The pancreas secretes enzymes that digest carbohydrates into the intestine. The inner surface of the intestine may be folded or have fingerlike projections that extend into the intestinal cavity. Such structures increase surface area for absorption. Foods not completely digested during passage through the intestine leave the body through the anal opening. The importance of proper protein nutrition cannot be overemphasized.
Proteins are also a significant source of dietary energy.
Fish are especially efficient at converting food to body tissues, so they need less food to grow than do many other animals. The diet of fish contains relatively little carbohydrate matter, the amount of protein is high as a percentage of the diet. The protein requirements of fish decreases as the fish grow larger. Protein requirements are dramatically affected by water temperature. When fish are held at cool temps.