Due to the different ability to accumulate pigments.
Once, "red fish" referred to sturgeon species such as beluga, sevruga, sturgeon, and sterlet. This name was not related to their color but indicated the special nutritional value of these fish.
Later, this term began to be used for salmon due to the red-orange color of their flesh. In the muscle tissues of these fish, pigments are accumulated from the shells of crustaceans they consume.
Salmon with a different diet have flesh that remains whitish-translucent, like most fish. Other fish species also eat crustaceans, but pigments do not accumulate in their tissues.
In contrast to red fish, the flesh of another group of salmonids — whitefish — began to be called "white fish."
