| Cyprinus carpio is the main cultural species in China.It is freshwater fish of the highest yield because of its wide distribution,large in size,fast growth and strong disease resistance.With the development of intensive and industrialized aquaculture,the breeding density and the feeding dose have increased greatly,which lead to the increase of residual feed and fish excrement and the deterioration of breeding environment.In this study,we carried out a 56-day breeding experiment using Cyprinus carpio(13.12±0.55 g)as the research object and feeding low-nitrogen feeds with different protein levels.The response mechanisms of Cyprinus carpio on different protein levels were investigated from growth characteristics,physiological and biochemical indicators and intestinal flora structure.The aim is to provide data and theoretical reference for optimizing formula of low-nitrogen feed,reducing farming costs and farming pollution of Cyprinus carpio.1 Effects of protein levels of low-nitrogen feed on growth of Cyprinus carpioFour kinds of experimental feeds were designed and producted,which protein levels were 29.95%,28.77%,27.60%,and 26.42% respectively(mark as Diet 1,Diet 2,Diet 3,and Diet 4 respectively).The breeding experiment was performed indoors for 56 days.The results showed that the average weight gain rate and specific growth rate of Cyprinus carpio fed Diet 3 were 377.67% and 2.60%,which were higher than those of Diet 1 and Diet 4significantly(P<0.05),and no significant difference in survival rates(P>0.05).The fullness of Cyprinus carpio fed Diet 3 was 4.14% which was higher than the other three groups significantly(P<0.05).There was no significant difference in liver body index between treatment groups(P>0.05).The results suggested that Cyprinus carpio grew best when the protein level of feed was 27.60%.2 Effects of different protein levels in low-nitrogen diets on physiological and biochemical indexes of Cyprinus carpioAfter 56 days of breeding tests,physiological and biochemical indicators of Cyprinus carpio in each test group were determined.The results showed that serum total cholesterol(T-CHO)and low-density lipoprotein cholesterol(LDL-C)in Diet 3 were significantly lower than those in the other three groups(P<0.05),and there was no significant difference in triglyceride(TG)groups(P>0.05),high-density lipoprotein cholesterol(HDL-C)was significantly higher than Diet 4(P<0.05),superoxide dismutase(SOD)activity was significantly higher than Diet 1(P<0.05),and malonaldehyde(MDA)activity was significantly lower than Diet 1(P<0.05),peroxidase(CAT)activity and alkaline phosphatase(AKP)activity were significantly higher than the other 3 groups(P<0.05),lysozyme(LZM)activity was significantly higher than Diet 1,Diet 4(P<0.05).The liver and pancreas superoxide dismutase(SOD)activity in Diet 3 was higher than the other three groups,but there was no significant difference(P>0.05);the peroxidase(CAT)activity was significantly higher than the other three groups(P<0.05);MDA activity was significantly lower than the other 3 groups(P<0.05).The results showed that the serum and liver-pancreas-related immune indexes of Cyprinus carpio were better when the protein level in the feed was 27.60%.3 Effects of different protein levels on the intestinal flora of Cyprinus carpio in low nitrogen feedIn this experiment,high-throughput sequencing(Mi Seq)technology was used to study the effect of different protein levels on the microflora structure of the foregut,midgut,and hindgut of Cyprinus carpio.The results showed that the bacterial diversity in the intestinal tract of Cyprinus carpio was relatively rich in each test group.Among them,the Shannon index and Simpson index in Diet 3 were higher than those in other groups,but there was no significant difference(P>0.05).Based on the level of phylum classification,4 phylums are the main types,such as Firmicutes,Proteobacteria,Actinobacteria,and Bacteroidetes;based on the genus level,Lachnospira,Faecalibacterium,Bifidobacterium,Bacteroides,Cetobacterium,Pseudoxanthomonas,Sutterella,Roseburia,Lachnoclostridium,Pirellula,and Neisseria.Among them,the richness of Bacteroides in Diet 3 in the anterior,middle,and posterior intestines is lower than the other three groups,and the richness of Faecalibacterium and Bifidobacterium in the anterior and midgut was higher than the other three groups,the richness of Lachnospira in the hindgut of the four groups was not significantly different.In Diet 3,Lachnospira is the most abundant and Bacteroides is the least abundant.In the hindgut,Faecalibacterium and Bifidobacterium are the most abundant,middle and posterior intestines are richer than foregut.As a result,rare species of Roseburia,Lachnoclostridium,and Sutterella were found in aquatic animals.The flora interacts to form the intestinal microbial environment of the fish.Adding different protein levels of feed will not affect the growth of the dominant intestinal flora and the richness and uniformity of species composition in the sample,which has important practical significance for the development of healthy Cyprinus carpio farming and optimization of feed formula. |
