Salmon live in the ocean for 1 to 7 years. Pacific salmon range as far south as Monterey, California and as far east as the coast of Siberia. When conditions are right, an unknown signal tells them to begin the migration home. Somehow they find their way across thousands of miles of ocean, even in overcast weather, when sun and stars cannot help their navigation. Reaching the coast, they pick up the scent of their home river with noses so sensitive that they can detect dissolved substances in parts per 3,000,000,000,000,000,000!
Once they enter fresh water, the salmon stop feeding. Their stored fat and muscle must last long enough to take them past numerous obstacles, and sustain them while they build nests, fight for dominance, and spawn. They undergo many physical changes: bright spawning colors appear, the males of some species get humped backs, hooked jaws, and sharp canine teeth. The digestive tract degenerates, and the ability to fight disease and heal injuries declines.
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