Effects And Mechanisms Of Lipid Levels On Marine Rainbow Trout(Oncorhynchus Mykiss) Cultured In Recirculating Aquaculture Systems

Rainbow trout(Oncorhynchus mykiss),is a cold-water fish belonging to salmon species,which is considered as one of the most widely farmed fishes all over the world.In China,there is a vast market prospect on trout farming to satisfy the population demands due to the shortage of high value aquatic products.In general,the major culture models of rainbow trout are: flowing water culture,net cage culture and recirculating aquaculture system(RAS).Although,the first two models were intensively investigated,the nutrient requirements in RAS is not clear until now.To make the matter worse,there are some lipid metabolism problems of rainbow trout cultured in this model.Therefore,the present study aimed to exploring the effects of dietary lipid level on growth,physiological metabolism and gene expression of some key enzymes of Donaldson rainbow trout strain,in order to provide both theoretical and fundamental bases as well as the practical references of nutrition regulation,reduction in abdominal fat deposition,mechanism on physiological metabolic and molecular nutrition of rainbow trout in RAS.Furthermore,the combination of animal physiology,biochemistry,molecular biology and eco-nutriology will help to better understand the effects of dietary lipid levels on growth,digestion and absorption enzymes,antioxidant ability,plasma immunity,fat metabolism and related genes expression of rainbow trout(Oncorhynchus mykiss)in RAS.For these reasons,four isonitrogenous diets containing 45% crude protein with increasing dietary lipid(L)levels of 12%,15%,18% and 21%(L12,L15,L18,L21)were fed to satiety to 25 fishes(333.25±20.71 g)for 77 days in triplicate groups.However,the following overview outlines the most important results and indications that included in this thesis as:1.The growth and body indices results showed that the weight gain rate(WGR)in low-fat group(L12)was higher than that in middle and high fat groups(L18,L21)by29.12%-34.35% respectively(P<0.05),while the feed conversion ratio(FCR)inboth low and middle groups(L12,L15)were lower than the other groups(L18,L21)by 12.95%-20.55% respectively(P<0.05).Meanwhile,the abdominal fat(AFR)as well as the visceral fat rates(VFR)in group L18 and L21 were increased by 5.34%and 9.34% respectively,when compared with L12 group(P<0.05).These results are clearly indicating that the lipid levels were the main factor affecting the fat deposition in visceral mass,while there was no such phenomenon happened on group L12.In conclusion,when dietary protein level is 45%,diets with low or moderate fat content are more suitable for the growth and feed utilization of trial fish,which also has effects on decreasing fat deposition around visceral organs and coming into being normal shape,and lipid requirement of rainbow trout cultured in RAS is no more than12%.2.The results of lipase activities in group L12 were higher than other groups by5.98% and 8.82% respectively(P<0.05).Also alkaline phosphatase(AKP)activities in groups L12 and L15 were higher than other two groups by 6.23% and 11.25%respectively(P<0.05).Ammonia nitrogen(NH4+-N)and nitrate nitrogen(NO2-N)concentrations were decreased with increasing lipid levels in the diet.Relative ammonia nitrogen concentrations in L12 were higher(0.127 mg/L/kg)than others by18.44%,23.30% and 24.91% respectively.Similarly,the relative nitrate nitrogen(0.004 mg/L/kg)in this group(L12)was higher than other groups by 140.05%,95.04% and 110.51% respectively.However,ammonia nitrogen absolute concentration was between 0.34-0.48 mg/L in daily detection,which was within the acceptable limits(<0.05 mg/L),so there was no bad influence to the trial fish.The results clearly showed that the feed intake and growth rate in L12 group were higher than other groups,and therefore the elimination of nitrogen was more efficient than other groups.The diets with higher lipid levels improving the nitrogen utilization was another reason.Overall,The digestion,absorption and water quality results also showed that the L12 and L15 groups had better digestion and absorption of fat and other nutrition that both lipase and AKP activities were significantly increased(P<0.05),which was the digestive physiological characteristics or mechanism that ensured these two groups growing better.3.The results of fat physiological metabolism in plasma and liver showed that,a)the activities of fatty acid synthetase(FAS)in liver were decreased with increasing the fat content in the diet,in contrary with the activities of lipolytic enzymes(LPL,HL,L-CPT1)that has been reversely changed.However,FAS activity of group L12 was significantly higher than groups of L18 and L21 with 7.5% and 8.7% respectively(P<0.05),while L-CPT1 activity of this group(L12)was lower than others by 16.35%,13.08% and 15.93% respectively(P<0.05).Meanwhile,activities of both LPL and HL of L12 and L15 groups were lower than L21 by 31.16% and 