Coho Salmon

Identification

Ocean-phase coho are bright silver with a dark blue-green back and small black spots. Two marks separate them from Chinook, which they most resemble: coho have white or grey gums at the base of the teeth rather than black, and spotting on only the upper lobe of the tail rather than both. The Alaska Department of Fish and Game gives white gums as the primary separator, and the Oregon Department of Fish and Wildlife specifies “a white gum line on the lower jaw with darker color both inside and outside of the gum line,” plus spots confined to the upper caudal lobe.

Spawning fish are unmistakable. Both sexes turn dark green across the head and back with deep brick-red flanks; males develop a pronounced hooked kype and a compressed, humped body. ODFW describes returning fish as having “red sides with dark greenish backs and heads.” Great Lakes adults, per the Michigan DNR, show the same dark blue-green back, silver sides and white belly, with small dark spots on the back, sides and typically the upper caudal lobe.

Range and distribution

ADF&G describes coho as occupying “nearly all accessible bodies of fresh water, from large trans-boundary watersheds to small tributaries,” from Southeast Alaska to Point Hope on the Chukchi Sea and up the Yukon. That adaptability is the defining ecological fact about the species: coho are the Pacific salmon of small streams, and they will use a drainage that would not support a Chinook run.

South of Alaska they occur throughout coastal British Columbia, Washington, Oregon and northern California. WDFW notes that coho can thrive “in small neighborhood streams and even urban waterways if conditions are suitable” — a trait that turns out to cut both ways. The southern limit is sharp and contracting. The California Department of Fish and Wildlife records that coho historically ran from the Smith River at the Oregon border to the San Lorenzo River in Santa Cruz County, but are “now absent from all tributaries of San Francisco Bay and many streams south of the Bay,” and occupy roughly two-thirds of identified historical habitat in the northern part of the state.

Habitat and water temperature

Coho spawn in low-gradient tributaries and the small-gravel margins of larger rivers. ODFW specifies “small, relatively low-gradient tributary streams with pea to orange-sized gravel,” with in-stream wood and vegetation providing the cover juveniles need. WDFW makes the same point and adds a behavioural detail: returning adults often wait at stream mouths for a rise in flow — typically after rain — before pushing upstream over obstacles.

Thermally, coho sit lower than Chinook. Richter and Kolmes (2005) give a juvenile rearing optimum of 12–15°C (54–59°F), incubation optimum of 2.5–6.5°C (36–44°F) at constant temperature and tolerable to 10.9°C (52°F), and a lethality threshold of 20–21°C (68–70°F) as a daily maximum. The generic EPA Region 10 criteria that apply to coho, summarised by the California Department of Fish and Game, are below 55°F (13°C) for spawning and incubation, below 61°F (16°C) for core juvenile rearing, below 59°F (15°C) for smoltification and below 64°F (18°C) for migration — all as seven-day averages of daily maxima.

The Alaska Department of Fish and Game’s temperature literature review notes that coho are comparatively tolerant of cold — performing better at 2°C and 5°C than several congeners — and correspondingly less resistant at the warm end, with an avoidance threshold above 15°C (59°F).

The practical consequence is that coho are more exposed to summer heat than their larger relatives, because the small tributaries they depend on have less thermal mass and warm faster than a mainstem. A creek that a coho parr must occupy through an entire summer can exceed the lethal band on a hot afternoon while the river it feeds stays within range. Regional warming compounds this: Isaak et al. (2018) recorded Northwest river warming of 0.18°C per decade overall and 0.27°C per decade in July.

Life history

Coho have the most rigid freshwater schedule of the Pacific salmon after pink and chum. Eggs are deposited in fall gravel, fry emerge the following spring, and — as WDFW puts it — the young “migrate to sea in their second spring, about 18 months after hatching.” ADF&G records juveniles spending one to three winters in streams before smolting, with the longer residence typical of northern, colder, less productive systems. That extended stream residence is why coho are so sensitive to summer habitat quality: the whole cohort must survive a freshwater summer, or several.

Ocean residence is short and predictable. Most fish return after about 18 months at sea as full-size adults. ADF&G notes that a fraction of males mature after only six months at sea and return as small “jacks,” a life-history hedge that keeps some male reproductive capacity in the population even if a full-size cohort fails. NOAA gives a typical lifespan of two to three years, with most fish reaching age three.

Coho are semelparous. They spawn once and die, and the death is programmed rather than accidental. Adults stop feeding on entering fresh water and complete migration, gonad maturation and spawning on stored energy alone. Research published by the U.S. Geological Survey on maturing Pacific salmon documents the mechanism: rising cortisol through the final months before spawning accompanies energy depletion and suppresses immune capacity, raising susceptibility to parasites and pathogens. The gut atrophies, the skin degrades and fungal infection follows. The Michigan DNR records the same outcome for Great Lakes fish: “both male and female adults die shortly after reproduction.”

