Effect Of Different Dietary Lipid Sources On The Growth,Antioxidation,Fatty Acid Composition And Gene Expression Of Juvenile Cobia

Fish oil has always been the preferred lipid source of aquatic feed.However,with the rapid development of intensive aquaculture and the continuous expansion of feed industry,as well as the continuous decline of marine fishery resources due to overfishing,fish oil production is in short supply and the price of fish oil is still high.Therefore,it is urgent to seek and develop new feed fat sources.Vegetable oil has the advantages of wide sources,low price,high quality and large yield.It is rich in 18-Carbon polyunsaturated fatty acids（C18 PUFAs）and is easily metabolized by fish,so it is considered an excellent alternative to fish oil.Due to the lack or weak ability of marine fish to synthesize long chain polyunsaturated fatty acids（LC-PUFAs）,and the content of LC-PUFAs in vegetable oil is very small,which is the key factor restricting the substitution of vegetable oil for fish oil.However,it has been found that some euryhaline fishes,such as Atlantic salmon（Salmo salar）and rabbitfish（Siganus concatenalu）were found to biosynthesize LC-PUFAs by C18 PUFAs,but different marine fish have different synthesis capabilities.Cobia（Rachycentron canadum）is euryhaline and carnivorous tropical marine fish with fast growth,strong disease resistance,high yield,high economic value,tender meat and rich in PUFAs.It is considered as one of the excellent marine fish species cultured in southern China.At present,ideal artifical compound feed for cobia is not developed.Therefore,it is necessary to in-depth systematic research on cobia culture biology,nutrition,physiology and biochemistry,including research on nutrition,physiology and biochemistry of lipid,especially PUFAs and LC-PUFAs.The experiment investigates the effect of different lipid sources on the growth performance,antioxidation and fatty acid composition as well as gene expression of key enzymes for biosynthesizing long chain PUFAs（LC-PUFAs）of juvenile cobia.After being acclimatized for 24 d,570 cobia juveniles,with the age of 47d,an initial body weight（BW）12.6±0.35 g,and body length（BL）11.86±0.21 cm,were selected for formal experiment.The juveniles were randomly assigned into five groups in triplicate,making a total of 15 tanks（400 L volume per tank）,with38 fish in each tank.The juveniles were fed using 5 different diets:（1）Control diet（CO）contained only basic ingredients;（2）fish oil diet（FO）was added fish oil supplementation alone;（3）safflower oil diet（SO）was added 6%safflower oil supplementation alone;（4）perilla oil diet（PO）was added 6%perilla oil supplementation alone;（5）safflower-fish oil diet（SO+FO）was added both 3%safflower and 3%fish oils supplements.The juveniles were fed for 12 weeks and sampled for analysis at 0 week and 12 weeks.The result were as follows:1.Different dietary lipid sources could significantly promote the growth performance of juvenile cobia.The survival rate（SR）of the juveniles fed various dietary PUFAs supplementation was significantly higher（P<0.05）than that of the fish fed CO（92.38%）and the SR of the fish fed SO+FO（100%）and FO（100%）was the highest.The body weight（BW）,relative wight gain（RWG）and specific growth rate（SGR）of juveniles fed various dietary lipid sources were significantly higher than those of the fish fed CO（P<0.05）,in which the BW（150±1.90g）,SGR(2.95±0.05%day^-1),and RWG（1091±26.7%）of the juveniles fed SO+FO was the highest.The protein efficiency ratio（PER）of the juveniles fed various dietary lipid sources was significantly higher（P<0.05）than that of the fish fed CO（1.38±0.03）.All of them,the PER of the juveniles fed SO+FO was the highest.The feed conversion ratio of the fish fed SO+FO（1.24±0.03）were significantly lower（P<0.05）than that of the fish fed CO（1.56±0.03）.The hepatosomatic index（HSI）of the juveniles fed various lipid sources was significantly higher（P<0.05）than that of the fish fed CO.The HSI of the juveniles fed SO was the highest.The lipid and moisture content of the juveniles fed various dietary lipid sources were significantly higher（P<0.05）than that of the fish fed CO.2.Different dietary lipid sources could considerably promote the nucleic acid synthesis of juvenile cobia.In the muscle of the juveniles,the RNA quantity（239.48±0.79μg/mg）in the fish fed SO+FO was significantly higher（P<0.05）than that of the fish fed PO（201.19±0.81μg/mg）,SO（194.86±0.93μg/mg）and CO（143.44±1.25μg/mg）.The RNA/DNA ratio in the fish fed SO+FO was significantly higher（P<0.05）than that of the fish fed CO,PO and SO.Linear regression analysis showed that there was a higher positive correlation coefficient between the SGR and RNA/DNA ratio of muscle in cobia juveniles.The order of correlation was:muscle R²>liver R²>brain R²>serum R²>heart R²>kidney R^2.3.Different dietary lipid sources could significantly promote antioxidant enzymes of juvenile cobia.The activity of superoxide dismutase（SOD）,catalase（CAT）,glutathione peroxidase（GPx）,and total antioxidant capacity（T-AOC）in the tissues/organs of the fish fed various dietary PUFAs supplementation were significantly higher（P<0.05）than that of the fish fed CO.The MDA in the tissues/organs of the fish fed various dietary lipid sources was significantly lower（P<0.05）than that of the fish fed CO.4.The fatty acid composition in the diet could significantly affect the fatty acid composition in fish body.Different dietary lipid sources could significantly affect the fatty acid metabolism of juvenile cobia,However,the degree of influence varies with different lipid sources supplementation.Different tissues/organs of the same fish have different fatty acid metabolism to the same lipid sources.The∑LC-PUFAs,∑n-6 PUFAs,∑n-3 PUFAs,and n-3/n-6PUFAs in the different tissues/organs of the juveniles varied with the same lipid sources.5.The expression ofΔ6 fatty acid desaturase（FADS2）gene in the muscle,liver,brain,heart,and kidney of the fish fed various dietary lipid sources was significantly higher（P<0.05）than that of the fish fed CO.The expression of FADS2 gene in the tissues/organs of the juveniles fed PO and SO was significantly higher（P<0.05）than that of the fish fed FO and SO+FO。The expression of fatty acid elongase 5（ELOVL5）gene in the muscle,liver,brain,heart,and kidney of the fish fed various dietary lipid sources was significantly promoted（P<0.05）by PO>SO>SO+FO,in which the expression was significantly higher（P<0.05）than that of the fish fed CO.The expression of ELOVL5 genes in the juveniles fed PO and SO was significantly higher（P<0.05）than that of the fish fed FO and SO+FO.In conclusion,different dietary lipid sources could significantly promote the growth performance,antioxidation,RNA/DNA ratio and the FADS2 and EVOVL5 genes expression of cobia juveniles.Different lipid sources also significantly affected the fatty acid composition of cobia juveniles.The best diet was 3%fish oil and 3%safflower oil supplementation.The optimum n-3/n-6PUFA ratio was 0.63 in the diet.Cobia may have the ability to synthesize LC-PUFAs,which is related to nutrition and environment.This study not only provides a scientific basis for the preparation of scientific,reasonable,safe,efficient and practical cobia feed,but also provides a theoretical basis for the sustainable development of artificial culture of cobia.