Because their lives are so closely entwined with the river, these fish are good indicators of the river's environmental health, scientists say. After hatching in freshwater tributaries, steelhead and coho spend a season or two in the river system before heading out to sea. They then live several years in salt water, ''bulking up'' on the Pacific Ocean's rich food supply. Adult fish can reach 20 pounds or more by the time they return to their home rivers, driven by the impulse to spawn. They swim upriver in winter and females lay their eggs in tributary riffles.
Males fertilize the eggs, which females then bury and guard.
The eggs develop in their gravel nests and hatch several months later. The young seek shelter in tributary pools and later work their way downstream. Unlike salmon, adult steelhead don't die after spawning, but return to the ocean. They may revisit the river to spawn several more times. Coho, also known as silver salmon, have a similar life cycle, but adults of both sexes die after spawning.