The gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax) require n-3 long-chain polyunsaturated fatty acids (LC-PUFA), eicosapentaenoic acid (EPA) and docosahexaenoic acid DHA, for optimal growth and health. Due to the rapid growth of global aquaculture the quantity of marine oils used in aquafeeds has been limited, yet the overall quantity of oil in an aquafeed has increased by the addition of vegetable oil (VO) to supply dietary energy. For aquaculture to continue to grow more fish must be produced with less marine ingredients, yet EPA and DHA must be maintained at levels above fish requirements. This project set out to re-evaluate the requirement for EPA and DHA in gilthead seabream and European seabass. Two dose-response studies were designed and executed where juvenile seabream and seabass were fed one of six levels of EPA+DHA (0.2 – 3.2 % as fed). Biometric data were collected and analysed to determine new requirement estimates for EPA+DHA for fish of two weight ranges (24 – 80 g and 80 – 200 g). The effects of the dietary LC-PUFA gradient on lipid composition and metabolism were also considered. This project found that the requirement for EPA+DHA declines with fish weight and that the current published EFA requirements are too low for both species when fed modern diet formulations. At a size range of 24 – 80 g, the period when a 3 mm pellet is consumed, the optimum requirement for growth is 1.3 – 1.5 % EPA+DHA, for both species. Beyond ~80 g (4.5 mm pellet) seabream require 1.20 – 1.25 % EPA+DHA, whereas seabass require 1.10 – 1.20 % EPA+DHA. Previous studies in both species, indicated that juveniles require approximately 1% LC-PUFA in their diets. In both species the addition of VO to the diet increased the level of lipid in the liver. Fatty acid and gene expression data showed that LC-PUFA biosynthesis was stimulated in key tissues, liver and mid-intestine, as FO was replaced by VO. The expression of lipogenic genes was also upregulated in the mid-intestine of both species but in liver only in seabream. The implications of this project are that EPA and DHA need to be supplied at a higher level when fish are < 80 g (3 mm pellet) and then in larger pellet sizes dietary FO can be reduced, whence optimizing the application of this commodity
