Atlantic salmon are ANADROMOUS - they lay their eggs in fresh water streams, and grow for several years before moving to ocean waters.
Hormones control the behaviour of the salmon.
When certain conditions of size in the growing salmon, and light conditions are right, hormones bring about 'downstream' behaviour.
It is thought that the lateral line has an important sensory role in maintaining a 'downstream' direction.
Special cells in the gills allow the Atlantic salmon to modify its physiology to adapt to salt water and fresh water at various times during the fish's life.
No one is certain how Atlantic salmon navigate in the ocean. They may use polarized light, their highly developed sense of smell, and even magnetic fields of the Earth.
Atlantic salmon sometimes travel thousands of kilometres to ocean feeding grounds. Many salmon from North American rivers travel to feeding grounds off Greenland and Labrador. Many salmon from more southerly Europe (Ireland, England, France, etc.) also travel to Greenland waters. Other European Atlantic salmon visit feeding areas near the Faroe Islands.
When ready to return to fresh water, it is likely that Atlantic salmon rely on their sense of smell, and perhaps some visual clues, to find their home rivers and enter them.
One Canadian researcher discovered that a salmon can detect one drop of a substance (Prostaglandin-A) in the equivalent of eight Olympic-sized swimming pools of water.
Hormones continue to control the upstream migration of the fish, back to its native stream - and even to the same stretch of water in which it was born.