26.81%,21.62% and18.92%(P<0.05)respectively,indicating that fishes fed on L12 and L15 diets have positively affected the fat utilization rate by increasing fat synthesis and decreasing lipolysis that satisfies the requirements for lipid in rainbow trout and promotes their growth consequently,and b)the dietary lipid levels have affected the transport of cholesterol(CHO)and the synthesis of some lipid.Overall,L12,L15 and L18 groups achieved higher lipoprotein-cholesterol HDL-C/LDL-C ratio than L21 with about65.56%,65.56% and 51.16% respectively(P<0.05)by elevating the content of high density lipoprotein-cholesterol(HDL-C)and decreasing the content of low density lipoprotein-cholesterol(LDL-C),thus the process of CHO entering liver was promoted.Meanwhile,group L12 made up the shortage of exogenous uptake by means of increasing endogenous formation of CHO and triglyceride(TG).Total cholesterol and TG contents in group L12 were significantly higher than other groups with 9.69%-15.72%,and 37.06%-59.55% respectively(P<0.05).In contrast,the HDL-C/LDL-C ratio in group L21 was the lowest(P<0.05),which slowed CHO transshipment to liver significantly,increasing the accumulation of CHO in peripheral tissues.4.So far,the antioxidant mechanism was significantly differed among the groups.High-fat group(L21)mostly made use of superoxide dismutase(SOD)and glutathione(GSH)to increase antioxidant ability.While,SOD and GSH contents in group L21 were significantly higher(P<0.05)than groups L12 and L15 by 4.16% and10.64%,30.89% and 43.50% respectively.Low-fat group(L12)mainly depended on CAT to increase antioxidant ability and decrease MDA content,the CAT content inL12 was significantly higher(P<0.05)than L18 and L21 with 8.50% and 19.39%respectively.Furthermore,this group ensured the minimum oxidative damage to the cell membrane,thus supplied internal condition for better growth performance.In this trial,there was no significant effect of dietary lipid level(P>0.05)on immunologic indices in plasma(containing total protein,globin and lysozyme).5.According to the gene expression results,FAS mRNA expression in muscle was decreasing significantly(P<0.05)with the increase of dietary lipid level.And low-fat group(L12)was significantly higher(P<0.05)than L21 group with 66.82%,while group L12 was significantly higher than others by 55.15%,48.29% and 55.15% in liver,respectively.On the contrary,LPL genes expression in liver was positively correlated with fat content in the diet.High-fat group(L21)was significantly higher(P<0.05 and P<0.01)than low-fat groups(L12,L15,and L18)by 113.59%,36.72%and 44.33%.There was the same trend in muscle.The result was similar to the change of fat metabolism enzyme activities in the liver.For the first time,the present results highlighted that the degree of molecular biological response to dietary lipid level was different between liver and muscle,which has not been reported before.Presented as FAS expression quantity changed faster in muscle than in liver,while LPL genes expression was opposite,which related to the importance of fat deposition in the muscle of trout.6.Preliminary finding are as follow:(1)Fat levels affected the transport of plasma cholesterol(CHO)of rainbow trout,diet with low fat content could promote peripheral CHO to transport into liver for hepatic metabolism,while diet with high fat level significantly slowed the transshipment of CHO to liver.(2)FAS gene expression changed faster in muscle than in liver with increasing fat level in diet,while LPL gene expression in liver was more sensitive than muscle.Fundamentally speaking,the degree of molecular biological response to fat nutrition was different comparing liver with muscle.In conclusion,this study showed that dietary lipid level had a significant effect on growth performance,abdominal fat deposition,fat metabolism and related genes expression of rainbow trout(Oncorhynchus mykiss)in the recirculating aquaculturesystem(RAS).On the one hand,the groups with low and middle fat contents improve the activities of FAS and reduce the activities of LPL by regulating gene expression of key enzymes in liver and muscle,and transport fat into liver effectively to promote fat metabolism,which satisfies the need on fatty acid,and reduces the abdominal fat deposition.On the other hand,these two groups enhanced antioxidant capacity by SOD and CAT to keep the healthy growth of the studied fish species.The results of this trial showed that the Donaldsons rainbow trout strain(330-600g)could achieve healthy growth without excessive fat supply,not more than 12%,in RAS with a dietary protein level of 45%,which is different with the results of previous studies that the diets of rainbow trout are always with higher fat content(15%-20%).The finding deals with the trouble of excessive fat deposition on rainbow trout by means of nutrition regulation,physiological metabolism and gene expression,and supplies the preliminary answer to the need for developing the special nutritional feed for different culture models.