The eggs remain buried in the gravel, develop and hatch after a couple of months. When the eggs hatch, ‘alevins’ emerge. They are not yet fully developed and remain in the gravel, slowly completing their development over the next two months.
During this time the alevins do not eat but live on the nutrients contained in a large orange belly, which is called a yolk sac. When the yolk sac is completely empty, the tiny fish swim up out of the gravel and begin actively feeding on insects, crustaceans and other minute organisms in the river.
These young fish, known as ‘fry’, grow rapidly in size to become ‘parr’, which are green-brown in colour and have distinctive fingerprint markings. Next the parr develop into silver-coloured ‘smolt’.
When the young fish become smolt, at 60–80 grams, their body chemistry changes to enable them to survive in seawater. At this point they migrate downstream to the ocean. The period in fresh water can be one or more years.
As the smolt move out into the ocean they mature into adult salmon and continue their migration. In the northern Atlantic waters they grow rapidly, benefiting from the abundant food sources available in the colder sea areas.
After one to five years at sea, the adult salmon return to freshwater rivers and streams. They normally return to the river system they came from. As they do so, the bodies of the salmon begin to change in preparation for reproduction. By now, the salmon are at least two years of age and likely to weigh 4 kg or more.
Each female has roughly 6,000 eggs and these begin develop and mature. The male's testes develop to provide the sperm (milt) to fertilise the eggs. The appearance of the males also changes as their skin becomes more colourful, their snouts grow and they develop prominent teeth to use in territorial disputes with other males.
After spawning, most of the adult fish die.