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The coho salmon, Oncorhynchus kisutch, is the second largest of the Pacific salmon. These fish
can reach lengths of 35 inches, and weights of up to 36 pounds. The other common name for this
fish is silver salmon. In the ocean, coho salmon have dark metallic blue or greenish backs with
silver sides and a light belly. There are small black spots on the back and upper lobe of the tail.
The gumline in the lower jaw has lighter pigment than that found in the Chinook salmon.
Spawning fish in Interior rivers are dark with reddish-maroon coloration on the sides. Males
develop a strongly hooked snout and large teeth.
Range And Abundance
Coho salmon can be found from Monterey Bay in California to Point Hope in the Chukchi Sea.
They range east in Interior rivers across the Alaska-Yukon border. They are found west to the
Anadyr River in Siberia, and south along the coast of Asia to Japan. Coho salmon populations
are less numerous than chum, pink and sockeye salmon, but they are still considered a relatively
Coho salmon enter spawning streams from August through November where they school in
pools or lakes for a number of weeks. Spawning takes place from late September through
January in shallow tributaries with gravel bottoms. The female
digs several gravel nests, called redds, and deposits between 2,400 to 4,500 eggs in them.
Fry emerge from the redd in May or June and remain in fresh water
from one to four winters before going to sea. Coho salmon smolts
tend to stay close to shore at first, feeding on plankton. As they
grow larger, they move farther out into the ocean and switch to a diet of small fish. Coho
salmon can stay at sea for two to three years.
High quality, troll-caught coho salmon are frequently frozen whole and
sold as a delicacy in Europe. Coho salmon are also extremely important as a sport fish in the state.
They are pursued by anglers both in marine and freshwater systems, adding millions of dollars annually
to the state's economy.
Coho salmon are an important subsistence fish for native people living along Alaska's
coast and interior rivers. These fish are harvested with gill nets,
beach seines and fish wheels and are eaten fresh, smoked, and canned.
Coho salmon produce between 1,500 to 5,700 eggs depending on the size of the fish.
These are deposited in a series of long gravel nests, called redds, dug by the female fish. The
eggs usually hatch in six to seven weeks.
Low oxygen levels, water pollution, and predation by fish, insects and birds are all threats at this
stage. Excess sediment in the water is extremely dangerous as it can smother eggs or cover the
redd trapping fish inside.
A newly hatched salmon is called an alevin. At this stage, it looks like a thread with eyes
and a huge yolk sack which provides all nutrition for the fish in the first weeks of its
life. Coho salmon alevins remain in the redd until the yolk sac is absorbed, then they emerge as
free-swimming fry in May or June.
Alevin require cold, clear, oxygen-rich water. Excessive sediment in the water is one of the
greatest dangers to salmon at this stage. It can reduce oxygen levels and cover the top of the
redd, trapping the tiny fish inside. Aquatic insects and other fish are an alevin's primary
Coho salmon fry usually stay in fresh water for two years in the Yukon River drainage. Some
fish in other areas may spend only six months in fresh water or, as in the Karluk River, up to four
Young coho salmon establish a territory in a pool that they defend aggressively from all other
fish. Their parr marks help them hide among the cover provided
by rocks, stumps, undercut banks and overhanging vegetation. Coho salmon fry feed almost
entirely from the water's surface on insects and spiders. As they grow larger, they can be a
serious predator of young sockeye salmon fry.
Many physical changes occur in a young salmon to help it make the transition from a freshwater
to saltwater existence. This process is called smolting. As the time for migration to the sea
approaches, the coho salmon fry replaces its parr marks, a pattern of vertical bars and spots
useful for camouflaging the fish in fresh water, with the dark back and light belly coloration
useful to fish living in open water. The fish's gills and kidneys begin to change so that they can
process salt water.
Coho salmon smolts remain close to shore for several months feeding primarily on plankton.
They gradually move into deeper, saltier water, and switch to a diet of small fish.
Most Alaskan coho salmon spend 2 to 3 years in saltwater. During their ocean existence,
their diet is 70% to 80% fish- primarily herring and sand lance.
Some fish migrate only a short distance into good feeding areas, and stay there; others travel
extensively. Coho salmon from California to British Columbia tend to travel north and spend the
summer along the central Alaskan coast. Most Alaskan fish are thought to make one complete
circuit of the north Pacific Ocean each year following a counter clockwise path with the
prevailing currents. As ocean temperatures increase in summer, the fish move north throughout
the north Pacific Ocean and into the Bering Sea. Most of these fish winter well south of the Gulf
Coho salmon may return to spawn anytime between August and December. The winter spawning
generally occurs in the southern part of the fish's range, with progressively earlier spawning
returns to the north.
Most coho salmon spawn in streams along the coast, but there are exceptions. Some coho
salmon migrate many miles up Interior rivers to spawn. Coho salmon in the Yukon River
drainage have been found spawning near Old Crow on the Porcupine River- more than 2,200 miles
from the sea. The fish select spawning sites with high water flow through the gravel which will
provide plenty of oxygen for their eggs.
Once a female salmon selects a spawning site, she rapidly pumps her tail to wash out a
depression in the stream gravels. After the eggs are laid and fertilized, the female uses the same
tail movements to completely cover the eggs with gravel. Over several days, she will lay several
more pockets of eggs like this in a line upstream.
Text by USFWS staff
Graphics used by permission of Harry Heine
Last modified 4, March, 2009
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