Salmon Survival: Threats and Remedies

By John Krist,
Senior reporter

U.S. Army Corps of Engineers. Fish ladders such as this one, at Ice Harbor Dam on the Snake River in Washington, are designed to ease passage of adult salmon and steelhead as they head upstream to spawn. Other structures have been installed at the dams to make it easier for smolts to avoid the dams on their way downstream.

Like their cousins in watersheds along the coasts of Washington, Oregon and California, Columbia River Basin salmon face numerous threats to their survival. Although their populations fluctuate from year to year as a result of natural changes in the ocean environment - variations in water temperature, currents and food supply - most scientists are certain that the long-term decline is due to human factors. Human beings have responded with several strategies intended to minimize those threats and reverse the decline.

THE THREATS

There are more than 250 dams on the Columbia River and its tributaries, and they contribute to high salmon mortality rates in several ways. Dams without fish ladders prevent adult salmon from reaching the cold, clear, gravelly streams high in the watershed where they spawn. (Grand Coulee Dam alone cut off access to 1,400 miles of salmon spawning streams.) Even with ladders, many adult salmon never manage to find their way around a dam. Long reservoirs behind the dams force young fish heading downstream to expend energy swimming instead of simply riding the current, and expose them to more predators; warm reservoir water also increases the likelihood of disease. When they reach the dams, young fish face other potentially lethal obstacles: They may be sucked into the generator turbines, where they are often mashed by the spinning blades or subjected to fatal pressure changes, or they may plunge over the spillway and into the boiling water below the dam where, stunned and often suffering from a condition like the bends, they are easy prey for predators.

Click here for a larger image of the Fish Passage graphic

Logging near salmon spawning streams can deprive the water of shade, causing the temperature to rise to levels lethal to eggs and fry. Sediment washed into streams from denuded hillsides or new logging roads can smother eggs and destroy spawning grounds by burying the gravel in which adult fish scrape their nests. Improperly positioned culverts can turn highways and roads into impenetrable barriers where they cross salmon and steelhead streams. Farm irrigation systems can dry up streams, and unscreened pumps can suck fish into ditches and fields. Contaminated runoff from fields and urban areas can also contribute to disease. Stream beds that have been channelized and the banks lined with riprap to control flooding deprive young fish of the cover - overhanging banks, vegetation - they need to hide from predators.

When salmon populations first began to plunge in the late 19th century, it was overfishing, not dams (none had been built yet) that was to blame. Commercial salmon and steelhead harvest in the Columbia Basin in the 20th century has ranged from 2.1 million fish in 1941 to 68,000 fish in 1995. Commercial, recreational and subsistence fishing in the Columbia Basin are carefully controlled, but salmon spend their adult lives at sea, where they are subject to additional fishing pressures.

U.S. Army Corps of Engineers. Transporting juvenile fish by barge is just one of many costly strategies the federal government has adopted in an effort to preserve dwindling salmon and steelhead populations in the Columbia River watershed. According to the General Accounting Office, the effort has cost U.S. taxpayers $3.3 billion over the past 20 years.

Early hatcheries were intended to offset the decline in salmon populations brought about by overharvesting in the late 19th century. As populations began dwindling again with the proliferation of dams in the Columbia watershed, hatcheries were called upon to offset mortality due hydropower operations. Biologists increasingly believe, however, that the abundance of hatchery-produced salmon is actually interfering with recovery of wild salmon populations. Research has shown that hatchery fish are less successful breeders and more prone to disease; by mating with wild salmon, they may introduce these genetic traits into the wild population and reduce its fitness for survival. Hatchery fish also compete with wild fish for food and habitat, overwhelming them with sheer numbers.

THE REMEDIES

While state and local governments, along with hundreds of private organizations, have addressed habitat damage in a variety of ways (new road construction standards, logging regulations and pollution-control efforts), federal efforts to reduce salmon mortality have focused on dams.

Click here for a larger image of the Fish Screens graphic

Fish ladders help speed migration of adult salmon upstream. Smolts are carried downstream in barges and trucks. At several dams, costly bypass structures have been installed: Screens divert young fish away from turbine intakes and into channels that lead them around the dam and empty in the river downstream. At some dams, operational changes have been adopted to send more water over the spillway at times of the year when smolts are migrating downstream, reducing the amount of time they must spend in the reservoir.