Oncorhynchus kisutch (Walbaum, 1792): coho salmon

Biological Description

Body elongate, streamlined, somewhat compressed laterally; scales cycloid, 121-148 in the lateral line. Head moderate, conical; snout rounded, both jaws strongly hooked inward in spawning individuals; mouth large, slightly oblique; well-developed teeth in both jaws. Gill rakers on first gill arch 19-25; dorsal fin rays 9-13; adipose small; caudal only slightly forked; anal fin rays 13-16. Color metallic blue on dorsal surface, silvery on side and ventral surface; numerous black spots on the back and dorsal lobe of the caudal fin; young with 8-12 long narrow parr marks; the pectoral, pelvic, and anal fins are tinged with orange. The anal fin is large, the first rays elongate and white with black behind. This color character is quite variable.

The coho salmon is most often confused with the Chinook salmon. Both species have distinct black spots on their backs and caudal fins. The coho has spots only on the upper lobe of the tail and white gums on the lower jaw, in contrast to the Chinook, which has spots on both lobes of the caudal fin and black gums around the teeth in the lower jaw.

Life History

Juvenile coho salmon grow to about four to seven inches before they become smolts and migrate to the ocean. While in the ocean for one to two years, this salmon may reach a length of 38 inches and weigh up to 31 pounds. Most adult coho salmon, however, weigh between 8 and 12 pounds when they return to their parent streams at the end of their life cycle.

The majority of coho reach sexual maturity in three years, but a few take four years. Mature fish enter the river tributaries in August and September and then spawn during October. In some areas, coho will migrate fairly long distances, but in the Columbia River they seldom travel upstream more than 150 miles. Spawning takes place in the gravelly areas of river tributaries where the current is fairly swift. On the spawning grounds, both the males and females are aggressive. A nest is built by the female by lying on her side and with a whipping operation of the tail excavates a long oval to round trough in medium to small gravel. A female may be accompanied to the spawning area by several males but usually the largest is dominant. A female may spawn in as many as three or four nests and probably with different males. The eggs are large and the number varies with the size of each females, but the average number of eggs per fish is about 2,000 to 2,500. Both sexes die after spawning. The fry usually hatch out in March or early April depending on water temperature and usually congregate in schools in the pools of the stream. In riffles, however, the young are aggressive, and this behavior results in displacement of excess fish downstream. This behavior may be significant in maintaining the numbers of newly emerged juveniles within the carrying capacity of the stream, causing the young to become more widely distributed in the stream. Fry then remain at least one year in fresh water. In some waters, fish have been known to remain for two years.

Geographic Distribution

The native range of the coho salmon is the Pacific Ocean and its numerous tributaries. Its range extended from Southern California northward to Point Hope, Alaska. In Asia it occurs from the Anadyr River, Russia, south to Hokkaido, Japan. The large coho populations along the west coast of Washington, Oregon, and California have in recent years been partially maintained by fish culture. The coho has been widely distributed in the Great Lakes, freshwater lakes and reservoirs from the New England states to the West Coast. The only self-sustaining stocks are presently in the Great Lakes, particularly in Lake Michigan.

Habits and Habitats

Like other Pacific salmon, coho are anadromous. They spawn in fresh water where the young send from one to two years before becoming smolts and migrating to the ocean. These feeding salmon usually remain in the ocean for about 18 months before maturing and returning to their stream of origin for spawning.

Coho salmon originating in the Columbia River drainage migrate both north and south along the Pacific coast, but most go south. These fish are important to the fisheries off northern California and Oregon. On the other hand, coho from Washington coastal streams migrate primarily northward. They are harvested mainly in the fisheries that operate off central and northern Washington. Coho from Puget Sound drainages do not go south of the Columbia River or north of Vancouver Island.

Diet

Coho fry are voracious feeders but probably do not turn to a diet of fish until they reach salt water. In reservoirs, juvenile coho feed primarily on zooplankton (57-75 percent) such as Daphnia and emerging insects (25-43 percent). In streams, juveniles feed primarily on insects such as Diptera (larvae, pupae, and adults), mayflies, and stoneflies. They also feed on worms, fish eggs, spiders, and fish. While at sea, coho feed primarily on fish such as herring, pilchard, anchovies, sand lance, rockfish, ratfish, and invertebrates such as crab larvae, euphausid shrimp, other shrimp, and squid. Growth in salt water is rapid and most fish are 8-12 pounds when they return to fresh water to spawn. In Idaho, fish held in captivity in freshwater lakes seldom exceed two pounds.

Idaho Conservation Status

Idaho Native or Import

Introduced. Coho was first stocked in Idaho in 1967. It has met with only nominal success in the majority of waters where it has been planted, but coho in Cascade Reservoir continue to furnish large numbers of fish annually to the creel. Its numbers must be maintained with annual releases of hatchery fingerlings. During the period 1962 to 1968, 9,846,000 eyed coho eggs were planted in the Clearwater drainage in an effort to establish a self-sustaining population. The small number of returning fish each year indicated that a natural run of fish could not be established so the program was discontinued.

Comments

Coho salmon are fourth in abundance among the Pacific salmons but they provide most of the harvest to sport fishermen. The coho is highly important as a commercial food fish throughout its range. It will strike most conventional baits and lures. The quality of the flesh is excellent, particularly from freshly-caught fish.

Credits

Simpson and Wallace 1982.Wydoski and Whitney 1979. Image Copyright Joseph Tomelleri. Used by permission.

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