Salmon are one of the few fish species which can live in salt and freshwater. They spend their juvenile lives in rivers, then migrate to the sea and return later to the place they were born. - The migration to the spawning habitat may commence up to a year before spawning takes place in autumn or winter. The salmon ceases to feed to direct all their energy to reproduction. The fertilized eggs are covered with gravel to a depth of several centimetres by the female. The parents then leave the eggs in the nest or "redd", and there is no further parental care. The eggs will hatch after about 180 days at normal water temperatures in March or April.
The just-hatched fish are called alevins, and still have a yolk sac attached to their bodies containing the remains of food supplied from the egg. When most of the their yolk sac has been consumed, the alevins become active and begin their journey up through the gravel.
Three to six weeks after hatching they are called fry. The small fish must rise to the surface of the water to take a gulp of air with which they fill their swim bladder. This critical period exposes the young to dangerous predators for the first time.
Fry quickly develop into parr with vertical stripes and spots for camouflage. They feed on aquatic insects and grow for one to three years in their natal stream. Once the parr have grown to 10–24 cm in body length, they undergo a physiological pre-adaptation to life in seawater while still in freshwater, by smolting (internal changes in the salt-regulating mechanisms of the body, and in the appearance and behaviour of the fish). The smolts change from swimming against the current to moving with it. This adaptation prepares the smolt for its journey to the oceans.
In spring, large numbers of smolts leave Irish rivers to migrate north along the slope current into the Norwegian Sea and the greater expanse of the North Atlantic Ocean. As they grow fewer predators are able to feed on them. Their rate of growth is therefore critical to survival.
Some Irish salmon will reach maturity after one year at sea and return to their river in summertime weighing from 1 to 4kg. If it takes two or more years to mature, the salmon will return considerably earlier in the year and larger at 3 to 15kg - becoming a highly prized fish but also a very rare one. Salmon exhibit a remarkable "homing instinct", by which a very high proportion are able to locate their river of origin using the earth's magnetic field, the chemical smell of their river and pheromones (chemical substances released by other salmon in the river).
Having spawned, the salmon are referred to as "kelts". Weakened by not having eaten any food since their arrival in freshwater and losing energy in a bid to reproduce successfully they are susceptible to disease and predators. Mortality after spawning can be significant, especially for males but some do survive and commence their epic journey again. In exceptional cases, some Irish salmon are known to have spawned up to three times!