No one knows what signals the fish to begin their spawning run, but when they do, they are drawn inexplicably back to the same stream they floated down as fry.
They revert again to freshwater fish. Their body shapes and personalities are altered. They stop eating. And they begin a single-minded journey home. Those who survive the pilgrimage-- against the odds of famished bears, immense dams, and plain old gravity -- are rewarded with a single opportunity to spawn in the shallow, fresh waters of their birth. Within a few days, they die, their bodies contributing nutrients to the environment that will nurture their tiny offspring.
There's not much in nature to compare with the Pacific salmon's lifelong odyssey, and why it has evolved its elaborate plan for procreation remains essentially a mystery. One theory is that the cold glacial streams of the Ice Age provided few nutrients for newly hatched fish, inspiring a journey tot he rich waters of the ocean. But the open ocean was also a dangerous place to spawn, promoting a return trip to the relative safety of the small freshwater streams. According to this view, the extra perils of the ocean, and the adaptations required to survive there, are more than offset by the extra weight boost the ocean environment provides.