The effect of protein levels on growth, postprandial excretion and tryptic activity of juvenile mulletMugil platanus(Günther)

Abstract

The objective of the present work was to determine the optimum dietary protein level for juvenile mul- lets. Five isocaloric diets were formulated to contain increasing levels (300, 350, 400, 450 and 500 g kg � 1 ) of crude protein (CP) corresponding to 18.7 MJ meta- bolizable energy kg � 1 . All diets were tested in tripli- cate. Each experimental unit was composed of a 50 L tank with 50 juveniles (meanSE initial weight and length equal to 1.17 � 0.02 g and 4.34 � 0.03 cm respectively). Diets were oiered ¢ve times a day until apparent satiation for 35 days. No signi¢cant diier- ence (P40.05) was observed in survival rate, feed e⁄ciency and body composition between treatments. However, weight gain, feed consumption and speci¢c growth rate were higher in ¢sh fed the 350 g kg � 1 CP level than those fed the highest protein content diet (500 g kg � 1 CP). The amount of postprandial ammo- nia excreted by mullet was linearly related to protein intake. Intestinal tryptic activity was inversely pro- portional to the percentage of dietary CP. It is likely that diets containingo350 g kg � 1 CP will be needed for on-growing mullet, especially when reared in ponds with abundant natural food.