Salmon migrate from the ocean to freshwater to spawn. This life pattern is called anadromy: salmon hatch in freshwater, grow there as juveniles, mature in the ocean, and return to rivers to reproduce.
Young salmon begin life in gravel-bottomed streams. They pass through stages known as alevin, fry, parr, and smolt. During the smolt stage, their bodies undergo major chemical changes that allow them to regulate salt and water as they move from freshwater through estuaries into the sea.
After spending roughly one to five years in the ocean, depending on species, adult salmon usually return to freshwater. Many navigate toward the river where they were born, a behavior called natal homing. Their sense of smell appears to help them recognize the correct watershed, although some fish stray into neighboring rivers.
The pattern has exceptions. Some salmon populations are landlocked and remain in freshwater throughout their lives. Salmon are also related to trout, char, and other members of the family Salmonidae. Eels can make an opposite-looking migration, traveling from freshwater toward the ocean to spawn.