Growth Performance, Body Composition And Metabolic Responses Of Blunt Snout Bream(Megalobrama Amblycephala) Yearlings Fed Practical Diets Differing In Protein, Lipid And Carbohydrate Levels

This study was conducted to determine the effects of dietary protein, lipid and carbohydrate levels on growth performance, body composition, physiological and biochemical parameters of yearling blunt snout bream, in order to providing theoretical basis for optimizing the feed formular for blunt snout bream, which included three series:Series1Growth performance and body composition of blunt snout bream (Megalobrama amblycephala) yearlings fed practical diets differing in protein and lipid levelsA3x3factorial design was used to determine the effects of dietary protein and lipid levels in practical diets on growth performance and body composition of blunt snout bream yearlings (average initial weight,50.37±1.27g). Nine practical diets were formulated to contain three protein levels (25%,30%and35%), each with three lipid levels (3%,6%and9%), respectively. Fish was held in floating cages (2mx1mx1m) and fed three times daily for8weeks. No interaction between dietary protein and lipid existed for growth performance and body composition (P>0.05). No significant difference (P>0.05) was observed in survival rate among all the treatments. Weight gain (WG), specific growth rate (SGR) and feed conversion ratio (FCR) were significantly influenced by both dietary protein (P<0.05) and lipid levels (P<0.05). WG and SGR of fish fed with25%protein were significantly (P<0.05) lower than that of the other groups; whereas, the opposite was true for fish fed with6%lipid (P<0.01). Although the lowest FCR was found in fish fed with35%protein and6%lipid, no obvious difference (P>0.05) was observed among the other treatments except for fish fed with25%protein3%lipid and25%protein9%lipid. Protein efficiency ratio (PER) and nitrogen retention efficiency decreased significantly with increasing dietary protein levels (P<0.05). In addition, PER was significantly (P<0.05) affected by dietary lipid levels with the highest observed in fish fed6%lipid. Energy retention efficiency increased significantly as dietary lipid levels increased (P<0.05). Condition factor improved significantly (P<0.05) as dietary protein and lipid levels increased. Intraperitoneal fat ratio and hepatosomatic index increased significantly (P<0.05) as dietary lipid levels increased. No significant difference (P>0.05) was observed in body composition among all the treatments in terms of dietary protein levels. Moisture, lipid and energy contents of whole-body were significantly (P<0.01) affected by dietary lipid levels. Contrary to moisture content, lipid and energy contents of whole-body increased significantly (P<0.01) with increasing lipid levels. Similar results were also observed in carcass and liver composition.Series2Digestive and metabolic responses of blunt snout bream (Megalobrama amblycephala) yearlings fed practical diets differing in protein and lipid levelsThe conceptual design of raising was the same as that of the first experiment. Activities of intestine protease, lipase and amylase were all significantly affected by both dietary protein (P<0.05) and lipid (P<0.01) levels. Protease and amylase activities of fish fed with25%protein were significantly (P<0.01) lower than that of the other groups, lipase activity of fish fed with30%protein was significantly (P<0.05) higher than that of fish fed25%protein. Activities of all three digestive enzymes increased significantly (P<0.01) as dietary lipid levels increased. Liver aspartate aminotransferase (GOT) activity was significantly (P<0.01) affected by dietary protein levels with the highest observed in fish fed30%protein, and decreased significantly (P<0.05) with increasing lipid levels. Activity of liver lipoprotein lipase of fish fed35%protein was significantly (P<0.01) lower than that of the other groups, activity of hepatic lipase and total lipase of fish fed30%protein was significantly (P<0.05) higher than that of the other groups. Total lipase activity decreased significantly (P<0.05) with increasing lipid levels. Plasma GOT activity increased significantly (P<0.05) as dietary lipid levels increased, alanine aminotransferase was significantly (P<0.05) affected by dietary protein levels with the highest observed in fish fed30%protein. Plasma cholesterol concentration increased as dietary protein levels increased, but not significantly (P>0.05) and decreased significantly (P<0.01) as dietary lipid levels increased. Similar results were also observed in triacylglycerols concentration although no significant difference was found among all the treatments. Series3Growth performance, body composition and plasma biochemical parameters of blunt snout bream (Megalobrama amblycephala) yearlings fed practical diets differing in carbohydrate levelsThis study aimed to evaluate the effects of dietary carbohydrate levels on growth performance, body composition and plasma biochemical parameters of yearling blunt snout bream (average initial weight,37.76±1.19g). Six isonitrogenous and isolipidic diets were formulated to contain different carbohydrate levels (20%,25%,30%,35%,40%and45%). Fish were held in several indoor aquarium tanks (3.0×1.0×0.8m) and fed3times daily for8weeks. The results showed that no significant differences (P>0.05) were observed in relative feed intake and survival rate among all the treatments. Weight gain (WG), specific growth rate (SGR), protein efficiency ratio and nitrogen retention efficiency were all significantly (P<0.05) affected by dietary carbohydrate levels with the highest observed in fish fed35%carbohydrate. Unlike the case of WG and SGR, the lowest feed conversion ratio was found in fish fed35%carbohydrate. Viscerosomatic index, hepatosomatic index, intraperitoneal fat ratio and lipid and energy retention efficiency all increased significantly (P<0.05) with increasing dietary carbohydrate levels. No significant difference (P>0.05) was found in whole-body and carcass protein and ash contents as well as in muscle glycogen content among all the treatments. Whole-body moisture, lipid and energy contents were all significantly (P<0.05) affected by dietary carbohydrate levels. Contrary to moisture content, whole-body lipid and energy contents all increased significantly (P<0.05) with increasing carbohydrate levels. Similar results were also observed in carcass moisture and lipid contents as well as in liver lipid content. Liver glycogen contents and plasma glucose levels were all significantly (P<0.05) affected by dietary carbohydrate levels with the highest found in fish fed40%carbohydrate, whereas little difference (P>0.05) was observed in plasma insulin concentrations. Contrary to plasma cortisol levels, both plasma lysozyme activities and immunoglobulin concentrations decreased significantly (P<0.05) with increasing carbohydrate levels.In conclusion, diet containing30%protein,6%lipid and35%carbohydrate with a protein to energy ratio of18.21g/MJ was suitable for blunt snout bream yearlings. Reduce protein levels appropriately leads to the increase of lipase activities both in intestine and liver, and increased dietary lipid level appropriately may enhance the digestive enzyme activities and reduce the proportion of dietary protein catabolized for energy. And an appropriate increase of dietary lipid or carbohydrate could produce protein-sparing effects.