HAVBRUKS-Havbruk - en næring i vekst

Atlantic salmon both store and produce marine omega-3 fatty acids (EPA and particularly DHA) when amount in the feed is limited. The main aim of this project was to determine how much EPA+DHA salmon needs, what the consequences are for growth and health if the content in the feeds is reduced, and how feeds can be optimized so that the fish will save EPA and DHA (by storing these while using other fatty acids for energy), and produce their own, by desaturating and elongating 18:3n-3 present in plant oils. In two long-term feeding trials during the seawater phase, Atlantic salmon were given low dietary levels of EPA and DHA from 1.3 to 7.4 % of total fatty acids (4-24 mg kg-1 feed), at 6°C and 12°C. The growth rate of the fish was negatively affected by low dietary EPA+DHA. The trial showed that salmon can be a net producer of DHA when fed low dietary levels, as retention of DHA was 120-200 %, meaning that we found up to twice as much DHA in the fish as it had been given from the feed. The retention of EPA was lower, and there were no differences in tissue concentrations between the diet groups. For DHA, reductions in tissue levels were observed with low dietary supply, most apparent in red blood cells. In retina and brain, effects on DHA were only seen at the low temperature. These results suggest that salmon have a specific requirement for EPA+DHA for optimal growth in sea and maintenance of DHA tissue levels at > 2.7 % of fatty acids. Furthermore, samples were analyzed from full-scale commercial production, using 5 vs 8 % EPA+DHA, and results were similar to findings from the experimental trials. During the grow-out period, fish were exposed to stress from repeated lice treatments and an outbreak of pancreas disease on the site. The lack of significant differences in growth or mortality between the diet groups, indicate that reducing the level of EPA+DHA from 8 to 5 % of dietary fatty acids, does not reduce fish robustness. A further trial conducted in this project investigated if retention of n-3 fatty acids were influenced by other FA classes in the diet, such as level of monounsaturated- (MUFA), saturated- (SFA) and n-6 FAs. The feeding trial used a mixture design, with rapeseed oil, palm oil and soybean oil, providing high MUFA, SFA and n-6, respectively. Linseed- and fish oils were used to balance the diets to similar contents of 18:3n-3, EPA and DHA. There were no differences in growth, but the palm oil diet gave increased feed conversion ratio and reduced fatty acid digestibility. Retention data showed that high dietary n-6 of up to 43 % of fatty acids did not negatively affect elongation and desaturation of n-3 FAs, but high n-6 fatty acids negatively affected the incorporation of n-3 fatty acids in the membranes.

The proposed project aims to develop strategies to maintain adequate levels of EPA and DHA in salmon fillet with low levels in the feed, to support the best possible utilization of this limited resource while ensuring that salmon is still a good source of these nutrients. This project will continue an ongoing collaboration between Skretting and NIFES in a project aiming to determine the requirement of EPA and DHA in salmon through the whole seawater production cycle, by feeding graded levels of EPA+DHA f rom about 1-5% of total lipid at two different temperatures (#217478). Unexpected results from this trial will be followed-up by re-running all diet groups at 12°C. Furthermore, material from a large scale CAC-trial testing two different levels of EPA and DHA will be utilized for in-depth studies on the consequences of low levels of these fatty acids on fish health, lipid metabolism and fatty acid retention. Furthermore, an additional feeding trial will be run to investigate possible effects of dietary n- 3/n-6 ratio and level of saturated fatty acids on fillet accumulation of EPA and DHA and on lipid metabolism and fish health. To extract additional information from all of these three feeding trials, in-depth analyses will be performed by subtraction libr ary and proteomics to elucidate molecular mechanisms. Additionally, the metabolic fate of C18 precursors will be quantified by in vitro studies on hepatocytes isolated from fish fed varying levels of EPA and DHA. Finally, transcriptional control of lipid homeostasis will be investigated by developing new methodology for Atlantic salmon in this area. Method development will be done in close collaboration with the existing project Nutritox #200506, taking advantage of method development and international co llaboration within this project.