Can farmed salmon retain flesh color under stress? Research points to a nutritional solution

Stress reduces color intensity of farmed salmon fillets, but high levels of vitamin A and astaxanthin in feed may help maintain pigmentation under pressure

Stress reduces the fillet color of farmed salmon, but adjusting antioxidant and vitamin levels in fish feed may help preserve pigmentation under challenging aquaculture conditions, concludes a new study from Nofima.

Salmon fillet color comes from the fish’s ability to store astaxanthin, a red pigment found in marine algae and passed along the food chain. Astaxanthin is a powerful antioxidant supporting the synthesis of vitamin A, essential for immune function, skin and mucous membrane health, fat metabolism and vision. To support fish health and growth – and meet consumer expectations – salmon feed is supplemented with astaxanthin and vitamin A.

Despite the increasing use of pigments in feed, fillet color has grown noticeably paler. In a recent study, Nofima researchers identified stress as a key factor in this loss of pigmentation, with one exception: stressed-out salmon fed high levels of both vitamin A and astaxanthin retained their color.

“The needs for vitamins and antioxidants may change when salmon are exposed to stress, and we still do not know enough about how the environment in which the salmon lives affects the color of the fillet,” said Trine Ytrestøyl, senior scientist at Nofima.

Stress triggers the formation of reactive compounds in the body that can damage cells, but antioxidants like astaxanthin help neutralize these harmful substances. When salmon are exposed to stress, their need for antioxidants increases. Throughout their life cycle, farmed salmon often endure significant stress – particularly during sea lice treatments, which involve crowding and reduced oxygen levels in the water, creating highly stressful conditions for the fish.

“When we examined data from commercial productions, we observed that salmon that had undergone many treatments for sea lice had paler fillets,” said Ytrestøyl.

Feed composition also plays a role in the color of salmon fillets: “We have previously seen that the marine omega-3 fatty acids EPA and DHA have a positive effect on fillet color. These fatty acids are reduced in the feed when marine feed ingredients are replaced with plant-based ingredients,” Ytrestøyl said.

Vitamin A, another antioxidant abundant in marine food chains, has also declined in salmon feed as marine ingredients have been replaced with plant-based alternatives.

“It is possible that salmon will produce more vitamin A from astaxanthin when there is little vitamin A in the feed, but the effect of this on the astaxanthin content in the fillet has not been studied in detail,” Ytrestøyl said, adding that the impact of stress on fillet color has not yet been fully investigated.

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To explore these factors, researchers conducted an experiment where salmon were fed diets with three different levels of vitamin A and two levels of astaxanthin, reflecting the range typically found in commercial feeds. The fish were then exposed to repeated stress over five weeks by crowding them and lowering oxygen levels in the water, mimicking conditions during sea lice treatment.

The results were notable: in salmon not exposed to stress, high levels of vitamin A in the feed reduced fillet color. When vitamin A was abundant, less astaxanthin was absorbed and digested in the intestine, possibly a regulatory response to prevent harmful excess levels of the vitamin.

In contrast, stressed salmon generally had lower astaxanthin levels in their fillets compared to the control group. However, there was one exception: fish that received feed with high levels of vitamin A and astaxanthin did not develop paler fillets under stress.

The findings indicate that stress exposure reduces fillet color in salmon and suggest that the fish’s need for vitamins and antioxidants may change under such conditions. This highlights the importance of considering both diet and environmental factors when aiming to maintain fillet quality in aquaculture.

“Stress should be reduced as much as possible, but it cannot be completely eliminated in aquaculture,” said Ytrestøyl. “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.”

Read more here.

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