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Riding the Silver Cycle
Coho salmon have been much in the news, as the federal National Marine Fisheries Service considers whether they will need to list most runs of the fish south of the Columbia River as "threatened" under the Endangered Species Act.
Coho, often called "silvers," have been declining in the wild throughout this century. Fishery scientists have identified 35 "stocks" (fish from a particular river system) at risk of extinction in the U.S., south of Alaska. Coho have vanished from more than half of their historic range--largely through exclusion by dams--and are threatened in most the rest. According to one estimate, healthy stocks can be found in only 6.5% of the coho's original habitat.
A quick tour of the coho's life cycle may help to illustrate its habitat needs, and the kinds of impacts that may threaten its survival.
Silvers begin life in late autumn as fertilized eggs deposited in gravel nests known as "redds." (A female coho may lay 2,500 eggs in her final act of life before dying.) To spawn, the fish need clean, pea-to-orange sized gravel. The developing eggs need clean, cool, highly oxygenated water. Sediment covering stream bottoms due to erosion can restrict spawning habitat or smother developing eggs.
When young fish (called "fry") emerge in the spring, they hide along the stream edges and in backwater areas. At this stage they need to find intact stream banks covered with overhanging vegetation. By summer, they have grown enough to move into the deeper pools in the middle of the stream which are formed when the current plunges over logs and other obstructions. They need cool water to thrive, ideally between 53-58 degrees F., and never above 68 degrees. Water temperatures can climb to dangerous levels for them when streams are left unshaded or too much water is taken from the stream for other uses. In areas devoid of large in-stream logs, there are also fewer deep cool pools in which to live.
During the winter the juvenile silvers avoid high water flows by moving into side channels and beaver ponds, or by hiding behind log jams, all the while feeding on insects. Their chances of survival during this phase are greatly reduced if streams are stripped of their wetlands, if stream meanders and beaver ponds have been eliminated, or if few logs remain in the stream channel. Salmon restoration efforts are often focused on re-opening old creek channels or creating small side ponds since the lack of appropriate winter-time habitat is often the limiting factor for coho survival in the streams.
In their second spring, coho are ready to go to sea (and at this point are called "smolts"). They still need clean water, of course, and they also need naturally high water flow to help carry them down to the oceans. Dams, by altering water flows, can interfere with this out-migration. Near the end of the their journey to the sea, smolts pause for a few weeks in estuaries, hiding and feeding in marsh channels while adjusting to saltwater. The loss of estuarine wetlands may leave them ill-prepared for ocean survival.
Silvers spend only about a year and a half at sea, but during this time they grow rapidly. While in the ocean they face predators such as birds (as small fish on the way out) and marine mammals (as larger fish heading home). In the past they also faced predators in the form of the sport and commercial fishermen, but there have been complete closures of ocean fishing for coho in recent years. While in the ocean they can also be heavily affected by changing ocean conditions, such as the periodic "El Nino" which brings warmer water up the coast from the south, which can limit food supplies and leave them too weak to survive.
Coho return to West Coast rivers in the early fall, first congregating in the estuaries (where pollution concentrations can be high) while re-adjusting to fresh water. Then they begin the return journey upstream, seeking to reach the small, gently sloped tributary streams where they prefer to spawn. Once again they need adequate water flows--many rivers are all but depleted in the fall by withdrawals for human uses. But if nothing has blocked the completion of their journey, the coho (like all salmon) spawn and then die, while their eggs settle into their redds to begin the cycle anew.
The coho's life-cycle, moving from stream to river to bay to the ocean and back again, touches the environment at every point. Everything that happens in a watershed affects the fish. Thus, there is no one simple thing that can be done to protect the remaining coho stocks or to restore damaged habitat. We all can play a role in protecting the remaining coho salmon by recognizing that everything we do that affects water affects the coho's migratory way of life and take due care.
For more information on how you can help salmon recovery efforts in the Pacific Northwest, please contact your local watershed restoration group or Pacific States Marine Fisheries Commission, 45 SE 82nd Dr., Suite 100, Gladstone, OR 97027-2522.