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27It has been known for almost 25 years now that inclusion of intact phospholipids in the diet could 28 improve culture performance of various freshwater and marine fish species. The primary 29 beneficial effect was improved growth in both larvae and early juveniles, but also increased 30 survival rates and decreased incidence of malformation in larvae, and perhaps increased stress 31 resistance. Determination of absolute dietary requirements has been hampered by the use, in 32 different dietary trials, of a wide range of phospholipid preparations that can vary greatly both in 33 phospholipid content and class composition. Larval studies have been compromised further by the 34 need on many occasions to supply phospholipid through enrichment of live feeds with subsequent 35 re-modelling of the phospholipid and fatty acid compositions. Generally, the levels of 36 phospholipid requirement are around 2 -4% of diet for juvenile fish and probably higher in larval 37 fish. The effects were restricted to young fish, as a requirement for dietary phospholipids has not 38 been established for adult fish, although this has been virtually unstudied. As the majority of 39 studies have used crude mixed phospholipid preparations, particularly soybean lecithin, but also 40 other plant phospholipids and egg yolk lecithin, that are enriched in several phospholipids, it has 41 been difficult to elucidate which specific phospholipid classes impart beneficial effects. Based on 42 the few studies where single pure phospholipid species have been used, the rank order for efficacy 43 appears to be phosphatidylcholine > phosphatidylinositol > phosphatidylethanolamine > 44 phosphatidylserine. The efficacy of other phospholipid classes or sphingolipids is not known. The 45 mechanism underpinning the role of the phospholipids in larval and early juvenile fish must also 46 explain their lack of effect in adult fish. The role of phospholipids appears to be independent of 47 fatty acid requirements although the presence of an unsaturated fatty acid at the sn-2 position may 48 be important. Similarly, the phospholipid requirement is not related to the delivery of other 49 essential dietary components such as the bases choline and inositol. Studies also suggested that the 50 phospholipid effect was not due to generally enhanced emulsification and digestion of lipids. 51Rather the evidence led to the hypothesis that early developing stages of fish had impaired ability 52 to transport dietary lipids away from the intestine possibly through limitations in lipoprotein 53 synthesis. The current hypothesis is that the enzymic location of the limitation is actually in 54 phospholipid biosynthesis, perhaps the production of the glycerophosphobase backbone and that 55 dietary supplementation with intact phospholipids in larvae and juvenile fish compensated for this. 56

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In order to enhance growth, survival and quality during early juvenile stages of marine fish it is important to avoid lipid oxidation problems that are known to cause pathologies and disease. The aim of the present study was to characterise and compare the antioxidant systems in juvenile marine fish of commercial importance in European aquaculture, namely turbot (Scophthalmus maximus), halibut (Hippoglossus hippoglossus) and gilthead sea bream (Sparus aurata). The experiment investigated the interaction of the dietary antioxidant micronutrient, vitamin E, with antioxidant defence systems. Fish were fed diets of identical unsaturation index supplemented with graded amounts of vitamin E. The relationships between dietary and subsequent tissue vitamin E levels were determined as well as the effects of vitamin E supplementation on lipid and fatty acid compositions of both liver and whole fish, on the activities of the liver antioxidant defence enzymes, and on the levels of liver and whole body lipid peroxidation products, malondialdehyde (thiobarbituric acid reactive substances, TBARS) and isoprostanes. Growth and survival was only significantly affected in sea bream where feeding the diet with the lowest vitamin E resulted in decreased survival and growth. A gradation was observed in tissue vitamin E and PUFA/vitamin E levels in response to dietary vitamin E levels in all species. The activities of the main radical scavenging enzymes in the liver, catalase, superoxide dismutase and glutathione peroxidase generally reflected dietary and tissue vitamin E levels being highest in fish fed the lowest level of vitamin E. The indicators of lipid peroxidation gave consistent results in all three species, generally being highest in fish fed the unsupplemented diet and generally lowest in fish fed the diet with highest vitamin E. In this respect, isoprostane levels generally paralleled TBARS levels supporting their value as indicators of oxidative stress in fish. Overall the relationships observed were logical in that decreased dietary vitamin E led to decreased levels of tissue vitamin E, and generally higher activities of the liver antioxidant enzymes and higher levels of lipid peroxides

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