Sexually maturing fish develop a light-pink or rose shading along the belly, and the males may show a slight arching of the back.

[3] Once the mature coho has reached three or four years old, it swims up freshwater rivers and streams to spawn (reproduce).

[8] Alevin no longer have the protective egg shell, or chorion, and rely on their yolk sacs for nourishment during growth.

Smolts are generally 100–150 mm (3.9–5.9 in) and as their parr marks fade and the adult's characteristic silver scales start to dominate.

The traditional range of the coho salmon runs along both sides of the North Pacific Ocean, from Hokkaidō, Japan and eastern Russia, around the Bering Sea to mainland Alaska, and south to Monterey Bay, California.

[14] Adult coho feed on a vast variety of prey items that depend on the region they reside in during their second year at sea.

Salmonid species on the west coast of the United States have experienced dramatic declines in abundance during the past several decades as a result of human-induced and natural factors.

[19] Its popularity is due in part to the reckless abandon which it frequently displays chasing bait and lure while in salt water, and the large number of coastal streams it ascends during its spawning runs.

When smoking coho it is best to use a cold-smoking rather than hot-smoking process, due to their lower fat content compared to sockeye and chinook.

Historically coho, along with other species, has been a staple in the diet of several indigenous peoples, who would also use it to trade with other tribes farther inland.

[23] The U.S. National Marine Fisheries Service (NMFS) has identified seven populations, called Evolutionary Significant Units (ESUs), of coho salmon in Washington, Oregon and California.

On May 6, 1997, NMFS, on behalf of the Secretary of Commerce, listed as threatened the Southern Oregon/Northern California Coast coho salmon ESU.

These reductions are due to natural and man-made changes, including short-term atmospheric trends (such as El Niño, which causes extremes in annual rainfall on the northern California coast), predation by the California sea lion and Pacific harbor seal, and commercial timber harvesting.

Some creeks were reported to have so many fish, "you could literally walk across on the backs of coho," claimed a Portland television station.