The life of a salmon follows a predictable pattern. It begins in the upper reaches of a freshwater river, as a small egg on a gravel bed in the wilderness, typically in the mountains. After a couple of years, the young salmon (known as a smolt) heads downstream and out to sea. When it reaches maturity, one to eight years later, the adult salmon can find its way back to the original stream or river, and uses its powerful sense of smell to locate the tributary where it was born.
After a strenuous swim upstream, the adult salmon breeds, spawns, and generally dies within a week, creating a nutrient-rich environment for the infant salmon that are about to hatch. And so the cycle begins again.
Imprinted at an early age:
I do not question that salmon drive by smell to where they need to find their home stream. But how in the world can you test how a salmon smells? "Tests in the 1950's found they are sensitive to the unique chemical odors of their home turf" which sounds good but how do you test the smell of a fish. It could still be GPS. Who knows? Whatever it is, it works and it works well.