GROWTH PERFORMANCE OF BLACK SEA BREAM Acanthopagrus schlegelii FED THE FREE OR DIPEPTIDE FORM OF LEUCINE  

Dietary leucine is essential for the optimal growth and health of fish and is necessary for protein synthesis in muscle tissues. Leucine deficiency can cause severe biochemical malfunctions that result in growth retardation. However, few studies have evaluated the effects of dietary leucine supplementation on aquatic animals. In the present study, therefore, we investigated whether juvenile black seabream could utilize the dipeptide form of leucine as a protein source and compared the efficacy of the dipeptide and free forms in terms of growth performance.

All experimental diets contained 0.4% leucine from fish meal. Six additional diets were prepared by adding incremental levels (0.3%) of two different forms of Leu to the control diet to be 0.7, 1.0, and 1.3% leucine level in diet. Leucinyl-Glycine (Leu-Gly) was used as the Leu source in a dipeptide form (D) and crystalline l-Leu was used as a free form (F). The conditioned experimental fish averaging at 3.23±0.003 g were then randomly distributed into twenty one 20 L tanks (15 fishes/tank) in a flow-through system with sand filtered seawater. Fish fed the leucine in dipeptide form (Leu-Gly) had significantly higher weight gain at all the three dietary leucine levels (0.7, 1.0, and 1.3%) than those fed free form, L-leucine. Weight gain and specific growth rate were significantly increased in fish fed the diets supplemented with free or dipeptide leucine compared to those in fish fed the control diet containing only the basal level of 0.4% leucine. The lowest feed conversion ratio (FCR) was found in fish groups fed the dipeptide diet. The highest FCR was found in fish groups fed the F-0.7% diets. The present study shows that the availability of AA could be better in fish when AA is provided as dipeptide forms than free forms, and that dipeptides can be used as AA source for AA requirement study in the fish.