STUDY ON THE OPTIMAL LEVELS OF DIETARY LEUCINE AND ISOLEUCINE FOR JUVENILE CHINESE MITTEN CRABS, ERIOCHEIR SINENSIS

We conducted two sets of experiments to determine the proper levels of leucine and isoleucine in the diet for juvenile Chinese mitten crabs, Eriocheir sinensis. The juvenile crabs with initial body weight of (0.900.02) g were fed with test diets for 60 days. The protein source of the test diets included casein, gelatin, fish meal and crystalline amino acids. The test diets were divided into 6 groups according to their graded levels of leucine (Leu-1~Leu-6: 0.87, 1.26, 1.64, 2.03, 2.39 and 2.81 g per 100 g dry diet), and according to the levels of isoleucine (Ile-1~Ile-6: 0.69, 1.21, 1.70, 2.19, 2.70 and 3.21 g per 100 g dry diet). The results showed that when the dietary leucine level was 2.39% (Leu-5), the crab exhibited the highest specific growth rate (SGR) and the highest crude protein content of the whole body. No significant differences were found in the survival rate (SR), the moisture, the crude lipid, and ash of the whole body between all Leu groups (P 0.05). The leucine, essential amino acids and total amino acids in muscles of the crabs in groups Leu-5 (2.39% Leu) and Leu-6 (2.81% Leu) were significantly higher than those in other groups (Leu-1 to Leu-4) (P 0.05). In terms of isoleucine, we observed the highest values in SGR and in the crude protein content of the whole body when the dietary isoleucine concentration was 2.19% (Leu-5). There were no significant differences between all Ile groups in terms of the SR, the moisture, the crude lipid, and ash of the whole body (P 0.05). Along with the increase in the Ile level in diets, the isoleucine, essential amino acids and total amino acids in crab muscles first increased and then decreased. The peak values of the parameters above were reached in Ile-5, but there were no significant diffe- rences between Ile-5 and Ile-4 (P 0.05). The broken-line model for the SGR against dietary amino acid levels was applied to estimate the optimal levels of leucine and isoleucine in diets for juvenile Chinese mitten crabs. The results were 2.36% for Leu and 2.25% for Ile of the total diet, or 5.88% for Leu and 5.72% for Ile of the total dietary proteins.