Depending on habitat (the place where the fish lives) and the innate preferences designed by evolutionary adaptation (to survive in a particular habitat one species of fish might eat plants, while another might eat shell fish. In this way, both species can survive in the same habitat) fish have numerous modes of feeding. Some fish are herbivores, eating only plants. Others are carnivores, eating meat. Still others are omnivores and consume plants and meat. Pacific Salmon are omnivores. They eat smaller fish, crustaceans, and insects.

When eating, food enters the digestive tract through the mouth. From the mouth, the food passes through a short tube called the esophagus to the stomach, which partially breaks down the food. From the stomach food passes through an organ humans do not have. This organ is called the pyloric ceca, a series of finger-like pouches. It is located were the stomach and intestine meet. Through the secretion of enzymes the pyloric ceca breaks down the food further and absorbs nutrients. The intestine completes the digestive process. The liver and pancreas secrete digestive enzymes to the intestine. (An interesting note: Herbivores tend to have longer intestines than carnivores. This is true of most animals.) Materials that pass the intestine undigested are eliminated through the anus.