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. The protein content of fish varies, but it generally amounts to 15 to 20 percent of the tissue. Water and fat account for most of the remaining 80 to 85 percent.