Marine Biology

Coho Salmon Life Cycle Alevins Fry Smolts



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Coho salmon (scientific name: Oncorhynchus kisutch) are found on both sides of the North Pacific Ocean. They range from California to Alaska in North America, through the Aleutian Islands, and from the Anadyr River in Russia, south to Hokkaido in Japan.

Adult coho salmon are bright silver with small black spots on their backs and on the upper part of their tail fin. The average weight is 8-12 pounds. The flesh of the coho salmon is light pink and when cooked to perfection, the fish has a very delicate flavor. They are highly valued as a food fish.

There are five stages in the life cycle of a coho salmon.

1. Eggs

The female deposits from 300 to 1000 eggs in a nest called a redd, which is a depression in the sand or gravel of the stream bed hollowed out by the female's tail.  The male fertilizes the eggs by covering them with milt (sperm). Then the female, using her tail, covers the eggs with sand. The embryos will incubate over the winter months. Depending on the water temperature, this period could last from 6 to 12 weeks. In late winter, the eggs hatch to become alevins.  

2. Alevins

As an alevin, the salmon has a strange appearance: large eyes, with a ballooning sack hanging from its tiny, thin body. The sac is the yolk of the egg from which it hatched and this yolk provides nourishment for the alevin during the first several weeks of life. After the all this nourishment has been absorbed, the fish must find its own food. It leaves the gravely bottom of the stream and swims to higher water. The alevin is orange with dark spots.

3. Fry

When it begins finding its own food, the fish becomes known as a fry. It is about an inch long and has an elongated orange body. It will swim about in the fresh water, feeding on tiny invertebrates and on the carcasses of  dead adult fish. Fry instinctively hide when startled, deal with river currents, learn to school together and acquire many other survival skills. They will spend 3 years in slow moving streams and lakes before migrating to the sea.

4. Smolt

A fry blossoms into a smolt when it is ready to head to the ocean were it will stay until it  becomes an adult. Its gills change enabling it to strain the salt from the ocean water.  Its color also changes to a shiny silver hue with dark spots on the upper part of its caudal fin. Now it is ready to migrate down the river to the ocean where it will spend the next phase of its life.

5. Adult

In the ocean, the adult coho salmon feeds, grows and gains weight quickly until it reaches maturity. It spends 18 months to 2 years at sea, until it is ready to spawn and produce the next generation. 

The Spawning Adult

The mature salmon will journey back to the fresh water spawning area where they began life. They prefer to swim upstream during the daylight hours, and are capable of jumping high in the air to clear rocks, falls and other obstructions. Spawning males develop bright red sides, greenish backs, and their jaws often become grotesquely hooked. Females also change color, but acquire paler shades than the males.

A short time after they spawn, the parent salmon die. The following Spring, the young  fry nibble on the remains of the decomposing bodies of the dead adult salmon, and so the life cycle of these remarkable fish begins anew. The average life span of the coho salmon as it passes through the 5 stages in the wild, is 4 to 6 years.  



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