Diet

Stream-rearing coho parr hold feeding stations in pools and take drifting aquatic insects, terrestrial insects and plankton. ADF&G describes freshwater juveniles feeding on aquatic insects and plankton, and ocean adults on “fish and squid.” In the Great Lakes, the Michigan DNR reports coho feeding primarily on rainbow smelt and alewife, competing mainly with steelhead for the same forage. As with all Pacific salmon, adults stop feeding on entering fresh water to spawn.

Great Lakes populations

Coho were the first Pacific salmon successfully established in the Great Lakes, and their introduction is the founding event of the modern Great Lakes sport fishery. The Michigan DNR records that coho were stocked in 1966 as smolts in the Platte River and Bear Creek, and that “the first spawning run in the fall of 1967” generated the response that built the fishery.

The motive was ecological. Alewife, an invader that had entered through the canal system, had exploded after sea lamprey destroyed the native lake trout that would otherwise have controlled them, producing mass die-offs that fouled beaches. The Great Lakes Fishery Commission records that Michigan stocked coho into Lake Michigan in 1966 to control alewife while creating a recreational fishery, and that “the newly introduced salmon consumed alewives, reducing their numbers such that die-offs did not occur.” Basin-wide stocking exceeded 25 million fingerlings a year by the early 1980s.

Great Lakes coho complete the full life cycle in fresh water. The Michigan DNR gives tributary spawning from early September to November depending on the stream, fry emerging in late spring, and juveniles remaining in tributaries until their second spring before descending to the lake as smolts — the same 18-month freshwater schedule as sea-run fish. Adults average around five pounds. The Commission notes that ecosystem changes since 2003, including zebra mussel invasion and zooplankton decline, have forced managers to reconsider whether to sustain Pacific salmon or prioritise native species recovery.

Angling

Coho behaviour in rivers is shaped by two facts: they are not feeding, and they are tied to flow. WDFW’s observation that returning adults hold at stream mouths waiting for a rise in water before moving up is the single most useful piece of natural history about the species. Coho runs are pulsed rather than continuous, and the pulse is a rain event. In small coastal drainages the fish may be stacked in tidewater for weeks and then move most of the way to the spawning gravel in a couple of days.

Once in the river, coho hold in pools and along the edges of woody structure — the same habitat their own juveniles use, and for the same reason: it offers cover and low-velocity water. Their reactions to objects in the water are aggression and territoriality, not hunger, which is why the aggressive, mobile responses coho are known for intensify as fish darken and approach spawning. Precocious jack males, returning after only six months at sea, appear in the run alongside full-size adults and account for a share of the small fish present.

Coho are among the shortest-lived Pacific salmon, and a run’s size is set almost entirely by two things: how many parr survived the freshwater summer eighteen months to three years earlier, and what ocean conditions did to the smolts. Coho abundance therefore swings hard between years in a way that is largely uncoupled from the previous year’s run.

Conservation

NOAA Fisheries lists four coho Evolutionarily Significant Units. Central California Coast is endangered; Lower Columbia River, Oregon Coast, and Southern Oregon/Northern California Coast are threatened. The Central California Coast ESU is one of NOAA’s national “Species in the Spotlight,” and its critical habitat was designated in 1999.

Oregon Coast coho has the most tangled listing history of the four, and it is often reported incorrectly. NOAA records the ESU as first listed threatened on August 10, 1998 (63 FR 42587), with a further listing determination on June 20, 2011 (76 FR 35755) and regulatory updates on April 14, 2014; protective regulations and critical habitat were established February 11, 2008. The intervening years involved litigation over the treatment of hatchery fish in listing determinations, which is why more than one listing date exists for what is nominally a single continuous listing. NOAA now describes Oregon Coast coho as among the closest to recovery of the 28 listed West Coast salmonids, with more native coho returning than at listing, though numbers swing with ocean conditions.

California adds a second layer. CDFW reports the Central California Coast ESU as endangered under both state and federal law, and notes that NOAA’s 2016 viability assessment of the Southern Oregon/Northern California Coast ESU found its ESA status “likely to move from threatened to endangered.”

Beyond the standard threats NOAA lists — dams, habitat degradation, habitat loss, harvest and climate change — coho face one that is largely specific to them. Researchers at Washington State University and the University of Washington identified 6PPD-quinone, formed when the tire antioxidant 6PPD reacts with ozone, as the agent behind acute pre-spawn mortality in urban streams. The WSU account of the December 2020 Science paper reports that “more than half of the coho salmon that return to Puget Sound’s urban streams die before they can spawn. In some streams, all of them die.” The sensitivity is strikingly species-specific — chum salmon in the same water are far less affected. For a fish that depends on small lowland streams threading through developed land, the exposure is structural: coho use exactly the drainages that carry road runoff.