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New studies show that salmon grow more and cope better with challenges such as infection and stress when fed on diets that are high in marine omega-3, especially during the seawater phase.

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  Maja Bævre-Jensen, Norwegian Seafood Research Fund (FHF)

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Limited access to fish meal and fish oil for use in salmon feed has meant that a proportion of the raw materials in the feed have been replaced with vegetable raw materials. The content of marine omega-3 has also decreased, which has changed the fatty acid composition in the salmon’s tissues and organs. Fat levels and specific fatty acids play key roles in many biological functions, and changes in fat composition in the feed can therefore affect growth, muscle quality and the health and robustness of the fish.

In a recently completed FHF-funded project led by Nofima called ‘OptiHealth’, scientists have looked more closely at salmon’s need for omega-3 fatty acids and whether there is a need to change the recommendations on feed composition. This is because salmon are currently exposed to more handling and challenging environmental conditions than was previously the case.

Omega-3 is particularly important during the sea phase

Throughout the project, various diets have been tested. Salmon were given feed in which the amount of omega-3 (EPA and DHA) varied from 1 percent to 3.5 percent. The highest level corresponds to a diet consisting of approximately 50% fish oil. The results consistently showed that the higher the level of omega-3, the better the salmon performed in all respects. The fish showed signs of improved resistance to disease, lower mortality rates, had fewer melanin spots, had better muscle quality and better growth – at least during the sea phase.

“It is very rare that we have such clear results. We measured many different parameters, and all of them pointed in the same direction”, project manager and senior scientist Bente Ruyter told Tekfisk in an article on 28.08.19.

The results showed that in terms of growth, feeding with a plant oil-rich diet resulted in better growth during the freshwater phase. However, the fish that had received this diet had no advantages over salmon that had received more marine oils when the fish were then transferred to seawater. In other words, it did not make these fish more robust in terms of withstanding sea transfer and life in seawater.

Freshwater feed did not affect the salmon’s robustness during the sea phase

Can adjustments to the feed recipe during the freshwater phase make the fish more robust to cope with seawater transfer? Norwegian salmon farming loses approximately 16-17% of its fish during the seawater phase, where a large proportion of this loss occurs immediately after transfer to seawater. A project led by the Norwegian Institute of Marine Research called ‘OptiNutr’ has tested various feed recipes during the freshwater phase and studied how the salmon coped with the transition to sea. One of the feeds was almost identical to a commercial feed, while the others had adjusted levels of amino acids, omega-3 and minerals and salts in various combinations. In this project, none of the adjusted recipes had effects on robustness, survival rates or growth after transfer compared to the commercial control feed. This may indicate that the freshwater feed that is already on the market today meets the salmon’s nutritional requirements during this phase of life.

EPA and DHA are important

The content of long-chain omega-3 fatty acids in diets is often stated as the sum of EPA + DHA, without really taking into account the ratio of the two fatty acids. Salmon DHA requirements have been thoroughly studied. EPA is mainly converted into DHA and is used for energy by the cells to a greater extent than DHA. Therefore, many have believed that DHA is more essential than EPA.

In the recently concluded FHF project called ‘Do salmon need EPA fatty acid for optimal immune response?’, the trials showed indications that EPA plays a unique and special role in the immune system of Atlantic salmon, especially when they are exposed to viral infections. The project is led by the Norwegian Institute of Marine Research. The background for this trial is that some of the alternative sources of omega-3 that can potentially be added to salmon feed contain a lot of DHA and not as much EPA. It was therefore important to find out whether it would be sufficient to add DHA from these sources to salmon feed, or whether a small amount of EPA in the feed could have unforeseen consequences. Although these studies have been conducted in cell lines and not in live salmon, they provide sufficient indications that EPA is also important for salmon.

Important to find alternative sources of omega-3

The fish that thrive the most and do best are fish that receive the highest level of marine omega-3 in their diet. This was most evident in the sea phase, and it turns out that salmon have a greater need for these essential fatty acids than anyone thought just a few years ago when it comes to maintaining fish health, quality and fish welfare. Due to the fact that there is limited access to fish oils and fish meal, the results of these studies provide a clear picture that the industry should look for alternative sources of omega-3 that are rich in both DHA and EPA. Examples of such sources are yeast, microalgae, krill or genetically modified plants.

“In the future, it will be possible to combine many sources. We know that the feed industry is already well underway regarding this job and has implemented the results of these studies in their new feed recipes. The omega-3 levels in commercial feeds are now higher than they were when we conducted our studies”, concludes Ruyter.

About the research

  • This article is written by the Norwegian Seafood Research Fund (FHF), which has also funded the mentioned research conducted by Nofima and the Norwegian Institute of Marine Research.
  • The article was first published at fhf.no.

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