Washington

On the up: salmon populations have reached record levels in the rivers of the US Pacific northwest. Credit: CORBIS

Salmon are swimming up streams in the Pacific northwest from the ocean in record numbers this spring, raising questions about the measures needed to conserve them.

So far, more than 345,000 spring and summer chinook salmon (Oncorhynchus tshawytscha) have passed the lowermost Bonneville Dam on the Columbia River. And more than 115,000 of these have already hurled themselves past eight major dams and reached the Snake River to spawn in fresh waters — a 25-fold increase on the average over the past 10 years.

Survival rates are also up. Scientists conservatively estimate that more than 21,000 wild chinook salmon, roughly 1.5% of recent years' smolt, will reach spawning grounds this season. This is more than double last year's figure of 8,900 and over 18 times the record low of 1,100 in 1995, according to fisheries biologist Charlie Petrovsky at the Idaho Department of Fish and Game.

Of the millions of smolt released from hatcheries in recent years to supplement the natural population, almost 1% survived the 1,000-kilometre journey. Survival rates in the past decade had been as low as 0.04%.

This year's salmon run is the largest on the Columbia River Basin since records began in 1938, and the best for wild chinook since the late 1970s. It supports theories that ocean climate cycles are a key influence on salmon populations, researchers say.

“The increase in run size is due to increased survival in the ocean,” says Ted Bjornn, a University of Idaho researcher who monitors runs.

Chinook are the most studied of the 26 salmon species listed under the US Endangered Species Act, and are considered a good barometer of salmon health in the Columbia River Basin, once the world's chinook capital.

But ecologists remain cautious. “These high returns should not cause us to assume the crisis is over and salmon are back,” says Peter Kareiva, a conservation biologist at the National Marine Fisheries Service's Northwest Fisheries Science Center in Seattle. He says they reflect a period, beginning in 1999, of cooler waters, more food and fewer warm-water predators. Periods of warmer surface temperatures would, he says, put the pressure back on the salmon population.

Hydroelectric development, mining, overfishing, irrigation and urbanization have all been blamed for declining populations of wild salmon in the region. But with the population apparently surging — and electrical power growing short in the region (see next story) — demands by environmentalists for dams on the Snake River to be demolished to conserve the salmon may lose momentum.

The new data are unlikely to quell controversy over management policies for the dams and rivers, however. In early May a coalition of environmental groups filed a suit against the National Marine Fisheries Service, alleging that its management plan for the salmon violates the Endangered Species Act. Environmentalists also claim that, under current policies, wild spring and summer chinook on the Snake River will be extinct by 2016.

But Kareiva says the picture is far more complex than this. “Our analyses point to serious extinction risks, but also to considerable opportunities for recovery if improvements are made in several different risk factors — hatcheries, habitat, harvest and hydropower.” Researchers estimate that full recovery of wild salmon will require a smolt survival rate of between 2 and 6%.