Establishing how feed, stress and the colour of salmon flesh are interconnected

Salmon flesh gets its reddish colour from a pigment called astaxanthin, which is added to the feed. According to the Norwegian Seafood Research Fund (FHF), pigment levels in salmon fillets have decreased in recent years, and some fish farms have reported that 2020 saw some of the lowest pigment levels ever measured in Norwegian farmed salmon.

In recent years, both the feed and the stress levels of salmon have changed. Farmed salmon eat feed that contains a lower percentage of marine-based raw materials, and frequent delousing stresses the fish. This may increase the fish’s need for the astaxanthin pigment, which is also an antioxidant. Therefore, the scientists will investigate how feed composition affects fillet colour and the salmon’s ability to manage stress.

“This project will provide the knowledge necessary for the future salmon industry to safely produce salmon using modern feed ingredients, without compromising fillet pigmentation and fish health”, says Kristian Prytz, Head of Aquaculture Industry/Processing at FHF.

Pigmentation is affected by feed composition

The research that is now underway builds on an FHF project where Nofima scientists and NTNU investigated the connection between the content of marine-based raw materials in feed and salmon fillet colour. When there are fewer marine-based raw materials in the feed, there are also lower amounts of different types of fats, such as phospholipids. Phospholipid is important for the transport of nutrients through the intestine, and can affect how salmon utilise pigments.

“The FHF project was very interesting, but we were also left with new questions that needed answering”, says Trine Ytrestøyl, senior scientist at Nofima.

In summary, the research shows that pigmentation can be affected when changing feed composition:

• The amount of phospholipid in the feed affected the salmon’s digestibility of astaxanthin and fat. The salmon grew more slowly if the levels of phospholipid in their feed were too low.
• Not having enough fish meal in the feed reduced salmon appetite, and fat accumulation was found in the intestine.
• When phospholipid was added to feed low in fish meal, it normalised salmon digestibility and growth.
• Salmon that received feed supplemented with marine-based phospholipid deposited less intestinal fat than salmon that received feed supplemented with phospholipid made from soya beans.
• Although pigment digestibility was similar in the two groups, the salmon that received soya lecithin had the reddest fillets.

“The results of this trial show that although intestinal uptake is a bottleneck regarding the utilisation of astaxanthin, what happens after intestinal uptake is also very important in relation to how much of the pigment ends up in salmon muscle”, says Ytrestøyl.

A new project has just started

In the new project, which is in collaboration with NTNU and Skretting, investigations will take place to find out whether there is a link between the content of vitamin A in the feed and the colour of salmon muscle, and whether salmon stress has an impact on pigmentation.

“The project will also include work focusing on the importance of feed phospholipids in relation to the colour of salmon flesh. We will compare plant-based phospholipids with marine-based phospholipids to see how this can affect the salmon’s utilisation of astaxanthin”, says Ytrestøyl.

Turning off genes in order to test the effect on pigmentation

Ytrestøyl, who has a PhD in pigmentation, is looking forward to delving even deeper into basic biology. In addition to feeding trials, cell models and new, advanced methods (gene editing with CRISPR/Cas9) will also be used to investigate biological mechanisms involved in in the salmon’s use of astaxanthin.

“Based on experiments using mammals, we have selected some candidate genes whose function we want to investigate through the use of gene editing (CRISPR). These are genes involved in the uptake, conversion and deposition of astaxanthin in various tissues and organs”, says Ytrestøyl.

Despite the fact that the colour of salmon fillets is an important characteristic when it comes to quality, there is still very little known about how external factors and biological mechanisms interact and lead to reduced pigmentation in salmon fillets.

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