Minimum requirements for omega-3 fatty acids in modern production of Atlantic salmon

There is a lack of knowledge about minimum requirements of EPA and DHA in salmon. The nutritional requirements of fish have largely been defined in terms of growth and survival. A definition of nutritional requirements should however also include information about fish health. An important focus of this project is to determine the minimum requirements of EPA and DHA in various life stages of Atlantic salmon. It is important to define some early symptoms of omega-3 fatty acid deficiency, since sub-optimal deficiencies may presumably lead to increased health risks, even when not manifested by obvious symptoms such as reduced growth and mortality. This project has conducted a study where performance and health of Atlantic salmon were followed from a fish size of 40 gram to 1kg. In the phase from 40 grams to 400 grams, the salmon was fed 14 diets with different levels of EPA or DHA, (1: 1) mixture of EPA and DHA and a commercial control diet. Salmon of 400 grams, from the14 different pre-diet groups, were further followed up to 1 kg with 3 different dietary levels of EPA and DHA. There were no significant differences in growth or mortality between the groups, with the exception of a reduced growth in EPA and DHA deficiency group. The capacities of the salmon to produce EPA and DHA from the shorter chain 18:3n-3, was relatively high when the dietary level of EPA and DHA was less than 1% in the feed, and markedly reduced with higher levels in the feed. Supplementation of the bioactive component lipoic acid to liver cells from the different dietary groups stimulated the capacity for EPA and DHA production. We observed significant changes in the fatty acid composition of the various tissues and organs over time. The heart and brain preserved their EPA and DHA levels to a larger extent than liver, skin, intestine and muscle, especially in phospholipids. When the levels of EPA and DHA were reduced in the different membrane phospholipids, they were to a large extent replaced with the pro-inflammatory n-6 fatty acids. Relatively small differences in tissue morphology between the dietary groups was observed, but the number of mucosal cells in the skin increased with increased level of EPA and DHA in the diet. Macrophages were isolated from salmon of 400 g, and subjected to treatment with various immune-stimulants and viral infection, in order to study the influence of fatty acid composition on the salmon immune response. Data suggests that salmon's immune system responds less in the deficiency group than in the groups fed with EPA and DHA in the diet. Transfer of fish of 1 Kg from land based tanks to sea cages, resulted in higher mortality in the deficiency group than in the fish groups fed moderate levels of EPA and DHA in the diet. In collaboration with the FHF project 900957, we have followed the fish further in sea until they reached approximately 4 kg in order to study the long-term health effects of low dietary level of omega-3 in fish feed.

The world production of fish oil, the major source of the essential fatty acids EPA and DHA, has been stable since the 1970s. Due to increasing demands for EPA and DHA as ingredients in human health products and fish feed, there is now a serious shortage of these fatty on the international markets. This has led to increasing substitution of fish oil by plant oils in Atlantic salmon feed and thereby reducing the EPA and DHA content of salmon. This reduction is probably unavoidable, so it is urgently neede d to know the minimum required levels of these fatty acids in fish diets for securing health and quality. It is known that 75-100% of the fish oil can be substituted by plant oils without negative effects on growth, as long as the diet contains fish meal . Today the level of fish meal in salmon diets is also decreasing leading to an even more pronounced reduction in EPA and DHA. This project will determine the minimum EPA and DHA requirements in diets for modern Atlantic salmon, with fast growth and low f eed conversion ratio, at different life stages in sea-water. The project will further assess if the capacity to produce EPA and DHA from 18:3n-3 is influenced by different dietary fatty acids, bioactive components and temperature. This project will use n ew advanced technologies to define some early symptoms of EPA and DHA deficiency, since sub-deficiency states can result in increased health risks, even when not manifested by overt symptoms and reduced growth.