| With the success of the artificial breeding technology of small yellow croaker?Larimichthys polyactis?,its artificial cultivation has developed rapidly.However,the domestic and foreign studies of small yellow croaker nutrition are still lacking.Therefore,it is important to investigation the protein nutrition physiology of L.polyactis for development of economic and efficient artificial feed and promotion of large-scale small yellow croaker culture.juvenile L.polyactis was chosen to be researched,aimed to evaluate the optimal dietary protein requirement,protein to lipid ratio,proportion of cottoned meal replacement for fishmeal.The research results can provide scientific basis and theoretical support for the research and development of L.polyactis compound feed.1.Study on the optimal dietary protein requirement of juvenile Larimichthys polyactis.Fish meal,soybean meal,and wheat gluten meal were used as protein sources,and fish oil,soybean oil,and soybean lecithins were used as the primary lipid source.Six isolipidic experimental diets were formulated to contain 30%,35%,40%,45%,50%,55%dietary protein,labeled as P30,P35,P40,P45,P50,P55.A total of 320 small yellow croaker?Larimichthys polyactis??initial weight 8.30±0.50 g?were randomly stocked into 16cylindrical fiberglass tanks?500 L?at a density of 20 fish/tank.Results shows that the weight gain rate?WGR?and specific growth rate?SGR?increased significantly with the increase of dietary protein level from 30%to 50%?P<0.05?,while the dietary protein level continued to increase,WGR and SGR had a decreased trend.FCR and PER significantly decreased with increasing dietary protein levels?P<0.05?.Trypsin activity in the intestine significantly increased with increased dietary protein levels,whereas amylase activity in the intestine showed the opposite trend?P<0.05?.Relative expression of hepatic LPL and FAS of fish on P40 diet significantly higher than those of fish on P35 diet?P<0.05?.The relative expression of FAS presented earlier increase and later decrease trend with increased dietary protein levels?P<0.05?,Relative expression of IGF-1 were found to be higher in fish on P50 diet than those of fish on other diets?P<0.05?.The relative expression of muscle GHR in fish on P40 diet was the highest?P<0.05?.Based on the Broken-line regression analysis of WG,the dietary protein requirement for L.polyactis was estimated to be 50.62%.2.Study on the optimal dietary protein to lipid ratio of juvenile Larimichthys polyactis.Fish meal,soybean meal,and wheat gluten meal were used as protein sources,and fish oil,soybean oil,and soybean lecithins were used as the primary lipid source.The formulated diets in a 3×3 factorial experimental design.Nine experimental diets were prepared with different dietary proteins?43%,47%,or 51%?and lipids?6%,9%,or 12%?,labeled as P43L6,P43L9,P43L12,P47L6,P47L9,P47L12,P51L6,P51L9,P51L12.A total of 540 small yellow croaker?Larimichthys polyactis??initial weight 16.02±0.50 g?were randomly stocked into 27 cylindrical fiberglass tanks?200L?at a density of 20 fish/tank,perform a 51-day growth experiment.Results shows that significant interactions were found between dietary protein and lipid levels in weight gain rate?WGR?and specific growth rate?SGR?,The highest WGR and SGR were observed in fish on P47L12 diet?P<0.05?.Protein efficiency ratio?PER?of fish significantly increased with the increasing of dietary lipid level?P<0.05?,Hepatosomatic index?HSI?and Viscera somatic index?VSI?of fish on L12 diet was significantly higher than that of fish on L6 and L9 diets?P<0.05?.Lipase activity in the intestine increased significantly as dietary lipid level increased,whereas trypsin activity in the intestine showed the opposite trend?P<0.05?.Relative expression of hepatic and muscle LPL significantly increased with increasing dietary lipid levels?P<0.05?,fish on P51 diet had markedly lower muscle LPL expression than those of fish on P43 and P47 diets?P<0.05?.The relative expression of FAS decreased with increasing dietary lipid levels?P<0.05?.Relative expression of IGF-1 in the liver and muscle significantly increased with increasing dietary lipid levels?P<0.05?.Interaction between dietary protein and lipid levels in the relative expression of the GHR gene was observed,relative expression of GHR significantly increased with increasing dietary lipid levels?P<0.05?.Under this experimental condition,our results indicate that a diet containing 47%protein with 12%lipid(P/E ratio of 24.55 mg protein kJ-1)is optimum for juvenile L.polyactis.3.Study on suitable proportion of cottoned meal replacement of fishmeal in diet of juvenile Larimichthys polyactis.Fish meal,cottoned meal,soy protein concentrate and wheat gluten meal were used as main protein sources,six isoenergetic and isolipidic diets were produced:a control diet?FM-based?and five diets that replaced FM by 12%,24%,36%,48%,60%with CM,labeled as FM,CM12,CM24,CM36,CM48,CM60.A total of 360 small yellow croaker?Larimichthys polyactis??initial weight 8.70±0.50 g?were randomly stocked into 18 cages?50cm×50cm×40cm?at a density of 20 fish/cage.Results shows that survival rate?SR?,weight gain rate?WGR?and specific growth rate?SGR?decreased with increasing dietary cottoned meal levels,and fish on CM60 diet was significantly lower than that of fish on other diets?P<0.05?.The FCR significantly increased with increasing dietary cottoned meal levels?P<0.05?.The whole body crude protein and lipid contents significantly decreased with increasing dietary cottoned meal levels?P<0.05?.Observation indicated that the structure of intestinal cells of fish on FM diet were clear and complete.And with increasing dietary cottoned meal levels,the structure of intestinal cells were damaged and vacuolization phenomenon appeared in the cytoplasm.The structure of the intestine of fish on CM60 diet was severely damaged,and nucleoli contracted.Relative expression of intestine LPL presented earlier decrease and later increase trend with increased dietary cottoned meal levels,relative expression of intestine LPL of fish on CM24 and CM36 diets significantly lower than those of fish on other diets?P<0.05?.Relative expression of intestine FAS presented earlier increase and later decrease trend with increased dietary cottoned meal levels?P<0.05?.Relative expression of intestine IGF-1 of fish on FM diet significantly higher than those of fish on other diets?P<0.05?.Relative expression of hepatic IGF-1 was found to be higher in fish on FM diet than those of fish on other diets?P<0.05?.Our results indicate that including too high proportion of cottoned meal in diet may inhibition of the growth performance,feed utilization.However,the effect of low dietary cotton meal substitution levels on growth is still within an acceptable range.Considering the trend of the FCR,we recommend that the proportion of cottoned meal replacement fishmeal in juvenile L.polyactis diet should not exceed 36%. |
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