Effects of dietary protein level on growth, feed utilization and digestive enzyme activity of the Chinese mitten crab, Eriocheir sinensis

A feeding trial was conducted using isoenergetic practical diets to evaluate the effects of the dietary protein level on growth performance, feed utilization and digestive enzyme activity of the Chinese mitten crab, Eriocheir sinensis. Four experimental diets were formulated containing 250, 300, 350 and 400 g kg-1 protein and 16 kJ g-1 gross energy. Each diet was randomly assigned to triplicate groups of juvenile crab with mean initial body weight 3.56 +/- 0.16 g and mean shell width 15.31 +/- 0.06 mm. Juvenile crab were reared in indoor flow-through system consisting of 12 plastic tanks (1.0 m x 0.6 m x 0.5 m) and fed diets twice daily at 6-8% of body weight for 12 weeks. Performance was judged on the basis of growth (specific growth rate of weight, SGR(G); specific growth rate of shell width, SGR(SW)), feed conversion ratio (FCR) and protein efficiency ratio (PER). A decreased FCR was observed with increasing dietary protein levels. Both SGR(G) and SGR(SW) significantly increased with increasing dietary protein levels up to 350 g kg-1, whereas there were no significant differences for protein levels from 350-400 g kg-1. Application of broken line regression analysis to SGR(G) provided an estimate of 347.8 g kg-1 dietary protein for maximal growth. The highest PER was observed in crab fed the diet containing 350 g kg-1 protein (P < 0.05). The percent survival was not affected (P > 0.05) by the different dietary treatments. No significant differences were observed in the apparent digestibility coefficients of crude lipid and dry matter among dietary treatments (P > 0.05). However, the apparent digestibility coefficients of crude protein and energy in crab fed different protein levels significantly increased with increasing dietary protein level (P < 0.05). Both amylase and protease activities in the intestine of E. sinensis were studied. The amylase activity decreased significantly (P < 0.05) with increased dietary protein level and protease activity increased. Regression analysis showed a negative effect of inclusion of dietary protein level on amylase activity (P < 0.05). However, protease activities were found to be positively correlated (P < 0.05) with dietary protein level. The protein content of the crab significantly increased with dietary protein levels up to 350 g kg-1 (P < 0.05), but no significant differences (P > 0.05) were founded with protein levels higher than 350 g kg-1.