Atlantic salmon in streams feed mainly upon aquatic insects, including larvae and nymphs of chironomids, mayflies, caddisflies, blackflies and stoneflies. At sea, Atlantic salmon eat a variety of marine organisms, including crustaceans such as euphausiids, amphipods and decapods, and such fishes as sand lance, smelt, alewives, herring, capelin, small mackerel and small cod (Scott and Crossman, 1973) (Table 1).
Digestive system:
The major divisions of the digestive tract are the mouth, esophagus, stomach, pyloric caeca, rectum and secretory glands, which include the liver and pancreas (Figure 3). The newly hatched fry (alevin) has a yolk sac attached to its stomach that supplies the essential nutrients stored for its subsistence until the digestive tract is functional and ready to accept exogenous food. Pyloric caeca, blind-ended finger-like projections (40–74) extend outward from the pyloric valve regions of the stomach and the anterior intestine (Rust, 2002). Their function resembles that of the intestine rather than the stomach, and digestion and some absorption occur in these regions. The small intestine contains the openings for the bile and pancreatic ducts and provides neutral to alkaline pH. Atlantic salmon have a full complement of digestive enzymes to hydrolyze protein, carbohydrate and lipid into smaller molecules for absorption. These enzymes are secreted in the pyloric caeca and small intestine and nutrient absorption occurs in both. The regulation of digestive function is under the combined actions of the nervous system and chemical signals that regulate motility of food through the alimentary canal, secretion of digestive enzymes and absorption of nutrients. Limited information exists on osmoregulation and the regulation of immune functions of the alimentary canal. However, the role of the distal intestine in osmoregulation and as a site for electrolyte secretion and absorption is widely recognized.
