The Coho Salmon, commonly called Silver Salmon, is a sought-after sport fish along the Pacific coast, ranging from Alaska down to central California, and in the Great Lakes as an introduced species. Its popularity stems from its aggressive fighting nature and the striking silver coloration it displays in the ocean. Like all Pacific salmon, the size an individual Coho reaches is highly variable, resulting from its unique life cycle and the diverse environments it encounters during its journey from freshwater streams to the open ocean and back. Understanding Coho size requires looking beyond simple averages to the biological and environmental factors that drive its growth.
Defining the Average Coho Size
A mature, full-term adult Coho Salmon typically returns to its natal stream after spending roughly 18 months in the marine environment. These adults, usually three to four years old, establish the baseline for the species’ average size. The majority of returning fish measure approximately 24 to 30 inches in length, translating to a weight of 8 to 12 pounds. While fish within this range are standard, a Coho reaching 15 to 20 pounds is generally considered a trophy catch.
Differences in Size Between Males and Females
The physical dimensions of a Coho Salmon are complicated by significant differences between the sexes, particularly as they approach the spawning season. While males and females may weigh similarly in the ocean phase, mature males develop pronounced secondary sexual characteristics that make them appear substantially larger. This sexual dimorphism includes the growth of a strongly hooked snout, known as a kype, and a slight humping of the back as they enter freshwater. The kype, along with enlarged jaws and teeth, serves to establish dominance hierarchies and compete with other males on the spawning grounds.
The presence of precocious males, often called “jacks,” also skews the average size. These are sexually mature males that return to spawn a full year earlier than their full-size counterparts. Jacks spend only about six months in the ocean and are considerably smaller, typically measuring 12 to 18 inches in length. Although small, they employ a “sneaker” spawning tactic, fertilizing eggs without engaging in the aggressive competition of larger males. Their early return contributes to the wide range of sizes observed in a single Coho run.
Environmental and Genetic Influences on Growth
The broad size range of Coho Salmon is heavily influenced by external factors encountered during their marine feeding period. Ocean productivity, specifically the availability of high-quality prey like small fish and squid, directly correlates with faster growth rates and larger final adult sizes. For instance, when upwelling brings colder, nutrient-rich water to the surface, it can lead to a boom in the food chain, resulting in significantly larger returns of Coho. Geographic location also plays a role, with Coho from the northern extent of their range, such as in Alaska, often showing a tendency toward greater average size than those found further south.
The difference between wild and hatchery-reared fish introduces another layer of variation, primarily due to differing selective pressures. Hatchery environments can sometimes accelerate early growth, but the fish may not ultimately reach the same maximum size potential as their wild counterparts. Furthermore, early growth is also affected by local conditions, such as limited food supplies in estuaries or the particular characteristics of the river where the juvenile fish spend their first year of life.
Record Holders and Maximum Potential
While the typical Coho Salmon weighs under 15 pounds, the species possesses a genetic potential that allows for the occasional colossal outlier. These rare, massive fish represent the absolute maximum size the species can achieve under optimal conditions. The heaviest documented Coho Salmon caught by an angler weighed 33 pounds, 4 ounces, showcasing a size nearly triple the species’ average.
In terms of length, the largest verified Coho have measured approximately 42.5 to 43 inches from nose to tail. These record-setting fish illustrate the biological upper limit of the species’ growth capacity. Such massive individuals are often a result of exceptional luck, where the fish benefited from several consecutive seasons of high ocean productivity and a genetic predisposition for accelerated growth.