Many species of salmon are anadromous—they spawn, or lay their eggs, in fresh water; the young migrate to salt water and grow up there; and the fish return to fresh water to breed after they reach maturity. Other populations or species of salmon are landlocked, spending their entire life cycle in fresh water. The migratory instinct of members of the salmon family is remarkably specific, each generation returning to spawn in exactly the same breeding places as the generation before it. Some salmon migrate hundreds or even thousands of miles to reach their spawning grounds. Even those species that do not migrate from fresh water to salt water spawn in the same freshwater streams as did their ancestors.
Or it may require a lifetime to complete, as in various species of Pacific salmon that are born in freshwater streams, travel to ocean waters, and then return to the stream where they were born to breed before dying. Who taught them to do that, since none come back to tell them? ONly God!