| At present,the taste,color,and nutrition of meat products have become more and more important to consumers.The growth and muscle characteristics of fish are affected by breeding and genetic characteristics,and more conventional methods is human intervention.Human-beings can change the growth pattern of fish by changing the environment factors and diet in aquaculture.In the study,Nile tilapia was used as a research model to explore effects of water environment(salinity,hypoxia)and dietary fat content on the growth,metabolism and meat quality of fishes.The combined effect of multiple factors is more suitable for actual aquaculture production,and all these results provide a new insight for understanding the environmental factors affecting flesh quality of fishes,and also provide a new reference for properly adjust the dietary lipid level under different aquaculture environment.The main results and conclusions of this study are as follows:(1).Influences of water salinity and dietary fat content on growth,nutrient composition and fillet quality of Nile tilapiaNile tilapia(Oreochromis niloticus)has some salinity adaptability,and the dietary fat content of tilapia has been increasing in recent years,but the effects of water salinity and dietary fat content on the growth,composition and fillet quality of Nile tilapia are still unclear.Thus,we designed the following experiment.Three different salinities(0,8‰,16‰)were set to raise Nile tilapia(Initial weight 5±?0.2?g),and each salinity-group tilapia was fed with two lipid-content diets,medium and high lipid diet respectively(MFD,6% and HFD,12%).After 8 weeks of breeding,the fish were sampled,and the growth,serum biochemical parameter,muscle nutritional composition and fillet quality related parameters were assayed.The results showed that,among MFD groups,the fish in medium salinity(8‰)group showed highest final weight(FW),carcass ratio(CR),mesenteric fat index(MFI)and content of muscle total protein,amino acids and lactate,but decreased viscerosomatic index(VSI),hepatosomatic index(HSI)and p H of muscle,compared with zero salinity(0‰)group and high salinity(16‰)group.And compared with zero salinity(0‰)group and middle salinity(8‰)group,the fish in MFD-high salinity(16‰)group showed increased condition factor(CF),VSI,HSI,whole fish lipid,hepatic TG and serum of glucose,lactate,TG and GOT,but reduced whole fish moisture,muscle moisture,whole fish ash,meat yield(MY)and centrifugal water loss.In HFD group,with the increase of salinity,FW,survival rate and MFI were gradually decreased,and FCR,VSI,HSI and CF were increased.In HFD group,the fish in medium salinity(8‰)group showed reduced muscle total protein and amino acid content.HFD-high salinity(16‰)cultured fish showed increased whole fish ash,hepatic TG,serum glucose,GOT,muscle total protein,muscle TG and PL content,and decreased whole body lipid,hepatic glycogen,MY,centrifugal water loss,p H.Regardless of freshwater and salt water,the fish in HFD group showed higher fat accumulation and muscle lactic acid content than in MFD group,however the reduced survival rate and meat yield.These results above showed that medium salinity increased weight gain,meat yield,total protein and amino acid content in Nile tilapia.In short,appropriate salinity can improve the growth and flesh quality of fishes.However,the high-fat diets in high salinity waters not only decreased the survival rate,but caused lower total protein,amino acid contents and p H value in fillet of Nile tilapia.In conclusion,the interaction of high salinity and high fat diet has a significant negative impact on growth and flesh quality of fishes.(2).Influences of hypoxia on nutrient composition and fillet quality of Nile tilapia fed with different dietary lipid contentPrevious studies in our laboratory have found that hypoxia(1~2mg/l)could alter the metabolism pattern in Nile tilapia.In the experiment,we used the oxygen content to explore the effect of hypoxia and high fat diet on fishes’ flesh quality.All fish(7?±?0.01?g)were divided into four treatment groups(3 tanks per group): medium fat diet(MFD,6%)-normoxia(7?±?0.1mg/l),MFD-hypoxia(1.1?±?0.1?mg/l),high fat diet(HFD,12%)-normoxia and HFD-hypoxia.After eight weeks feeding trial,in MFD group,hypoxia significantly deceased the feeding intake,weight gain,survival rate,whole body protein liver,glycogen,and blood glucose levels,while increased HSI(hepatosomatic index),MFI,hepatic TG,serum lactic acid content,GPT(glutamyl aminotransferase)and GOT(aspartate aminotransferast)activity.Therefore,under medium fat levels,hypoxia stress did harm to the growth of tilapia,enhanced glycolytic metabolism,and caused excess lipid deposit in body.At normoxic levels,high-fat diets significantly increased weight gain,HSI,MFI,whole body lipid,liver TG and serum lactic acid content,but reduced feed intake,survival rate,whole body protein and whole fish moisture.Therefore,the medium fat-hypoxia group and the high fat-normal oxygen group are not suit for tilapia growth.The interaction significantly affects feeding intake,HSI,MFI,whole body protein,whole body lipid,hepatic glycogen,hepatic TG and serum lactic acid,and HFD-hypoxia group had lowest food intake and survival rate,but did not cause more adverse harm to body composition and serum biochemistry than individual factor.In muscle of MFD group,hypoxia significantly reduced total protein,moisture,p H,meat yield,PL(phospholipid content),DAA(delicious amino acid),FAA(flavored amino acid),EAA(essential amino acids),TAA(total amino acid)content,as well as the proportion of Σn-3 PUFA and DHA in total lipid,while increased shear force,centrifugal water loss,cooking water loss,and deposition of TG.Hypoxia downregulated the expression of collagen synthesis(COLIA1 and COLIA2)and lipolytic genes(CPT1b)in tilapia muscle fed with medium fat,but significantly up-regulated fat synthesis(FAS)and myocyte growth factor(Myo G).In short,under medium fat feeding,hypoxia reduced the nutritional content of muscle,and increased hardness and increased water loss,which damaged muscle quality.Under normoxic environment,in muscle,high-fat diets reduced total protein,moisture,PL,p H,DAA,FAA,EAA,TAA,and relative expression of key genes for collagen synthesis(COLIA1 and COLIA2),but increased glycogen,TG,centrifugal water loss,cooking water loss,and relative expression of FAS and Myo G.In conclusion,the HFD-nomoxic group and the MFD-hypoxia group have similar effects,both reduced the total protein,EAA,PL content and p H value in muscle,but increased water loss rate in Nile tilapia.The two-factor interaction significantly affected total protein,moisture,PL,EAA,TAA,p H,centrifugal water loss,cooking water loss,COLIA1,COLIA2,CPT1 b,FAS.In addition,moisture and PL content in the HFD-hypoxia group were significant higher than the HFD-normoxic group and MFD-hypoxic group,and the interaction of the two factors has not aggravated the damage to muscle quality.In short,both low oxygen and high fat diet are harmful to the healthy growth of fish and significantly affect the muscle quality,and the interaction of the two factors results in more mortality,but the effects will not be exacerbated in the flesh quality.This study focuses on the two most commonly and important environmental factors,and the effects of environmental changes on tilapia growth and muscle quality were systematically evaluated.The experimental results found that adverse environmental changes will limit the growth of fish and cause the loss of fish’s nutritional value.Besides,in adverse environment,HFD will further restrict the growth of fish,the nutritional value of fish will be more severely destroyed than individual factor,but the combined effect of low oxygen and HFD has not increased alteration in meat quality.Therefore,this study provides an important basis for feed researchers to adjust the change in dietary fat content under different breeding conditions,and also provides a reference for aquatic workers to evaluate the balance between production and fish quality. |
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