Lysine requirement and efficiency of lysine utilization in turbot (Scophthalmus maximus) juveniles

A study was undertaken to estimate dietary lysine requirement and efficiency of utilization in turbot juveniles. Seven isonitrogenous (8.0% N DM) and isolipidic (15% DM) diets were formulated to contain half of dietary nitrogen provided by intact protein and the other half provided by crystalline amino acids. Experimental diets included graded levels of lysine ranging from 2.5 to 6.4 g 16 g⁻¹ N. Step-wise lysine levels were achieved at the expense of sodium glutamate. Except for lysine, overall amino acid profile of the experimental diets was made similar to that of the whole-body composition of turbot juveniles. Each experimental diet was fed by hand, twice daily, to apparent visual satiation, to triplicate groups of 17 fish (initial body weight of 18.1 g) for 8 weeks. Final body weight and weight gain (g kg ABW⁻¹ day⁻¹) of fish increased with the increase of dietary lysine levels. Voluntary feed intake was not affected by dietary lysine level except for group fed the 3.5% lysine diet which was higher than in groups fed the 2.5 and 3.2% lysine diets. At the end of the trial, there were no significant differences among groups in whole-body protein content, AA composition or visceral index. Hepatosomatic index linearly decreased with the increase of dietary lysine level. N retention (g N kg ABW⁻¹ day⁻¹ or % N intake) linearly increased with dietary lysine level but specific activity of glutamate dehydrogenase was not significantly affected by dietary treatments. However, lysine utilization efficiency (expressed as lysine retention in percentage of lysine intake) was inversely related to dietary lysine level, indicating diminishing returns to additional lysine intake as the maximum response was approached. A four-parameter saturation kinetic model was used to fit weight gain (g kg ABW⁻¹ day⁻¹) against dietary lysine level. Based on that model, dietary lysine level meeting 95% of the lysine requirement for maximum weight gain of turbot juveniles was estimated to be 5.00 g 16 g⁻¹ N. Based on this value and on the A/E ratio determined from turbot whole-body amino acid profile an estimation of the EAA requirements of turbot juveniles was calculated.