Triplicate groups of European sea bass (Dicentrarchus labrax L.), of initial weight 90 g, were fed four practical‐type diets in which the added oil was 1000 g kg−1 fish oil (FO) (control diet), 600 g kg−1 rapeseed oil (RO) and 400 g kg−1 FO, 600 g kg−1 linseed oil (LO) and 400 g kg−1 FO, and 600 g kg−1 olive oil (OO) and 400 g kg−1 FO for 34 weeks. After sampling, the remaining fish were switched to the 1000 g kg−1 FO diet for a further 14 weeks. Fatty acid composition of flesh total lipid was influenced by dietary fatty acid input but specific fatty acids were selectively retained or utilized. There was selective deposition and retention of docosahexaenoic acid (DHA; 22:6n‐3). Eicosapentaenoic acid (EPA; 20:5n‐3) and DHA were significantly reduced and linolenic (LNA; 18:3n‐3), linoleic (LA; 18:2n‐6) and oleic (OA; 18:1n‐9) acids significantly increased in flesh lipids following the inclusion of 600 g kg−1 RO, LO and OO in the diets. No significant differences were found among different treatments on plasma concentrations of prostaglandin E2 and prostaglandin F2α. Evaluation of non‐specific immune function, showed that the number of circulating leucocytes was significantly affected (P < 0.001), as was macrophage respiratory burst activity (P < 0.006) in fish fed vegetable oil diets. Accumulation of large amounts of lipid droplets were observed within the hepatocytes in relation to decreased levels of dietary n‐3 HUFA, although no signs of cellular necrosis was evident. After feeding a FO finishing diet for 14 weeks, DHA and total n‐3 HUFA levels were restored to values in control fish although EPA remained 18% higher in control than in the other treatments. This study suggests that vegetable oils such as RO, LO and OO can potentially be used as partial substitutes for dietary FO in European sea bass culture, during the grow out phase, without compromising growth rates but may alter some immune parameters.

No abstract

Within a decade or so insufficient fish oil (FO) will be available to meet the requirements for aquaculture growth. Consequently, alternative sources are being investigated to reduce reliance on wild fish as a source of FO. Vegetable oils (VO) are a feasible alternative to FO. However, it is important to establish that alternative dietary lipids are not only supplied in the correct quantities and balance for optimal growth, but can maintain immune function and prevent infection, since it is known that the nutritional state of the fish can influence their immune function and disease resistance. A way of maintaining immune function, while replacing dietary FO, is by using a blend of VO rather than a single oil. In this study, juvenile European sea bass (Dicentrarchus labrax) were fed diets with a 60 % substitution of FO with a blend of rapeseed, linseed and palm oils. Two oil blends were used to achieve a fatty acid composition similar to FO, in terms of energy content, and provide a similar balance of SFA, MUFA and PUFA. Fish were fed the diets for 64 weeks, after which time growth and fatty acid compositions of liver and blood leucocytes were monitored. The impact of the dietary blends on selected innate immune responses and histopathology were also assessed, together with levels of plasma prostaglandin E 2 . The results suggest that potential exists for replacing FO with a VO blend in farmed sea bass feeds without compromising growth, non-specific immune function or histology.

Echium oil (EO) is a vegetable oil in which percentages of stearidonic acid (STA,18:4n-3) often exceed those of its n-6 series equivalent γ-linolenic acid (GLA, 18:3n-6). STA is elongated to 20:4n-3 in fish cell cultures, suggesting that EO could be included in diets for marine fish to increase tissue 20:4n-3 and 20:3n-6 and, thereby, modulate eicosanoid metabolism. Thus, the present study aimed to test the hypotheses that dietary EO would increase tissue 20:4n-3 and 20:3n-6 and modulate immune function and eicosanoid production in juvenile Atlantic cod (Gadus morhua L.) fed a diet where fish oil (FO) was replaced by EO. Duplicate groups of juvenile cod (initial weight ca. 4g) were fed for 18 weeks on fish meal based diets (55% protein and 16% lipid) that differed in oil source (FO or EO). There were no significant differences in growth and feed efficiency, hepato-somatic index, percentages of liver and flesh lipids and lipid class compositions for cod fed FO and EO. Percentages of 18:4n-3, 18:3n-6 and 20:3n-6 in the total lipids of flesh and liver were higher, and percentages of 20:5n-3 and 20:4n-6 were both lower in fish fed EO than in those given FO. In flesh, the increased 18:3n-6 and 18:4n-3 were primarily located in phosphatidylcholine and, to a lesser extent, phosphatidylethanolamine, whereas 20:3n-6 concentration was highest in phosphatidylinositol. Desaturation of 18:3n-3 (to tetraene products) and 20:5n-3 to 22:6n-3 in hepatocytes was very low but was increased by dietary EO. EO significantly decreased the production of prostaglandin F from gill cells stimulated with calcium ionophore A23187, and reduced head kidney macrophage activity, but had no effect on serum lysozyme activity or basic haematology. In conclusion, dietary EO may have beneficial effects on some immune parameters including eicosanoid metabolism in marine fish although this may be primarily due to decreased 20:4n-6 rather than increasing tissue levels of 20:3n-6 or 20:4n-3