Upon reaching sexual maturity, adult coho salmon migrate inland from the sea. They spawn in small, low-gradient freshwater streams of the Oregon Coast and lower Columbia after fall rains raise river levels (typically from September through December). While the vast majority of coho make this trek at three years of age and weighing an average of 5-10 pounds, a small percentage of juvenile male coho reach sexual maturity early and return to their spawning grounds as jacks at two years of age.
Offspring hatch in the streams, then migrate seaward as smolts after a year of freshwater rearing. Once at sea, the survival of the juvenile coho is closely related to the upwelling of cool, nutrient-rich waters.
There are three evolutionary significant units (ESUs) for coho salmon in Oregon: Lower Columbia, Oregon Coast and Southern Oregon. Data are currently provided on the site for Oregon Coast and Lower Columbia Coho.