The Coho Salmon (Oncorhynchus kisutch) is a species of Pacific salmon recognized for its sleek profile and aggressive behavior. Known colloquially as the Silver Salmon because of its bright, reflective ocean coloring, this fish links the vastness of the North Pacific Ocean with the cold-water streams of the continent’s western edge. The species plays a significant part in the natural food web and supports the economic activity of coastal communities across its range.
Physical Characteristics and Identification
Adult Coho Salmon typically average between 8 and 12 pounds and measure 24 to 30 inches in length, though some individuals can reach weights up to 36 pounds. In the ocean, their coloration is a striking dark metallic blue or greenish-blue on the back, transitioning sharply to brilliant silver on the sides and belly, which contributes to their popular nickname. As the fish return to freshwater to spawn, their appearance changes dramatically, with the sides turning a deep reddish-maroon and the back darkening.
Anglers and biologists distinguish Coho Salmon from other Pacific salmon, especially the larger Chinook, by several features. One reliable identifier is the mouth: a Coho has a black mouth with a distinct white or light-colored gumline, contrasting with the all-dark gums of a Chinook. Coho Salmon usually only exhibit small black spots on the upper lobe of their tail fin, whereas Chinook Salmon typically have spots covering both the upper and lower lobes.
The Unique Life Journey
The life of a Coho Salmon is defined by its anadromous nature, involving migration between freshwater and saltwater environments over about three years. The cycle begins with eggs laid in gravel nests, or redds, in the upper reaches of streams. These hatch into alevin that remain in the gravel until their yolk sacs are absorbed. The juvenile fish, known as fry or parr, then spend a lengthy period in their natal stream, typically 16 to 18 months, feeding on insects and small crustaceans.
This extended freshwater residency differentiates them from species that migrate to the ocean much sooner. As they prepare to enter the ocean, the juveniles undergo smoltification, a physiological transformation that adapts their bodies to survive in saltwater. Once in the Pacific, the young adults spend around 18 months feeding and growing rapidly, sometimes migrating over a thousand miles from their home stream.
Upon reaching full maturity, the adults undertake the return journey to the exact stream where they hatched, guided by their imprinted sense of smell. Coho Salmon are semelparous, meaning they reproduce only once in their lifetime, expending all energy on the upstream migration and spawning process. After the female deposits her eggs and the male fertilizes them, the adult fish die, completing their life cycle.
Habitat and Geographic Range
The Coho Salmon range extends throughout the North Pacific Ocean, encompassing coastal waters and tributary streams from Alaska down to Monterey Bay in central California. Highest abundance in North America is found along the coasts of Alaska, British Columbia, and the Pacific Northwest states down to central Oregon. The species has also been successfully introduced into the Great Lakes region, where self-sustaining populations now exist.
Freshwater rearing habitat requires cold, clean, and well-oxygenated streams, often smaller than those preferred by other salmon species. Juvenile Coho rely on complex stream habitats featuring deep pools, stable gravel substrates for spawning, and ample in-stream cover from large woody debris and undercut banks. The health of the population is directly tied to uninterrupted access to these high-quality stream environments, as degradation can severely impact the survival of the young fish.
Ecological and Economic Importance
Coho Salmon function as a keystone species that links marine and terrestrial environments. After spawning, the decomposing carcasses of the adults deliver marine-derived nutrients, such as nitrogen and phosphorus, back into freshwater streams and riparian zones. This nutrient subsidy supports a wide array of life, including insects, birds, and mammals like bears and eagles, and contributes to the growth of streamside vegetation.
The species holds economic value, supporting commercial and recreational fisheries across the Pacific coast. Commercially, Coho are harvested using methods like trolling, seining, and gillnetting, and they are prized as a high-quality, medium-red salmon. Recreational angling is a significant draw, contributing to local economies through tourism and gear sales. Due to habitat loss, several distinct populations, or Evolutionary Significant Units, are currently listed as Threatened or Endangered under the Endangered Species Act.