Stressed salmon become paler, with one exception, according to new research. (The SalmoFan colour chart in the picture). Photo: Nofima

Stressed Salmon Show Paler Fillets, Despite Increased Pigment in Feed

New research by Nofima reveals how stress affects pigment absorption in farmed Atlantic salmon

Tromsø — Despite higher levels of pigment in commercial salmon feed, the distinctive pink hue of salmon fillets appears to be fading. New research conducted by the Norwegian Institute of Food, Fisheries and Aquaculture Research (Nofima) has identified a key reason: stress.

The pigment astaxanthin, which gives salmon its pink color, is also a powerful antioxidant and a precursor to vitamin A, essential for immune function, metabolism, and vision. Although both astaxanthin and vitamin A are routinely added to salmon feed to promote fish health and desirable coloration, researchers have now found that stress significantly impacts the fish’s ability to retain this pigment in muscle tissue.

“Even though more pigments are being added to feed than before, stressed salmon fillets are still turning out paler,” says Trine Ytrestøyl, senior scientist at Nofima.

Stress from Sea Lice Treatments Linked to Pigment Loss

Farmed salmon are often exposed to high-stress events, especially during sea lice treatments, when fish are crowded and oxygen levels drop. These conditions lead to oxidative stress in the body, increasing the demand for antioxidants like astaxanthin.

“When we analyzed data from commercial farms, we found that salmon undergoing multiple sea lice treatments had noticeably paler fillets,” explains Ytrestøyl.

Feed Composition Matters

The study also examined how changes in feed ingredients affect pigment retention. Marine omega-3 fatty acids, like EPA and DHA, have been shown to support better fillet coloration. However, their presence in feed has declined as fishmeal and fish oil are replaced with plant-based ingredients.

<-- Trine Ytrestøyl has researched salmon and how stress combined with different feeds affects the red colour. The fish in the photo was not part of the research. Photo: Terje Aamodt/Nofima.

Vitamin A, which is naturally abundant in marine ecosystems, has similarly declined in modern salmon diets due to the shift toward plant-based inputs.

“There’s a hypothesis that when less vitamin A is available in the feed, salmon might convert more astaxanthin into vitamin A,” says Ytrestøyl. “But the implications of this conversion for fillet pigmentation remain unclear.”

Experimental Findings: A Complex Interaction

In a controlled experiment, salmon were given feeds with three levels of vitamin A and two levels of astaxanthin. These fish were also exposed to stressors that mimic sea lice treatments—crowding and oxygen depletion—several times a week over five weeks.

Key findings included:

• Unstressed salmon fed high levels of vitamin A absorbed less astaxanthin, resulting in paler fillets. This suggests a regulatory mechanism to avoid excess vitamin A, which can be toxic in high concentrations.

• Stressed salmon had lower astaxanthin levels in their fillets overall, except in one group: those fed with high levels of both vitamin A and astaxanthin. These fish maintained normal fillet coloration despite stress.

This indicates a differential nutritional requirement under stress, where elevated levels of certain nutrients may mitigate the negative effects of environmental pressure.

Rethinking Nutritional Strategies in Aquaculture

The study highlights the importance of tailoring salmon nutrition to environmental conditions. As Ytrestøyl emphasizes, “While we should strive to minimize stress in aquaculture systems, it can’t be entirely eliminated. That means we must adapt feed formulations to support fish health and fillet quality under real-world conditions.”

The findings come from the project “Dietary Factors and Physiological Mechanisms Interact and Control Pigmentation of Salmon Muscle,” funded by the Norwegian Seafood Research Fund (FHF). The project is a collaboration between Nofima, NTNU (Norwegian University of Science and Technology), and feed producer Skretting.

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