Salmon are born and die in a river, but they migrate out to sea, where they grow and spend most of their life, before they return to their birthplace to spawn. The life cycle of the average salmon is about seven years.
Eggs are laid in the gravel at salmon spawning areas, where the adult fish meet. The female digs a small hole with her tail and lays up to 10,000 eggs, depending on the kind of salmon. The male will stand guard and then fertilize the eggs.
When eggs hatch, these tiny salmon are called alevin. They can move their tail to move around in the gravel, but they can't swim yet. Alevin are identified by the sack hanging from their stomach--it's the egg sack that feeds them at this stage until the sack is used up.
Once the egg sack is gone, the salmon are called fry until they move into salt water. The largest salmon, called chinook or king salmon, will start moving downriver to saltwater quickly into the fry stage. Another kind of salmon, which migrates upriver in the spring, is called a coho or silver salmon. These stay in the river for about a year and then venture out to the estuary, which is where fresh and salt water mix near the ocean.
Once salmon reach salt water in the estuary, they're in the smolt stage. Salmon must adjust to the salt water before moving into the ocean. This is called smoltification and involves changes in their bodies.
Salmon spend as long as seven years maturing at sea, depending upon their species. Even within a given species, some return earlier than others. Fishers call these jack salmon because of their smaller size. When the salmon are ready to return, they fight their way upstream to where they hatched to begin the salmon life cycle again. Most die after spawning because they can't live in the fresh water for too long and they're weakened from the long, upstream journey. A few make it back to sea when they spawn lower in the river where they can return.