Effects Of Dietary Protein Levels With Various Stocking Density On The Growth,Plasma Parameters And Gene Expression Of Juvenile Blunt Snout Bream,Megalobrama Amblycephala

Proteins are the most abundant macromolecules found in biological systems and important components of all living organisms.However,protein is one of the most expensive nutrients in fish feeds.It affects growth performance,feeds cost and health of cultured fish,as well maintenance of a good farming environment.Stocking density is also a critical factor affecting fish wellbeing in the aquaculture industry.It affects the feeding behavior and the metabolic rate of the fish.Blunt snout bream,Megalobrama amblycephala,commonly known as Wuchang bream,is an herbivorous commercially freshwater fish species with a long cultured history in China,because of its high larval survival rate,fast growth,tender flesh,high diseases resistance.Therefore,this study was conducted to investigate the effects of essential amino acid(EAA)supplemented dietary protein levels with various stocking density on the growth performances,feed utilization and physiological responses of juvenile blunt snout bream.To determine the effects of EAA supplemented dietary protein levels with various stocking density on growth performances,whole body compositions,plasma biochemical compositions,gene expression and nitrogen emission of juvenile blunt snout bream.A total of 1080 health juvenile blunt snout bream with an average initial weight of 4.01±0.01g were divided into nine triplicate dietary groups.The 3×3 factor experimental design was used to prepare three isoenergetic(18 KJ g-1)experimental diets containing three EAA supplemented dietary protein levels(30%,33%,and 36%)with three stocking density(20,40,and 60 fish/m3).Upon completion of the experimental feeding trial which lasted for 70 days,the first objective of the study,which was to investigate the effects of EAA supplemented dietary protein levels and stocking densities on the growth performance and whole body compositions were undertaken.The results showed that growth performance and feed utilization parameters were not significantly influenced by dietary protein levels.While growth parameters in terms of final average weight(FAW),weight gain rate(WGR),and feed conversion ratio(FCR)were negatively affected by stocking density.The highest growth performance and feed utilization of juvenile blunt snout bream were obtained at the lowest stocking density(20 fish/m3)and fed with 33%EAA supplemented dietary protein.The best survival rate value was obtained at 40 fish/m3.Meanwhile,whole body moisture content was increased with the increasing EAA supplemented dietary protein levels.While lipid content of the fish was decreased with the increasing dietary protein levels.As well as whole body moisture content was significantly decreased with the increasing stocking density(P<0.05).Whereas lipid content was increased with the increasing stocking density,and protein content also increased to some extent with the increasing stocking density.Plasma parameters are important diagnostic tools that can be used as an effective and accurate index to assess and monitor the physiological and pathological changes in fishes.Due to that,plasma indices such as total protein(TP),glucose(GLU),total triglyceride(TG),alanine transaminase(ALT),aspartate aminotransferase(AST),and total cholesterol(TC)were determined.Dietary protein levels supplemented with EAA affected plasma glucose(GLU)level positively and cholesterol(TC)level negatively.While,plasma protein(TP),triglyceride(TG),cholesterol(TC),and aspartate aminotransferase(AST)concentrations were positively affected by stocking density,while there was no effect of their interaction except plasma protein(TP)(P>0.05).The activity of alanine transaminase(ALT)was not affected by dietary protein level,stocking density,and their interaction(P>0.05),In the meantime,total ammonium nitrogen(TAN)emission was significantly increased with increasing dietary protein level and stocking density(P<0.05).Growth and developments of aquatic animals mainly depend on the protein synthesis and deposition and protein synthesis is an important component of the processes involved in the growth response.However,TOR pathway genes closely connected to protein synthesis and play a key role in this process.The relative expression levels of TOR,IGF-1,4E-BP2,and S6K1 genes in the liver of juvenile blunt snout bream were not affected by EAA supplemented dietary protein levels and their interaction(P>0.05).While the expression level of S6K1 was significantly increased with the increment of stocking density.Likewise,the expression levels of TOR,IGF-1,and 4E-BP2 genes in the liver of juvenile blunt snout bream were significantly decreased with the increasing stocking density to some extent but not continues(P<0.05).Generally,the overall results presented here indicate that low dietary protein level supplemented with EAA and optimum stocking density can sustain optimum growth performance,feed utilization and healthy physiological responses in blunt snout bream.Also,it has an advantage in the reductions of fish feed costs and total ammonium nitrogen(TAN)emission.As a result,using low dietary protein level supplemented with EAA and optimum stocking density has economic and environmental benefits in blunt snout bream production.