Salmon flesh gets its reddish color from a pigment called astaxanthin, an antioxidant that is added to the feed in farmed salmon, Norway's Nofima applied research institute explained.

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, Nofima said in its announcement.

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, which may increase their need for astaxanthin, Nofima said.

Therefore, researchers are investigating how feed composition affects fillet color 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,” said Kristian Prytz, head of aquaculture industry/processing at FHF.

Pigmentation affected by feed composition

The research underway now builds on an FHF project that teamed up Nofima scientists with researchers at the Norwegian University of Science & Technology (NTNU) to investigate the connection between the content of marine-based raw materials in feed and salmon fillet color.

When there are fewer marine-based raw materials in the feed, there are also lower amounts of different types of fats, such as phospholipids, Nofima explained. Phospholipid is important for the transport of nutrients through the intestine and can affect how salmon utilize pigments.

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

In summary, Nofima said the research shows that pigmentation can be affected when changing feed composition in the following ways:

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

New project

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 vitamin A content in the feed and the color of salmon muscle and whether salmon stress has an impact on pigmentation, Nofima said.

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

Ytrestøyl, who has a doctorate in pigmentation, said she is looking forward to delving even deeper into basic biology. In addition to feeding trials, the researchers will use cell models and new advanced methods — such as gene editing with CRISPR/Cas9 — to investigate biological mechanisms involved in in the salmon’s use of astaxanthin, the announcement said.

“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,” Ytrestøyl said.

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