Effect Of Dietary Lipid Levels On Growth Performance, Blood Biochemical Indices And Digestive Physiology Of Juvenile Yellowcheek Carp (Elopichthys Bambusa)

Yellowcheek carp(Elopichthys bambusa) was recognized as a potential species for Chinese aquaculture. But mass production and intensive culture of the fish using the artifical diets were not practised yet. One cause of the situation was the lack of information about nutriology study of Elopichthys bambusa. Dietary lipids play an important role in fish nutrition as a source of energy and essential fatty acids to maintain biological structure and normal function of cell membranes. Furthermore, there were limited informations on the effect of dietary lipid levels on evacuation rates of fishes. The present study, performed in the flow-through water circulation system of the experimental fish culture base (Huazhong Agricultural University, Wuhan), was carried out to evaluate the effect of dietary lipid levels on growth performance, blood biochemical indices, digestive enzymes activities and digestive tract evacuation of juvenile yellowcheek carp (Elopichthys bambusa). There were two experiments in this study, and the main results of the study were as follows:1 Effect of dietary lipid levels on growth, blood biochemical indices and digestive enzymes activities of juvenile yellowcheek carp(Elopichthys bambusa)Five iso-nitrogenous (45.64±0.20% crude protein) and iso-energetic diets (16.82±0.11 MJ/kg gross energy) with graded levels of lipid at 2.79%(A),5.85%(B), 8.71%(C),11.65%(D),14.77%(E) were formulated to feed the fish, to investigate the effect of dietary lipid levels on growth, body composition, food utilization, blood biochemical indices and specific activities of digestive enzymes existing in hepatopancreas or intestine of yellowcheek carp(Elopichthys bambusa). Three hundred fingerlings (average weight 8.26±0.81 g) were randomly distributed in five treatments in triplicate groups with 20 fish per replicate and fed with test diets twice daily to apparent satiation for 56 d. A recirculated filtered rearing system was used for rearing the fish. With the increase of dietary lipid levels, weight gain rate (WGR) and specific growth rate (SGR) increased firstly and then decreased. WGR and SGR in treatment B were significantly higher than that in treatments A and E (P＜0.05). Protein efficiency ratio (PER) increased with increasing levels of dietary lipid up to 6% and then showed a declining tendency thereafter, which was going against the trend of feed conversion ratio (FCR). FCR in treatment B was 0.94, which was remarkably lower than that in treatments A, D and E (P<0.05). Hepatosomatic index (HSI), whole body lipid content and liver lipid content were significantly affected (P<0.05) by dietary lipid levels, while mesenteric fat index (MFI), condition factor (CF), whole body moisture and ash contents showed no significant differences (P>0.05). HSI in treatment A was significantly lower than that in treatments C, D and E (P<0.05). The trend of whole body protein content in the experiment was similar to SGR. Survival rate (SR) in all treatments was more than 95%. The levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP) and total bile acid (TBA) in serum increased with the increase of dietary lipid levels, while total protein (TP), total cholesterol (CHO), triacylglyceride (TG), high density lipoprotein cholesterin (HDL-C) and low density lipoprotein cholesterin (LDL-C) decreased with the increase of dietary lipid contents. With the rising of dietary lipid levels, specific activities of protease and lipase in digestive tract of the fish reduced after a increasing, but amylase activity showed no significant changes (P>0.05). Specific activities of all the three digestive enzymes in hepatopancreas were highest in group A (P<0.05), while the activity of protease in hepatopancreas exhibited a decreasing with the increasing of dietary lipid levels. There were no significant differences among the activities of lipase and amylase in hepatopancreas in groups B, C, D and E. Analysis of SGR by quadratic model regression suggested that the dietary lipid requirement for optimal growth in juvenile Elopichthys bambusa was 7.71%(%wet weight). Lipid metabolism and the secretion of digestive enzymes in hepatopancreas were negatively affected by the high dietary lipids. 2 Effect of dietary lipid levels on digestive tract evacuation of yellowcheck carpFive iso-nitrogenous (46.71±0.58%) and iso-energetic (17.30±0.32 MJ/kg) experimental diets containing 2.85% (group A),5.79% (group B),8.73% (group C), 11.67% (group D) and 14.61% (group E) of lipid were fed to Elopichthys bambusa to investigate the effect of dietary lipid levels on digestive tract evacuation speed, intestine somatic index (ISI,%) and intestine-body length ratio (ILR,%) of Elopichthys bambusa. Evacuation measurements of digestive tract were performed by serial slaughter before and after the 56 days of feeding experiment. The data of evacuation were fitted by exponential model and liner model. Furthermore, the relationship between dietary lipid levels and instantaneous evacuation rates of Elopichthys bambusa was researched by linear and nonlinear regression analyses in this study. The water temperature was 27.8±1.1℃during the experimental period. The results were as follows:digestive tract evacuation data of Elopichthys bambusa were better explained by exponential model than liner model. There were no significant effects of dietary lipid levels on digestive tract empty speed before the 56 days of the feeding experiment (P=0.858). After the process of feeding, significant effects of dietary lipid levels on digestive tract empty speed of Elopichthys bambusa were observed (P<0.01), and the relationship between dietary lipid levels and the instantaneous evacuation rates could be best explained by the quadratic model, y=0.2509-0.0003x-0.0002x2 (R2=0.9879, P<0.01). After the 56 days of the feeding experiment, percentages of feed remaining in digestive tract in group E at 13 h after the fish satiated was significantly higher than that in group A (P<0.05) and group B (P<0.01). At the end of the experiment, ISI in group D was significantly lower than that in group A (P<0.05) while this index in group E was significantly lower than that in groups A, B and C (P<0.05). These results suggested that: the lipid contents in iso-nitrogenous and iso-energetic diets had no remarkable direct effects on digestive tract instantaneous evacuation rate of Elopichthys bambusa, but the long-term feeding with high lipid diets could reduce instantaneous evacuation rate by weakening the smooth muscle of digestive tract. The slow empty speed of Elopichthys bambusa feeding with high lipid diets might influence growth performance by inhibiting the return of appetite.