The right kind of fat

Omega-3 oils really do make a difference in salmon feed, Vincent McDonagh reports.

New research by Nofima, Norway’s highly respected Food Research Institute, has confirmed that increased levels of omega-3 in salmon feed can lead to healthier and stronger fish.

Fish with more omega-3 in their diet are also better able to cope with challenges such as stress and infection, which is especially important during the sea phase of their life cycle.

Scientists at the organisation have carried a recent detailed study into the content of feed and how it impacts on salmon.

They found that limited access to fishmeal and fish oil for use in salmon feed means that some ingredients have been replaced by plant-based raw materials, leading to a lower content of marine omega-3. This, the study found, has changed the fatty acid composition in the salmons’ tissue and organs.

The study has been supported financially by FHF, the Norwegian fisheries and aquaculture research fund, and led by Nofima’s “OptiHealth” programme researchers. They looked closely at the salmons’ need for omega-3 fatty acids and asked whether there is a need to change recommendations regarding feed composition.

FHF says salmon today are exposed to more handling and challenging environmental conditions, adding that fat levels and specific fatty acids play key a role in many biological functions. This means that if the composition of the fat in feed is altered, it can affect both growth, muscle quality and the health and robustness of fish.

Omega-3 on the menu

Different diets were tested throughout the project. Salmon were given feed in which the amount of omega-3 (ie EPA and DHA) varied from 1% to 3.5%. The highest level corresponded to a diet with about 50% fish oil.

“Overall, the results showed that the higher the omega-3, the better the salmon performed in every way,” FHF declares.

The fish showed signs of improved disease resistance and lower mortality. And they had fewer melanin spots, improved muscle quality and better growth, at least in their sea phase.

The results also showed, however, that feeding salmon with a vegetable-oil-rich diet delivered improved growth in the freshwater phase, although it did not make the fish better equipped for the seawater cycle.

It poses the question: “Can adjustments in the feed recipe in the freshwater phase make the fish better able to withstand the seawater release?”

Norwegian fish farms generally suffer a loss of around 16-17% in the sea phase, with a large proportion occurring immediately after the transfer.

A project led by the Norwegian Institute of Marine Research has tested different feed recipes in the freshwater phase and studied how salmon handles the transition into the sea. One of the recipes was close to commercial quality, while others had adjusted levels of amino acids, omega-3, minerals and salts in various combinations.

None of the adjusted recipes had an effect on robustness, survival or growth when compared to the commercial-quality feed.

The researchers think this may indicate that the freshwater feed already on the market today covers a salmon’s nutritional needs in this phase of its life.

The content of the long-chain fatty acids is often stated as the sum of EPA + DHA, without mentioning the ratio between the two fatty acids, but it has long been thought that DHA is the more essential.

Building up the immune system

But FHF says another experiment, led by the Institute of Marine Research, has shown that EPA has a unique and special role in the immune system of Atlantic salmon, especially when they are exposed to viral infection.

The reason for this study is because there is the potential to add a lot more DHA and less EPA to some feeds. It is important to find out whether it would be sufficient to add DHA from these sources or whether a small proportion of EPA could have unforeseen consequences. While the studies were not carried out on live fish, they did show that EPA is very important for salmon.

A high level of marine omega-3 in a fish’s diet is likely to improve its quality, FHF maintains, and this was most evident in the sea phase.

It also turns out that when it comes to maintaining fish health, quality and welfare, salmon have a greater need for essential fatty acids than was thought only a few years ago.

As supplies of fishmeal and fish oils are limited, however, the study concludes, the industry should look for alternative sources of omega-3 that are rich in both DHA and EPA. These sources could include yeast, microalgae, krill or genetically modified plants.

Bente Ruyter, a senior researcher at Nofima, believes that in the future the industry will be able to find a number of additional sources.

She adds: “We know that the feed industry is already well under way with this job and has included the results of these studies in their new feed recipes. In fact, the omega-3 levels in feed are now higher than when we did our studies.”

Bente Ruyter