Effects Of Protein Levels On Growth Performance, Digestive Functions And Protein Metabolism Of Juvenile Yellow Cheek Carp (Elopichthys Bambusa)

A 56-day growth experiment was conducted in a recirculated filtered rearing system to determine the effects of dietary protein levels on growth performance, digestive functions and protein metabolism of juvenile Elopichthys bambusa (5-20g), at the water temperature of 27.8±1.7℃. White fishmeal and casein were used as dietary protein source to formulate isocaloric experimental diets at five protein levels (41%,45%,49%, 53%and 57%). The results are as follows:1. Effects of protein levels on growth performance:No significant difference was observed among groups on survival rate, which ranged from 95.56%to 100%. As dietary protein level increased, weight gain ratio and specific growth rate of the fish improved, and reached a peak at 49.5%(significant differences were found among groups, P<0.05), then maintained a plateau. A similar trend was found with feed efficiency up to 53.2%, however, the feed efficiency of 57% group was significantly lower than 49% group (P<0.05). Protein efficiency ratio of 49.5% group was significantly higher (P<0.05) than other groups.2. Effects of protein levels on body composition of juvenile Elopichthys bambusa:There was no significant effect of protein levels on whole body moisture of juvenile Elopichthys bambusa (P>0.05). The body protein was increasing firstly and then reducing with the increasing of dietary protein levels. Compared with the first group (41%,17.11 %), the third group (49%,18.72%) increased significantly (9.4%, P<0.05). The body lipid was reducing with the increasing of dietary protein level, and was significantly different in all groups (P<0.05). As the increasing of protein level, the body ash was improving gradually (P<0.05).3. Effects of protein levels on the related indices of hepatopancreas and intestine of juvenile Elopichthys bambusa:With the increase of dietary protein level, the hepatosomatic index of juvenile Elopichthys bambusa decreased from 1.71%(group 1,45%) to 1.30%(group 5,57%) (P<0.05). There were significant differences among the groups 41%/45%/49% and 57% (P<0.05), while no significant difference between the second and third group (P>0.05). Intestine weight to body weight ratio and intestine length to body length ratio showed a similar trend with the hepatosomatic index, whereas the intestine weight to body weight ratio and intestine length to body length ratio declined more gently.4. Effects of protein levels on the enzyme activities in hepatopancreas and intestine of juvenile Elopichthys bambusa:No significant changes on amylase activities of juvenile Elopichthys bambusa hepatopancreas were observed with the dietary protein level (P>0.05). With the dietary protein levels rising from 41% to 49%, the protease and lipase activity tended to increase gradually, and had the maximum value at 14.27/0.108 U/mg prot respectively. The AST and ALT activity in hepatopancreas had a similar trend to protease activity, and reached the maximum value at 3.00,2.10 U/mg prot respectively.Intestinal protease, lipase and amylase activities of juvenile Elopichthys bambusa showed different trends with the increasing dietary protein. With the increasing dietary protein level, protease activity increased first, and then decreased; each group showed significant differences (P<0.05). Group 3 (49%) had the highest (29.84 U/mg prot) activity, which was 40.6% higher than the low-protein group (21.22 U/mg prot). No significant changes on lipase activities of juvenile Elopichthys bambusa hepatopancreas were observed with the dietary protein level increased (P>0.05). Amylase activity was gradually decreased with the increasing levels of dietary protein, and significant changes were observed among group 1,2,3 and 4, while there was no significant difference between group 4 and 5 (P>0.05).Results of the experiment indicated that protein levels have a positively promotion to the growth performance, the deposition of body protein and secretion of digestive of the juveniles; meanwhile, protein levels promoted the increase of activities of AST and ALT. Broken-line model and quadratic curve model were used to analyze weight gain ratio and protein efficiency ratio, respectively. It was found that the suitable protein requirement for juveniles of Elopichthys bambusa was 48.4%-50.2%.