Study On Mechanism Of Carbohydrate Utilization For Juvenile Cobia, Rachycentron Canadum

Feeding experiments were conducted to evaluate the effects of nutritional physiology of different carbohydrate source, different protein/carbohydrate ratio and different carbohydrate levels on growth performance, feed utilization and hepatic carbohydrate metabolic enzyme activities. Results of the present study are presented as follows:1. An 8-week feeding trial was conducted to investigate the effect of dietary carbohydrate sources on growth performance, survival, feed utilization and hepatic carbohydrate metabolic enzyme activities of juvenile cobia. Six experimental diets were formulated to contain 20% glucose, sucrose, maltose, dextrin, corn starch, and wheat starch, respectively. The results indicated that fish fed the wheat starch and dextrin diets had significantly better weight gain, specific growth rate and protein efficiency ratio compared to those fed the other diets. However, fish fed the glucose diet had the significantly lower survival and condition factor than those fed the other diets. There were significant differences in total plasma glucose and triglyceride concentration in fish fed diets with different dietary carbohydrate sources. Haematocrit, hemoglobin, red blood cell and leukocyte were significantly affected by the dietary carbohydrate sources. The activities of glucose-6-phosphate dehydrogenase (G6PD), 6-phosphofructokinase (PFK) and fructose-1, 6-bisphosphatase (FBPase) were significantly affected by the dietary carbohydrate sources, while fish fed the glucose diet had higher G6PD, PFK and FBPase activities than those fed the other diets. These data indicated that dextrin and wheat starch were the optimal most adequate carbohydrate sources for juvenile cobia.2. An 8-week feeding trial was conducted to investigate the effect of dietary protein/carbohydrate ratio on growth performance, survival, feed utilization and hepatic carbohydrate metabolic enzyme activities of juvenile cobia. Isolipidic, isoenergetic diets with 45/15(LC),40/22(MC),35/29(HC) percent protein/carbohydrate were assessed,The carbohydrate source was wheat starch and dextrin, respectively. The outcomings showed that the highest weight gain was obtained in the LC wheat starch group, no significant difference in the MC and HC groups between different carbohydrate sources. The results suggested that wheat starch had better protein-sparing effect in the LC group, and there were no significant differences in MC and HC groups. Better specific growth rate (SGR) and feed efficiency(FE) were observed in LC groups. There were no significant differences of condition factor, hepatosomatic index, viscerosomatic index among fish fed with the same carbohydrate source. The activities of G6PD, PFK and FBPase were significantly affected by the dietary protein/carbohydrate ratio, the highest activity of PFK was obtained in HC group. The FBPase activity in the LC wheat starch and MC dextrin groups were significant higher than the other groups. Higher activity of G6PD was obtained in LC groups, while the lower values were obtained in HC groups.3. An 8-week growth trial was conducted to determine the effect of dietary carbohydrate levels on growth performance, survival, feed utilization and hepatic carbohydrate metabolic enzyme activities of juvenile cobia. Six isonitrogenous, isolipidic diets using wheatstarch as carbohydrate source were formulated to contain six carbohydrate levels (10%, 14%, 18%, 22%, 26% and30%). The results showed that there were no significant differences in WG and SGR of fish fed with diets containing different carbohydrate levels. The protein efficiency ratio (PER) and FE were not affected by different carbohydrate levels. No significant differences were observed in whole fish, liver composition and morphological index. The PFK activity positively corrected by the carbohydrate levels, the peak was found in 30% group. There were no significant differences of FBP activity in all treatments. The activities of glucose-6-phosphate dehydrogenase positively corrected by the carbohydrate levels except the 10% group, the lowest value was found in 14% group.