Astaxanthin is a carotenoid. Like many carotenoids, it is a colorful, fat/oil-soluble pigment. Astaxanthin can be found in microalgae, yeast, salmon, trout, krill, shrimp, crayfish, crustaceans, and the feathers of some birds.
While astaxanthin is a natural nutritional component, it can be found as a food supplement. The supplement is intended for human, animal, and aquaculture consumption. The commercial production of astaxanthin comes from both natural and synthetic sources.
In aquaculture it is used to assist captive fish population to keep their natural bright colors, particularly reds, oranges and yellows. Cecil is feeding his brook trout the supplement to make sure his brookies stay colorful. You often see it added to tropical fish food so they maintain their colors as well. Krill, shrimp and scuds often have large amounts of natural astaxanthin in them. The BOWs where these species are present in high numbers often have vividly colored trout.
As an interesting side note, astaxanthin is what gives flamingo's their pinkish color. Without a diet of the shrimp the flamingos eat, they'd be just plain old white.
