Coho Salmon

During their ocean phase, coho salmon have silver sides and dark-blue backs with spots on their back and upper tail lobe. During their spawning phase, their jaws and teeth become hooked. After entering fresh water, they develop bright-red sides, bluish-green heads and backs, dark bellies and dark spots on their backs. Sexually maturing fish develop a light-pink or rose shading along the belly, and the males may show a slight arching of the back. Mature adults have a pronounced red skin color with darker backs and spots, with females having darker shades than males.

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. Coho salmon have also been introduced in all the Great Lakes, as well as many landlocked reservoirs throughout the United States. Coho salmon were first introduced in Lake Erie in the 1920s to control the lake's alewife population. Large-scale stocking began in 1966, when 660,000 fingerlings were introduced in Lake Michigan. Wisconsin began a program of introduction of 500,000 fry a year, mainly on the coastline of Lake Michigan. By 1970, the species was present in all of the Great Lakes. Ontario and Minnesota adopted stocking programs in 1969, but later abandoned them.

In their freshwater stages, coho feed on plankton and aquatic invertebrates in the benthos and water column, such as Chironomids, midge larvae, and terrestrial insects that fall into the water. Upon entering the marine environment, they switch to a diet of plankton and fish, with fish making up most of their diets after a certain size. Introduced populations in the Great Lakes feed primarily on alewife and smelt. Great Lakes salmon are known to compete with native lake trout, brook trout, and brown trout for food and space when resources are scarce. Due to emerging earlier and having a larger hatching size, coho salmon typically have an advantage in these confrontations.

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.

Once the mature coho has reached three or four years old, it swims up freshwater rivers and streams to spawn. Coho will swim up streams as far as they are physically able, usually reaching areas of water as little as 5 centimetres (2.0 in) deep. Females seek out beds of gravel at the head of a riffle, where they turn on their sides to dig a nest with movements of their tail, creating round or oval depressions roughly the same lengths and width as the fish. This process is repeated for up to seven nests, each called a redd. Females become extremely aggressive with each other over nesting sites, and with males until these are dug. Males then fight for the right to mate. Once a female has chosen a mate, usually the largest male, she lays her eggs onto the redd, while he simultaneously releases milt (sperm) onto the eggs. Unchosen males also sneak in to release milt at this time.

The total North Pacific harvest of coho salmon in 2010 exceeded 6.3 million fish, of which 4.5 million were taken in the United States and 1.7 million in Russia. This corresponds to some 21,000 tonnes in all. Coho salmon are the backbone of the Alaskan troll fishery, though the majority are caught by the net fishery (gillnet and seine fishing). They average 3.5% by fish and 5.9% by weight of the annual Alaska salmon harvest. The North Pacific yields of pink salmon, chum salmon and sockeye salmon are about 15 times larger by weight.