The instinct that takes the Salmon on the long trip upstream is the fact that they go to a sort of headquarters of a stream to Spawn. This helps a new Salmon to be born and grow.
When the Salmons enter fresh water, they are in better condition, but as soon as they reach the fresh water, they stop feeding.
They often search for an exact place to deposit their eggs. The young Salmons remain in fresh water for about an year and then descend from the streams and enter the salt water. Hence, the cycle begins all over again.
Below Salmon info from Wikipedia (Read More):-
Salmon is the common name for several species of fish of the family Salmonidae. Several other fish in the family are called trout; the difference is often attributed to the migratory life of the salmon as compared to the residential behaviour of trout, a distinction that holds true for the Salmo genus. Salmon live in both the Atlantic (one migratory species Salmo salar) and Pacific Oceans, as well as the Great Lakes (approximately a dozen species of the genus Oncorhynchus).
Typically, salmon are anadromous: they are born in fresh water, migrate to the ocean, then return to fresh water to reproduce. However, there are rare species that can only survive in fresh water habitats. Folklore has it that the fish return to the exact spot where they were born to spawn; tracking studies have shown this to be true but the nature of how this memory works has long been debated.
http://en.wikipedia.org/wiki/Salmon
