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Due to its traditionally good availability, digestibility and high content of n ) 3 HUFA, fish oil is the main lipid source in fish feeds. However, world demand for this product has grown significantly in recent years, whereas its production, based on fisheries landings, is static. The purpose of the present study was to assess the effect of partial replacement of fish oil in compound diets for gilthead seabream and seabass, by several vegetable oil sources, on growth, dietary fatty acid utilization and flesh quality. Five iso-energetic and isoproteic experimental diets were formulated (25% lipid content). Fish oil was the only added lipid source in the control (FO) diet, and it was included in the other experimental diets at a level high enough (40% of FO diet) to keep the n ) 3 HUFA levels well over 3% in order to cover the essential fatty acid requirements of these species. Fish oil was replaced by soyabean oil (SO), rapeseed oil (RO) and linseed oil (LO) or a mixture (Mix) of them. Feed intake in all dietary groups was in the range of results obtained for commercial diets in both species, and growth and feed utilization were very good. The results show that, providing a minimum content of essential fatty acids in the diet, it is possible to replace up to 60% of the fish oil by SO, LO and RO or a mixture of them in diets for seabream and seabass, without compromising fish growth. Fatty acid composition of liver and muscle reflected that of the diet, but utilization of dietary lipids differed between these two tissues and was also different for the different fatty acids. Despite reduction in dietary saturated fatty acids by the inclusion of vegetable oils, their levels in fish liver were as high as in fish fed the fish oil diet, whereas, in muscle, levels were reduced according to that in the diet. Linoleic and linolenic acids were accumulated in the liver proportionally to their levels in the diet, suggesting a lower oxidation of these fatty acids in comparison to other 18C fatty acids. Regarding eicosapentaenoic acid (20 : 5n ) 3; EPA), docosahexaenoic acid (22 : 6n ) 3; DHA) and arachidonic acid (20 : 4n ) 6; ARA), these essential fatty acids were reduced in the liver at a similar rate, whereas DHA was preferentially retained in the muscle in comparison with the other fatty acids, denoting a higher oxidation particularly of EPA, in the muscle. Some other PUFA increased despite their low dietary levels in seabream fed LO diets and in seabass fed SO diet, suggesting the stimulation of delta-6 and delta-5 desaturase activity in marine fish. Despite differences in fatty acid composition, fillet of fish fed vegetable oils was very well accepted by trained judges when assessed cooked. KEY WORDS

To determine the effect of long term feeding of diets with high levels of vegetable oils gilthead seabream juveniles were fed for 7 months with several diets containing vegetable oils at two inclusion levels. Anchovy oil was the only lipid source of FO (fish oil) diet, whereas in diets 60RO, 60LO and 60SO, 60% of fish oil was substituted by rapeseed (RO), linseed (LO) or soybean (SO) oils, respectively, and 80% was substituted by linseed or soybean oils in diets 80LO and 80SO. Afterwards, all fish were fed only a fish oil diet to determine the progressive evolution of fillet fatty acid profiles, since fish muscle fatty acid composition changes by feeding vegetable oils affecting its nutritional value for human health.Results showed that substitution by vegetable oils of up to 60% fish oil in diets for gilthead seabream does not affect growth and feed utilization even after a long feeding period. However, 80% substitution of fish oil significantly reduced growth. Feeding vegetable oils reduced muscle contents of docosahexaenoic acid (DHA) and arachidonic acid (ARA) to a lower degree than their reduction in the diet, denoting their importance, reduction of eicosapentaenoic acid (EPA) in muscle being more pronounced. Moreover, re-feeding with a FO diet for 60 days effectively recovered muscle DHA and ARA contents, whereas those of EPA were not recovered even after 90 days. Linoleic acid was strongly retained even after bwash outQ. Fish fillets were very well accepted by the panel of judges. Dietary soybean oil inclusion produced significantly less shiny and less hard fillets, but more juicy and adhesive than FO fillets, with a slight earthy flavour when fish were fed 80SO diet. However linseed oil inclusion only increased shininess when fish were fed 80LO diet. A significantly higher chroma value was obtained for 80LO fish fillets, although it was not noticeable to the human eye. D

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In the present study 75 g European sea bass were fed for 8 months with different diets (22% dietary lipid content) containing vegetable oils at two inclusion levels. The control diet (Diet FO) contained anchovy oil as the only lipid source; in diets 60RO, 60LO, and 60SO of fish oil was substituted by rapeseed, linseed or soybean oils, respectively; finally, in diet 80LO, 80% of the fish oil was substituted by linseed oil. Fish were fed to apparent satiation three times a day. All fish were individually weighed once per month. Lipid and fatty acid composition of diets and fish fillets were determined at the beginning, middle and end of the experimental period. Once the commercial size was reached, all fish were fed a 100% FO containing diet during 150 days.No significant differences were found in feed intake. Fish fed the diets containing 80% linseed oil or 60% rapeseed oil had significant ( P b 0.05) lower growth. Flesh fatty acid composition of total lipids reflected the fatty acids in the diets. Flesh contents of n-3 HUFA were reduced to about 45% in fish fed diets 60RO, 60LO or 60SO and to about 50% in diet 80LO. This reduction was markedly higher for EPA than for DHA. High levels of oleic, linoleic and linolenic acids were found in fish fed RO, SO and LO respectively.After 150 days of re-feeding period with a 100% fish oil diet, DHA levels were restored in those fish previously fed diets containing vegetable oils, but EPA levels remained lower when compared with fish fed 100% FO diet. Flesh content of linoleic and linolenic acids remained higher in those fish previously fed soybean and linseed oil containing diets, respectively.

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