAA/NEAA ratio of 50/50 is necessary to promote maximum growth performance of juvenile European sea bass. However, this ratio should be increased to 60/40 if maximization of feed, protein and energy utilization is to be considered.