The life cycle of salmonids consists of three main stages. During early life, juvenile salmon mature in their natal stream until approximately 18 months of age. At the end of juvenile life a morphological transformation called smoltification occurs. Next, the fish migrate to the ocean, where they feed and mature for one to four years. Adult salmon then navigate, using olfactory cues, to their natal stream where they spawn and expire.
Numerous offspring remain to start the cycle anew.
Behaviorally, the salmon shift from being territorially protective to gregariously interactive, permitting migration in large schools to marine waters. After two to four years in the ocean, spawning migration begins. Morphological, physiological, and behavioral changes mark this stage. In order to accommodate the ripened sperm and eggs, the gastrointestinal tract dissolves. Feeding ceases and the nourishment for the upstream voyage comes from mobilization of fat reserves, further conserving body cavity space.