The coho salmon (Oncorhynchus kisutch; Karuk: achvuun) is a species of anadromous fish in the salmon family and one of the five Pacific salmon species. Coho salmon are also known as silver salmon or "silvers". The scientific species name is based on the Russian common name kizhuch (кижуч).
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. The coho salmon is also a symbol of several tribes, representing life and sustenance.
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. Coho salmon average 20 to 28 inches (50.8 to 71 cm) and 7 to 11 pounds (3.2 to 5.0 kg), occasionally reaching up to 36 pounds (16 kg). Size can vary depending on age and geographic location. Males tend to be slightly larger than females. Mature adults also develop a large kype (hooked beak) which is used to attract a mate during spawning, with males having a more pronounced kype than females. The coho salmon's lower jaw can be distinguished by a light shade at its superior edge.
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. A number of specimens, (more than 20), were caught in waters surrounding Denmark and Norway in 2017. Their source is currently unknown, but the salmon species is farmed at several locations in Europe, making it probable that the animal has slipped the net at such a farm.
The eggs hatch in the late winter or early spring after six to seven weeks in the redd. Once hatched, they remain mostly immobile in the redd during the alevin life stage, which also lasts for six to seven weeks. Alevin no longer have the protective egg shell, or chorion, and rely on their yolk sacs for nourishment during growth. The alevin life stage is very sensitive to aquatic and sedimental contaminants. When the yolk sac is completely resorbed, the alevin leaves the redd. Young coho spend one to two years in their freshwater natal streams, often spending the first winter in off-channel sloughs, before transforming to the smolt stage. 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. Smolts migrate to the ocean from late March through July. Some fish leave fresh water in the spring, spend summer in brackish estuarine ponds, and then return to fresh water in the fall. Coho salmon live in salt water for one to three years before returning to spawn. Some precocious males, known as "jacks", return as two-year-old spawners. Spawning males develop kypes, which are strongly hooked snouts and large teeth.
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. Adult coho feed on a vast variety of prey items that depend on the region they reside in during their second year at sea. Spawning habitats are small streams with stable gravel substrates.
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 (reproduce). Once reaching a suitable location, females "dig" a divot in the riverbed by flexing her tail and loosing rocks from the riverbed. This will be one of many of her nesting sites (sometimes she will create six or seven), each called a redd. Females are extremely aggressive during this time towards other females over nesting sites, and towards males until she finishes digging. Males will then fight over females for who gets to mate with the female. Once the female has chosen a mate, usually the largest male, she will lay her eggs onto the redd, while the male simultaneously releases milt (sperm) onto the eggs. Other males will also sneak in to release their milt during this time. Once the female has laid all her eggs, she will cover the eggs with rocks and pebbles using her tail. Salmon have a trait called semelparity, where the adults will then stop eating as their body slowly deteriorates until they die.
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. Four of these ESUs are listed under the U.S. Endangered Species Act (ESA). These are the Lower Columbia River (threatened), Oregon Coast (threatened), Southern Oregon and Northern California Coasts (threatened), and Central California Coast (endangered). The long-term trend for the listed populations is still downward, though there was one recent good year with an increasing trend in 2001.
The Puget Sound/Strait of Georgia ESU in Washington is an NMFS "Species of Concern". Species of Concern are those species for which insufficient information prevents resolving the U.S. National Oceanic and Atmospheric Administration's concerns regarding status and threats and whether to list the species under the ESA.
On May 6, 1997, NMFS, on behalf of the Secretary of Commerce, listed as threatened the Southern Oregon/Northern California Coast coho salmon ESU. The coho salmon population in the Southern Oregon/Northern California region has declined from an estimated 150,000–400,000 naturally spawning fish in the 1940s to fewer than 10,000 naturally producing adults today. 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.
More than 680,000 coho salmon returned to Oregon in 2009, double that of 2007. The Oregon Department of Fish and Wildlife required volunteers to herd fish into hatchery pens. Some creeks were reported to have so many fish, "you could literally walk across on the backs of coho," claimed a Portland television station. Lower temperatures in 2008 North Pacific waters brought in fatter plankton, which, along with greater outflows of Columbia River water, fed the resurgent populations. The 2009 run was so large, food banks were able to freeze 40 tonnes (39 long tons; 44 short tons) for later use.
