Astaxanthin in farmed trout

Astaxanthin is a naturally occurring pink-red carotenoid produced by microalgae in the aquatic environment. These microalgae are consumed by all kinds of zooplankton and crustaceans. In turn, zooplankton and crustaceans are eaten by many fish species, including trout and salmon.

In trout and salmon, astaxanthin is bound to actomyosin in the muscles, which gives the flesh a pink colour. Salmon and trout that live their adult lives in the sea eat large quantities of crustaceans and therefore develop a rich pink colour. Fish cannot produce astaxanthin on their own, so when pink flesh is required in farmed fish, the pigment needs to be added.

Astaxanthin is not just a pigment, however. It has many biological functions that support the development, growth and health of trout.

Although there are other, similar pigments, astaxanthin is the most efficient carotenoid to pigment the flesh of trout. Astaxanthin is lipid soluble, and its pigmentation efficiency is aided by an increased dietary lipid level. The fatty acid profile can have an impact, as feeds with higher levels of polyunsaturated fatty acids (PUFA) tend to exhibit greater pigmentation efficiency. Increased levels of vitamin E have also appeared to increase pigmentation efficiency, possibly due to this vitamin’s antioxidative capacity, which helps to preserve astaxanthin. The pigmentation efficiency of astaxanthin is, however, rather low overall. It usually does not exceed 20%, but it does increase in rainbow trout and salmon as they get larger.

The astaxanthin used in trout feed can either be natural, via cultured microorganisms or synthetic. Well-known organic astaxanthin sources are produced by the microalgae Haematococcus pluvialis, the bacterium Paracoccus carotinifaciens or the yeast Phaffia rhodozyma. Both natural and synthetic astaxanthin work well and have the same properties.

Levels of astaxanthin inclusion

Typically, for practical purposes, trout finishing diets contain astaxanthin levels ranging from 60 to 80 ppm, as these concentrations yield the best results. In most markets, trout with a visual colour rating of 27–28 on the Salmofan™ colour chart are preferred. To achieve this pigmentation in trout weighing up to 500 grams, it is necessary to feed them astaxanthin roughly until their weight doubles. In the case of larger trout, astaxanthin should be fed until the fish’s weight is increased by 30–50%.

Multiple factors can influence pigmentation efficiency, including the astaxanthin level in the feed, feed intake, feed conversion ratio (FCR), duration of astaxanthin feeding, fish size, maturation stage and oxidative stress. To achieve the desired flesh colour, astaxanthin must be fed for a sufficient period of time. This is necessary due to genetic and individual variations in pigmentation, including the variation of feed intake among individual fish.

Changing colours during trout maturation

Most of the time, the stability of astaxanthin in the flesh is quite high, and if a choice is made to switch to feed without astaxanthin, the muscle astaxanthin level will remain unaffected for up to two months. This changes when trout are sexually mature. Astaxanthin is then transferred from their muscles to their eggs during the reproductive phase through hormonal changes, and the colour of the meat fades, while the eggs become deeply pink. The transferred astaxanthin protects the eggs against harmful UV rays. Astaxanthin transferred to the eggs will also transfer to the skin of the broodstock, which helps their camouflage in natural conditions. This will eventually lead to a lighter flesh colour. For large, pigmentated trout, it would be appropriate to either use late-maturing rainbow trout strains or triploid fish. The latter do not mature and therefore do not have the maturation challenges that accompany diploid fish.

Feed production and storage conditions

Astaxanthin is a powerful antioxidant, preventing the oils in feed from oxidation. As such, though, it is labile to light, heat and oxygen. During the feed production process, precautions must be in place so that astaxanthin quantities are meeting the required levels after feed production. Also, the importance of specialty feed additives must not be overlooked. Chelated minerals, in particular, have been proven to support stability in astaxanthin and vitamins better than their inorganic counterparts (Figure 1).

Figure 1. Astaxanthin survival depending on chelated minerals and unchelated minerals. Research completed in the Alltech Coppens Aqua Centre.

Once the feed is in storage, astaxanthin will react with oxygen whereby it protects the oils in the feed from oxidation and therefore its level can slowly decrease over time.

Feed containing astaxanthin should be stored in a cool, dry place, away from direct sunlight, to prolong shelf life. At Alltech Coppens, we have a shelf-life management protocol of our Astax products. Also, farmers are advised to practise a storage management protocol such as the “first in, first out” principle. Once opened, a bag of feed containing astaxanthin should be finished quickly.

Astaxanthin is much more than a pigment, not only resulting in nicely pink meat in farmed trout but also supporting the fish’s health and growth.

Further reading

Bjerkeng, B. 2000. “Carotenoid pigmentation of salmonid fishes — recent progress.” In Cruz-Suárez, L.E., Ricque-Marie, D., Tapia-Salazar, M., Olvera-Novoa, M.A., and Civera-Cerecedo, R. (eds.). Avances en Nutrición Acuícola V. Memorias del V Simposium Internacional de Nutrición Acuícola, Mérida, Yucatán.

Hertramph, J. and Piedad-Oascua, F. 2000. “Handbook on ingredients for aquaculture feeds.” Kluwer Academic Publishers.

Seposo, S. 2022. “Application of astaxanthin on fish feed for enhanced pigmentation efficacy on rainbow trout (Oncorhynchus mykiss) fillet, a review of related literature.” Internship, Alltech Coppens.

Storebakken, T. and No, H.K., 1992. Pigmentation of rainbow trout. Aquaculture, 800: 209-229.