Effects Of Dietary Phosphorus And Stocking Density On Growth Performance And Phosphorus Excretion For Juvenile Blunt Snout Bream(Megalobrama Amblycephala)

Blunt snout bream,Megalobrama amblycephala,also known as Wuchang bream,is an herbivorous freshwater fish species with a long history of raising in China because of its high larval survival rate,fast growth,tender flesh.However,the unreasonable addition of feed dietary in intensive commercial culture might lead to growth inhibition and water phosphorus eutrophication.Hence,juvenile blunt snout bream was chosen as the research model animal to investigate effects of dietary phosphorus and stocking density on growth performance and phosphorus excretion for this species,including growth performance,whole body composition,blood biochemistry and phosphorus excretion experiment.The main research contents are as follows:1 Effects of dietary phosphorus and stocking density on growth performance and whole body composition for juvenile blunt snout breamDietary protein was supplied by fish meal,soybean meal,rapeseed meal and cottonseed meal.Soybean oil and soybean lecithin powder were used as lipid sources.Monocalcium phosphate(MCP)was used as a phosphorus source,which was formulated to three diets at graded levels of phosphorus(0.81%,1.15%,1.43%).1080 fish(initial weight 3.75±0.01g)were selected,and randomly sorted to 27 cages(size:1m×1m×1m).Experiment is designed by using 3×3 factor,inculding three stocking density treatment groups(20,40,60 fish/m3)and each gradient was fed three gradient(0.81%,1.15%,1.43%)phosphorus levels at triplicates for 10 weeks.The results showed that final weight,weight gain rate,specific growth rate,protein efficiency ratio and protein conversion ratio were significantly elevated with the increase of dietary phosphorus or decrease of stocking density(P<0.05),while feed conversion ratio was in the opposite trend(P<0.05).At stocking density of 20 fish/m3 and 1.43%dietary phosphorus level,the maximum final weight,weight gain rate,specific growth rate,protein efficiency ratio,protein conversion ratio,and minimum feed conversion ratio were obtained.Significant interactions between dietary phosphorus and stocking density were observed with indexes of growth performance and feed utilization(P<0.05),but not whole body composition(P>0.05).Moisture,ash content and phosphorus concentration in vertebrae were improved significantly with increasing dietary phosphorus level(P<0.05),while the trend of crude lipid content was opposite(P<0.05).However,dietary phosphorus had no significant effect on crude protein content and calcium concentration in vertebrae(P>0.05).Only crude lipid content was significantly elevated as enhancing dietary phosphorus level(P<0.05).Therefore,based on the growth and feed utilization,the optimal phosphorus level and stocking density is 1.43%phosphorus in the diet at 20 fish/m3.Meanwhile,1.43%dietary phosphorus level could reduce crude lipid content and improve bone mineralization for juvenile blunt snout bream.2 Effects of dietary phosphorus and stocking density on blood biochemistry and phosphorus excretion for juvenile blunt snout breamAt the end of the feeding trial,plasma was collected for measuring the contents of blood biochemistry,which were carried out with an automatic biochemical analyzer(Mindray BS-400,Mindray Medical International Ltd.,Shenzhen,China).The results showed that dietary phosphorus and stocking density only had significant interactions in triglyceride,total protein and complement 3(P<0.05).Serum phosphorus,calcium concentration,total protein and immunoglobulin M content were enhanced significantly with increasing dietary phosphorus level(P<0.05),while serum triglyceride content,aspartate aminotransferase,alanine amiotransferase and alkaline phosphatase activities showed a converse trend compared with those(P<0.05).However,there was no significant difference between 1.15%and 1.43%dietary phosphorus levels in alkaline phosphatase activity total protein,immunoglobulin M and complement 3 content(P>0.05).Compared with stocking density of 40,60 fish/m3,contents of immunoglobulin M and complement 3 were suppressed significantly(P<0.05),while content of total protein showed an opposite trend(P<0.05).Content of triglyceride in 60 fish/m3 stocking density was increased significantly compare with that in 20 fish/m 3 stocking density(P<0.05).The activities of aspartate aminotransferase and alanine aminotransferase were enhanced with the increase of stocking density(P>0.05).Maximum total protein content and relatively large triglyceride content were obtained at stocking density of 40 fish/m3 and 1.43%dietary phosphorus levels.Content of complement 3 was highest at stocking density of 20 fish/m3,and relatively best at 1.15%and 1.43%dietary phosphorus levels.Aspartate aminotransferase and alanine aminotransferase had the lowest activity value at stocking density of 20 fish/m3 and 1.43%phosphorus level.The concentration of ammonia nitrogen and total phosphorus in the water increased significantly with the increase of stocking density(P<0.05).The minimum value of total phosphorus in the water was appeared at stocking density of 20 fish/m3.Only total phosphorus concentration in the water increased significantly with the increase of dietary phosphorus level(P<0.05).However,when stocking density was at 20 fish/m3,there was no significant difference in total phosphorus concentration in the water between 1.15%and 1.43%dietary phosphorus levels(P>0.05).There was phenomenon that operculum appeared obvious concave deformation at 0.81%phosphorus level.Therefore,on the basis of triglyceride and total protein content,the optimal stocking density and dietary phosphuras level should be 40 fish/m3 and 1.43%phosphorus level.Considering immune as evaluation index,the optimal combination is stocking density of 20 fish/m3 and 1.43%phosphorus level.20 fish/m3 stocking density achives the lowest phosphorus emissions in the water.In summary,the increase of dietary phosphorus and reduction of stocking density can promote growth performance,immunity and reduce lipid content and feed conversion ratio for juvenile blunt snout bream.Meanwhile,the increasing phosphorus level could reduce crude lipid content and improve bone mineralization for juvenile blunt snout bream.However,the increased dietay phosphorus level and stocking density result in increasing phosphorus emission.