When cooked, these blocks of fibers flake. The raw muscle of fresh fish is translucent and slightly rubbery, and its color varies. Most species have white or light-colored flesh, but some species, such as tuna, mackerel, and bluefish, have darker flesh. Generally, darker flesh has a higher fat content. Most fish have a layer of dark, fatty tissue just under the skin, but the amount varies enormously among species.
Fish use darker muscle for steady, continuous swimming.
The lighter muscle serves as an energy reserve for sudden bursts of speed. Fish have very active digestive systems. They do not chew their food, and they can gorge themselves when a large amount of food is available. Feeding causes digestive enzymes to pour into their stomachs, and the digestive process accelerates greatly when they feed heavily. Fish Composition Fish muscle is mostly water, protein, and fat.