The flesh of all salmon, irrespective of their origin – farmed or wild – is a pink colour because they consume a carotenoid antioxidant that comes from their diet.
Carotenoids are a naturally occurring group of pigments that pass on colour to the tissue of a variety of organisms. More than 600 naturally occurring carotenoids have been identified in plants and animals – everything from tomatoes to flamingos. They even produce the colours of autumn leaves.
Carotenoids found in fish belong to a group known as xanthophylls and include astaxanthin. Astaxanthin is the major carotenoid naturally found in wild salmon and crustaceans such as shrimp and lobsters, and is responsible for their pink-red pigmentation.
Diets containing carotenoids have also been shown to increase the growth rate and survival of juvenile fish – in both starter feeds and at the fingerling stage.
Astaxanthin is made by microorganisms such as yeast and algae. These materials are eaten by small fish and crustaceans, which in turn are eaten by salmon. It is when salmon and crustaceans eat these smaller fish and crustaceans that they accumulate sufficient levels of the carotenoid – deposited in the muscle but also in their skin and eggs – to change their colour.
Synthesised pigments are commonly used in a variety of animal feeds as standard practice, including feeds for farmed fish. In cases where organic feeds are formulated, the pigment is derived from bacteria.
Because salmon cannot make their own astaxanthin and must consume it via their diet, it has been the norm for many years to feed salmon raised in seawater with diets that contain nature-identical astaxanthin. This means that while the astaxanthin is synthesised, it is a pure version of what is eaten by wild salmon – providing health benefits to the fish and the familiar colouring that appeals to end-consumers all over the world.
It should be noted, however, that longstanding regulations relating to feed additives have stipulated that synthetic astaxanthin cannot be given to salmonids in the first six months of their life.
Whether obtained naturally or synthesised in a nature-identical form, astaxanthin is also an antioxidant and a precursor to vitamin A. These two functions help to protect tissue against oxidative damage, stimulate the immune system, and improve fertility and growth.
As female salmon prepare for breeding, xanthophylls are transported to the ovaries where they improve the maturation rate of the oocytes (eggs/ova). The pigments become part of the yolk sac.
When the eggs are released, the pigments have two further functions, protecting the eggs from damage by light and helping the male to find them.