Source: FHF

Factors Affecting Pigment Coloration in Salmon

Previous studies have indicated that genetics control as much as 60% of the variation in fillet color, but also feed composition and management-related stress can affect this.

Image illustrating the color of the fillets of the pale and red genetic lines. Source: Norwegian University of Life Sciences

Pale or uneven red color in salmon muscle is increasingly being reported as a quality problem for Norwegian farmed salmon. The pigment level has steadily decreased in recent years, and in 2020 there are fish farmers reporting the lowest pigment levels ever measured in Norwegian farmed salmon. Increasing the pigment addition to the feed does not seem to solve the problems, so more knowledge is needed to find other solutions.

Genes control pigmentation

Salmon probably respond to stress by consuming astaxanthin as an antioxidant. To solve the pigmentation problem, the astaxanthin content in the feed has been increased, but due to the low absorption through the intestine, this has not resulted in a sufficient improvement of muscle coloration.

Astaxanthin concentration in muscle of individual fish from the PALE and RED lines (n=80 in total). Source: Norwegian University of Life Sciences

The objective of this project was:

1) using salmon knock-out (KO) lines to study the effect of single candidate genes on coloration and to identify metabolic networks that are affected by these genes.

2) using selected salmon hatchling lines (red/pale) in a seawater experiment to quantify pigment uptake and examine whether repeated exposure to hypoxia/stress affects coloration.

Transition from seafood to vegetable

Source: Norwegian University of Life Sciences

The experiments confirmed that the beta-carotene oxygenase 1-like gene is probably the most important gene controlling pigmentation. Gene expression analysis also revealed that astaxanthin metabolism and underlying gene synthesis pathways are linked to cholesterol biosynthesis, vitamin D synthesis, and lipid metabolism. Considering that vegetable oil-based diets affect genes that control lipid metabolism, the results may indicate that the transition from marine to plant-based diets may have affected astaxanthin metabolism as well.

The seawater experiment showed no loss of astaxanthin or fillet color after repeated starvation and hypoxia prior to slaughter. This observation applied to both the red and pale genetic lines. The stress-exposed groups of fish also had lower body weight and fitness factor compared to non-stressed controls.

Sockeye salmon: genetic effects

Cluster of differentially expressed genes in the liver of bco1 KO. Colored outlines around genes indicate whether the gene is up- or down-regulated; red means it is down regulated and blue means it is up regulated. The more intense the color, the stronger the up/down dimming. Source: Norwegian University of Life Sciences -->

The project has discovered important molecular mechanisms and genes that control pigment coloration in salmon muscle, through experiments with fish using different genetic lines selected for different degrees of coloration. Furthermore, the results indicate that repeated stress has a negative effect in a production context, but that this is independent of pigmentation. The overlapping networks of genes that control astaxanthin and lipid metabolism highlight that different sources of fat in the feed may be important in achieving satisfactory pigmentation.

Author/Source: Stine Falk-Petersen /FHF 

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