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.
Fat content varies greatly among species and fluctuates seasonally within species.
Fat content depends on the season and on the fish's size, age, and sex. It tends to be higher during warmer months, when food is more plentiful. Fish provides important nutrients for the human diet. Fish protein is a complete protein that contains all the essential amino acids. It is also easily digested.