If it weren't for an added colorant, it would be grey or white like most of the other fish on display at the store.
The next time you're passing a seafood counter, take a good look at the salmon filets. That deep red color you see, that rich hue that makes the fish so attractive to certain shoppers, is not naturally occurring in farmed fish. It comes from an additive that's mixed into fish feed. In fact, if fish farmers didn't add it, the farmed salmon would be grey. Suddenly that doesn't seem nearly so appetizing, does it?
The red color found in wild salmon comes from their diverse and natural diet of crustaceans, such as krill and shrimp. These little critters contain a reddish compound called astaxanthin, the same that turns flamingos pink. Quartz reported that the spectrum varies with the species:
"Since Alaska’s sockeye salmon are closer to the Bering Sea’s teeming krill, they’re the reddest of all. Salmon further south— Coho, king, and pink, for instance — eat relatively less krill and shrimp, giving them a lighter orange hue."
But farmed salmon hunt for none of these crustaceans. Kept in pens, they are fed a mix of ground-up anchovies and herring, fish oil, corn gluten, food processing byproducts such as wheat and soy, and, of course, astaxanthin in additive form, either derived from crustaceans or formulated in a lab.
This food coloring is the most expensive component of fish feed, accounting for 20 percent of its cost, but according to salmon farmer Don Read, who works in British Columbia, "If we didn’t do it, customers wouldn’t buy it... Consumers buy what they are comfortable with. They won’t go into the store to buy white salmon." Read told TIME that he wishes he and other fish farmers didn't have to use the colorant, as it would save significant amounts of money, but "that's not the way it works."
I recently spoke out against eating fish and the complex problems associated with aquaculture, and my opinion on those issues has not changed; but I do think it's important for customers to be aware of what's in their food, and to understand that farmed/domesticated/processed versions are never the same as the real, wild thing, no matter how hard we try to replicate it.