Influence Of Replacing Fish Oil With Schizochytrium Sp.Oil On Growth Performance,Health Status And Lipid Metabolism In Juvenile Grass Carp,Ctenopharygodon Idellus

Fish oil is an important source of n-3 long-chain polyunsaturated fatty acids(LC PUFA),with shortage of resources and high price,therefore,finding economically viable and environmentally sustainable fish oil alternatives is an important task for the global aquaculture industry.In recent years,some microalgae products rich in n-3LC PUFAs such as Schizochytrium sp.(Sc)have become a promising alternative source.While extracting the refining Sc oil,the lipid-extracted by-products have also been obtained(stearin oil and wax grease oil).In this study,grass carp(Ctenopharygodon idellus)was used as the research object to evaluate the effects of dietary Sc oil on the growth,body composition,antioxidant activities and lipid metabolism of juvenile grass carp.Evaluated the effects of fish oil and two Sc oil by-products,stearin oil and wax grease oil and stearin oil with exogenous EPA on growth and health of grass carp.Provide basic information for the replacement of fish oil and the use of by-products of Sc industrial.Trial 1 In order to investigate the effects of replacing fish oil with Sc oil(stearin oil and wax grease oil)on growth performance,antioxidant status and lipid metabolism of juvenile grass carp a basic diet without n-3LC PUFA was used as a control group,the three treatment groups provided fish oil(FO),stearin oil(SO)and wax grease oil(WGO)with 0.52% n-3 LC PUFA were fed grass carp(12.71 g ± 0.99g)for 56 days.The results showed that(1)the specific growth rate of the FO,SO and WGO groups was significantly higher than that of the control group,and SO group was obviously higher than that of FO group and the WGO group(P<0.05);(2)The crude protein content of muscle and whole fish was notably increased,the whole fish crude lipid content and adipocyte size of intraperitoneal fat in FO,SO and WGO groups were significantly lower than those in the control group(P<0.05);(3)the FO,SO,and WGO groups significantly increased the levels of DHA and n-3 LC PUFA in muscle of grass carp(P<0.05),which preferentially deposited on the polar lipids of the muscle,in terms of fatty acid health index,there was no significant difference in thrombogenicity index(TI)among the groups(P>0.05),the atherogenicity index(AI)of SO group and WGO group was significantly higher than that of FO group,and the hypocholesterolemic/Hypercholesterolemic ratio(h/H)was significantly lower than that of FO group(P<0.05);(4)Total antioxidant capacity(T-AOC)in serum of FO,SO and WGO groups was obviously increased,T-AOC in hepatopancreas increased significantly in FO group and SO group(P<0.05).Superoxide dismutase(SOD)activity in FO group was apparently higher in hepatopancreas and serum than in other groups(P<0.05).In hepatopancreas,the FO and SO groups had significantly higher maleic dialdehyde(MDA)content than that in control group(P<0.05),and there was no significant difference in serum MDA among the groups(P>0.05);(5)In FO,SO and WGO groups,the ATGL expression level was markedly up-regulated in adipose tissue,the expression level of amino acid catabolism-related gene GLDH was obviously down-regulated in muscle,and the expression of GLDH in SO group was notably lower than that in FO group(P<0.05).The above results showed that fish oil,Sc oil(stearin oil and wax grease oil)can promote the growth performance of grass carp,enhance the nutritional value of fish,significantly inhibite the crude lipid content of whole fish,and reduce the adipocyte size.Stearin oil and wax grease oil had no negative effect on the antioxidant status of experimental fish,and fish oil exhibited better antioxidant effect and fatty acid health index on fish than Sc oil.Dietary stearin oil may show better growth performance than dietary fish oil by saving energy required for de novo synthesis of DHA,promoting lipolysis energy supply and inhibiting protein catabolism.In summary,Sc oil(stearin oil and wax grease oil)can be used as a source of n-3 LC PUFA in grass carp diets.Trial 2 Equipped with five groups of iso-nitrogen and isoenergetic diets,which were the same as the basic formula of Trial 1,and fed grass carp(22.70g±0.80g)for 49 d.The0.52% n-3LC PUFA of the diets were provided with fish oil(F-O);Sc sterin oil(S-O);sterin oil(DHA): EPA =3:2(SE1-O);sterin oil(DHA): EPA = 1: 1(SE2-O);EPA(E-O),respectively.The results showed:(1)the crude protein content of muscle in E-O group was apparently lower than that of S-O group(P<0.05);(2)In muscle,the EPA proportion of E-O group was significantly higher than S-O,SE1-O and SE2-O groups(P<0.05),the DHA content of S-O group were notably higher than those of F-O group,SE1-O and E-O groups(P<0.05),the percentage of C18: 3n-3 in SE2-O group and E-O group was obviously lower than F-O group(P<0.05),there was no significant difference in AI among the groups(P>0.05),the TI of SE2-O group was significantly higher than that of F-O group,h / H was markedly lower in S-O group and SE2-O group than in F-O group(P<0.05);(3)The content of MDA in liver,the activity of SOD and CAT in muscle and liver had no significant difference among the groups(P>0.05);in muscle,the MDA content in E-O group was significantly higher than other groups(P<0.05);(4)the adipose size of intraperitoneal fat in E-O group was notably higher than that in F-O group and SE1-O group(P<0.05),and the ATGL and CPT1 m RNA levels in E-O group were apparently down-regulated(P<0.05);in muscle,the fatty acid desaturase FAD m RNA level was markedly higher in E-O group than S-O group(P<0.05),the fatty acid elongase ELO m RNA level was obviously down-regulated in S-O group(P<0.05).The aboveresults showed that there was no significant difference on the growth performance,n-3LC PUFA content,antioxidant activity,and adipose tissue hydrolysis of grass carp when used Sc oil alone or combined with EPA.It may be because EPA was not easy to accumulate in grass carp,and it was more inclined to be oxidized or converted to DHA.When EPA was used as the only source of n-3LC PUFA,the hydrolysis and ?-oxidation of adipose tissue and the content of crude protein of muscle were reduced.In addition,high levels of DHA in the diet may reduce the body's ability to synthesize LC PUFA.Therefore,grass carp may need DHA more than EPA,so Sc oil can be used alone in feed industry without compatibility with EPA.In summary,when Sc oil was used as a source of n-3LC PUFA,which increased redistribution and utilization of lipids,and showed better growth-promoting performance than fish oil.When Sc oil was used combination with EPA,it had no improvement on growth,antioxidant capacity and lipid metabolism than the use of Sc oil alone,maybe related to that EPA was not easy to accumulate in grass carp,and it was more inclined to oxidize or convert to DHA.Therefore,Sc oil can be the sole source of n-3LC PUFA in grass carp diets.