The Effect Of Carbohydrate To Growth And Metabolism On Cobia

Feeding experiments were conducted to investigate the effects of dietarycarbohydrate levels on growth performance, body composition, tissue composition,plasma parameters and hepatic carbohydrate metabolic enzyme activities in cobia(Rachycentron canadum L) at two different growth stages (mean initial weight172.56±0.80g and598.44±4.60g). In addition, the effect of re-feeding diets withdifferent carbohydrate levels following fasting on plasma glucose, total protein,triacylglycerol, total cholesterol, low-density lipoprotein-cholesterol (LDL-C) andhigh-density lipoprotein-cholesterol (HDL-C) contents in cobia was carried out. Initialweight was172.56±0.80g, and the treatment was fasted for24h after10weeks offeeding. Six isonitrogenous (45%) and isolipidic (9.4%) diets were formulated tocontain0%,6%,12%,18%,24%,30%carbohydrate, respectively. Fish at two differentgrowth stages,170g and600g were distributed to seawater floating net cages(1.5m×1.5m×2.5m) at a density of20and10fish per net cages, respectively. Triplicategroups of fish were fed randomly by each diet. The results are following：1. Fish fed high-carbohydrate diets (24and30%) showed higher SGR and FER thanthose fed low-carbohydrate diets (0%). With increasing dietary carbohydrate, theenergy using for growth increased in both stages. Significantly higher lipid contents ofwhole-body, liver and muscle were observed in fish fed diets containing higher starch(24,30%) compared to lower starch (0%) at stage of170g, but not for600g stage.Both liver and muscle glycogen were significantly changed by dietary treatments. Theapparent digestibility coefficient (ADC) of dry matter and ADC of energy wererelated to the dietary carbohydrate, while protease activities showed no significantdifferences in both stages. Compared with the fish fed the low-carbohydrate diet(0%), the activities of hexokinase (HK) and pyruvate kinase (PK) in the liver, theactivities of Amylase in the intestine and the content of glucose (GLU) andtriglyceride (TC) in the plasma of the fish fed high-carbohydrate diets (24,30%) weresignificantly higher at170g stage, but did not for600g stage. In concision, based onthe SGR and FER, diets containing23.14-25.30%and23.24-23.51%carbohydrate(wheat starch) were appropriate for the cobia with170g and600g initial body weight,respectively. In addition, adding more carbohydrate could increase the fat of thewhole body and liver and the glycogen content of the liver and muscle, the ADCs ofthe dry matter and energy, and increase the contents of the fat of muscle, blood glucose, triglyceride in the plasma and the activities of the HK and PK in the liver,and the activities of Amylase in the intestine of the small-size fish significantly, butthese phenomena did not for big-size fish.2. The results showed that the glucose levels of cobia after re-feeding with each dietrised from0to5h, peaking at5h, and then started to decline till7h, when the glucoselevels were similar to those at0h. The total protein levels in serum were significantlyinfluenced neither by different diets nor by different times. The triacylglycerol levelsin serum increased from0to5h, and then began to decline. At7h, the levels weresimilar to those at0h. In the group of0%, the contention of cholesterol in strumincreased from0to2h, and then decreased durning the following5h. The level after7h was similar to that at0h, but in other groups, the levels increased from0to5h, andthen began to decline, while it is7h recovered to those at0h. The levels of LDL-Cincreased from0to5h, and then began to decline. At7h, the levels of LDL-Crecovered to those at0h. However, the levels of LDL-C had significant differenceneither among different diets nor different time. In the group of24%, the level ofHDL-C at5h was significantly higher than those at0h and24h.No significantdifferences were found in other groups for the levels of HDL-C. In conclusion, theglucose levels increased, and then declined after re-feeding for the cobia with170ginitial weight. The peak of glucose increased with the increasing of dietary starchlevel up to12%and remained nearly the same thereafter. The levels of triacylglyceroland cholesterol in serum increased after re-feeding, but there were no significantdifferences in the LDL-C and HDL-C levels in serum after re-feeding for the cobiawith170g initial weight.