Optimising dietary fatty acids and lipids of Atlantic salmon to secure their health and welfare through varying environmental conditions

Fish oil (FO) and fishmeal are the major sources of omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in Atlantic salmon diets. When fish oils decrease in fish feed, the fatty acid (FA) composition of tissues and organs in Atlantic salmon changes. Since fats and specific FAs have central roles in many biological functions, changes in dietary FA composition is associated with changes in fatty acid profile in different tissues, which may have an important impact on fish robustness and its nutritional value for human consumption. Overall, the main goal of the project has been to gain knowledge about the dietary requirement of fatty acids and lipid levels needed for production of a robust Atlantic salmon. A land-based feeding trial was conducted in order to elucidate how the genetic background of the fish and different ratios of omega-3 FAs in the diet influence the endogenous capacity to produce the healthy EPA and DHA FAs. Particularly, this study investigates the consequences of replacing dietary FO with a plant oil from Camelina sativa (CO) on three Atlantic salmon groups either non-selected (NS) or selected for high and low delta-6 desaturase capacity (the capacity to produce EPA and DHA); evaluating the potential effect of an early feeding in freshwater prior to transfer to seawater. The experimental fish groups were then fed diets with different ratios of alfa-linolenic acid (ALA), EPA, DHA, and EPA+DHA in the seawater phase. Results showed that fish fed the CO diet had higher growth rates in the freshwater phase, especially in the high delta-6 desaturase group. However, whole body DHA and EPA levels decreased compared to fish fed the FO diet, with no impact on mortality after seawater transfer. The improved growth induced by the CO diet in freshwater levelled off in the seawater period with no differences between the genetic groups. Instead, higher dietary levels of DHA in the seawater phase increased growth rates of all three genetic groups indicating the importance of a omega-3 FA rich diet during this period. These results provide a better understanding on how to implement genetic selection to nutritional studies in order to improve endogenous EPA and DHA production and to improve EPA and DHA metabolic capacities by nutritional programming in freshwater prior to seawater transfer using plant oil-based diets. Furthermore, an experiment has been carried out in sea cages at the Gifas Research station in Nordland, where the main objective has been to provide new knowledge about the levels of fats and fatty acids required to ensure the production of a robust Atlantic salmon throughout the seawater phase from a fish size of 200 grams up to a slaughter size of 5 Kg. In the study, groups of salmon received four different feeds. All the feeds contained low levels of marine protein (basic feed). The feeds differed in that they had four different levels of fish oil as a source of EPA and DHA. It was 1, 1.3, 1.6 and 3.5 percent EPA and DHA respectively as a proportion of the feed. The results showed a clear pattern; increasing proportion of omega-3 fatty acids in the feed improved fish health, growth and muscle quality with lower incidence of black melanin spots, stronger red fillet colour and higher proportion of EPA and DHA (omega-3). In summary, the present project has generated new knowledge on the requirement and utilization of EPA and DHA in Atlantic salmon under different rearing conditions and life stages. This knowledge will help the salmon industry to produce salmon without compromising fish growth, welfare and muscle quality.

The project organisation has secured close communication between research institutes and universities, feed producers and salmon industry. Results have been presented at 1 international conference, 2 seminars and 2 dialogue meetings for the industry in collaboration with FHF. Some of the latest results was presented at AquaNor 2019 and gained a lot of interest from both feed and fish producers as well as public in general. These results have further been published and discussed in several popular-scientific dissemination publications, including Norwegian newspapers and radio debates. One scientific article is already published and two more will soon be submitted to peer reviewed journals. New knowledge from the project, on requirement and utilization of EPA and DHA under different rearing conditions and life stages, will enable the salmon industry to produce diets for salmon without compromising fish health and welfare and muscle quality.

The proposed project aims to generate new knowledge regarding dietary fatty acid and lipids for Atlantic salmon to secure fish health and welfare through challenging environmental conditions and different life stages. Due to shortage of fish oil, such knowledge is necessary for sustainable growth of the salmon industry while securing fish health. In this project, EPA and DHA requirements during long-term growth in sea cages will be fine-tuned in the area between 10-16 g/kg of the diet, where knowledge is currently lacking. The potential impact of lipid level on the requirement will be studied. Shorter term land-based trials will distinguish the health effects of the level of n-6 fatty acids from the n-6/n-3 ratio and the n-3 level, as well as give information about the effects of the different n-3 fatty acids (EPA, DHA and ALA) alone, in combination and in different ratios. Furthermore, the impact of genetic background and life stage of the fish will be studied. All trials will include assessment of the retention and deposition of EPA and DHA in whole fish and different tissues, as well as evaluation of health status and study of lipid metabolism. Additionally, as the dietary fatty acid composition needs to be adequate for fish exposed to the various challenges in commercial aquaculture productions, our land-based trials will include tests of how the fish cope with hypoxia/ chronic and acute stress, a gill infection challenge and vaccination with subsequent evaluation of immune responses.