The color of the salmon fillet comes from the fish’s ability to store the pigment astaxanthin in its muscle. Astaxanthin is produced by marine algae and found throughout marine food chains. It’s a powerful antioxidant, and salmon can also convert it into vitamin A—a nutrient that plays a vital role in immune function, skin and mucous membrane health, fat metabolism, and vision.
To support salmon health and growth, both astaxanthin and vitamin A are added to salmon feed. The pink color of the fillet is also important for its visual appeal in stores.
“However, the need for vitamins and antioxidants may increase when salmon are exposed to stress, and we still don’t fully understand how environmental factors influence fillet color,” said Trine Ytrestøyl, senior scientist at Nofima.
Stress from sea lice treatment
Stress triggers the production of reactive compounds in the body that can damage cells. Antioxidants help neutralize these compounds, reducing harm to the organism. For salmon under stress, such as during sea lice treatments, the demand for antioxidants like astaxanthin may rise.
Throughout their life, salmon experience significant stress. Sea lice issues often require repeated treatments, during which fish are crowded and oxygen levels in the water drop—conditions that are extremely stressful for the animals.
“In our research using data from commercial facilities, we observed that salmon undergoing multiple sea lice treatments had noticeably paler fillets,” Ytrestøyl said.
Feed composition matters
The ingredients used in feed can significantly impact fillet color. “We’ve previously found that marine omega-3 fatty acids, EPA and DHA, positively influence fillet color,” Ytrestøyl explained. “But these fatty acids are often reduced in feed as marine ingredients are replaced with plant-based alternatives.”
Vitamin A, which is abundant in marine food chains and also acts as an antioxidant, has also declined in salmon feed due to this shift toward plant-based sources.
While salmon can synthesize some vitamin A from astaxanthin when dietary levels are low, the effect of this process on fillet pigmentation has not been thoroughly studied. Similarly, little is known about how stress affects the deposition of astaxanthin in muscle tissue.
In a recent experiment, salmon were fed diets with three different vitamin A levels and two levels of astaxanthin. These concentrations reflected the variation found in commercial feeds. The salmon were then subjected to stress conditions mimicking sea lice treatment—being crowded and exposed to low oxygen levels several times a week over a five-week period.
The study is part of the project Dietary Factors and Physiological Mechanisms Interact and Control Pigmentation of Salmon Muscle, funded by the Norwegian Seafood Research Fund. The project is a collaboration between NTNU – the Norwegian University of Science and Technology, Skretting, and Nofima.
Results
When salmon were not stressed, the highest level of vitamin A in the feed had a negative effect on the fillet color. Less astaxanthin was absorbed in the intestine and digested when there was a lot of vitamin A in the feed. This may be a form of regulation to protect the salmon from excessively high levels of vitamin A, which has been shown to be harmful.
Salmon exposed to stress had less astaxanthin in the fillet compared to the control group. But there was one exception: salmon that had received feed with a high content of both vitamin A and astaxanthin did not develop paler fillets as a result of stress. Thus, there was a different effect of high vitamin A content in the feed when the salmon were stressed compared to when they were not. This may indicate that the requirements for vitamins and antioxidants change when salmon are exposed to a stressful environment.
The research results show that salmon exposed to stress develop paler fillet color, and that the need for antioxidants may change when salmon are subjected to stress.
"Stress should be reduced as much as possible, but it cannot be completely eliminated in aquaculture. To ensure the fish have good health and quality, it is important that the amount of vitamins and antioxidants in the feed is adjusted to the challenges the fish face,” concluded Ytrestøyl.
Download a fact sheet about the results (in Norwegian) below.
