Chinook salmon are a fish of superlatives. The biggest of all salmon, they can reach weights of 100 pounds or more. In California's Central Valley, perhaps 2 million Chinook once spawned and died in the rivers each year.

These salmon are marvelously adapted to the hot Valley climate, with runs entering the rivers in the fall, late fall, winter and spring. With such diversity and abundance, they were mainstays of California fisheries.

Today, Central Valley salmon populations have reached record- low numbers. The spring and winter runs have been listed as threatened or endangered. Commercial fishing for all runs has been shut down.

The causes of the salmon population collapse are multiple and complex, an apparent "perfect storm" of interacting factors, man-made and natural. To supplement the wild populations, hatcheries produce approximately 31 million young salmon per year. Yet today, hatchery fish are part of the problem.

Hatchery fish are less-adapted for survival in the wild, but they can compete with and interbreed with wild salmon in rivers, overwhelming wild fish by their sheer numbers and weakening the offspring.

They are wonderful food, but when it comes to spawning and perpetuating the species, they are no substitute for truly wild salmon.

In previous years, up to 90 percent of salmon caught off California by fishermen were born in a hatchery. Unfortunately, the fishermen have never had an easy way to tell hatchery from wild fish. As a result, wild salmon that are endangered species are caught as well as hatchery fish, with negative effects on their beleaguered populations.

A major step toward solving the problem: Mark all hatchery fish. First, hatcheries could remove a small fatty fin on the back whose loss does not affect survival. Its absence is easy to detect on adult salmon. Second, hatcheries could inject a wire tag into each salmon's snout that encodes their hatchery of origin and other information. This marking technique has a long history of successful use worldwide.

If all hatchery fish have the mark and tag, the currently closed fishery could probably be reopened, albeit in a limited fashion. Fishermen could keep all marked fish. Unmarked fish would be released into the wild.

The inadvertent catching of some wild fish would cause some to die. Yet more would live. The marking and tagging program would also pay large benefits in improved management of salmon populations by providing more information on how salmon use the ocean and on the impacts of the fishery on wild fish.

This is not a new idea. The benefits of marking programs have long been recognized in Washington and Oregon, where over 100 million juvenile Chinook salmon are marked each year. Around the Great Lakes, agencies are beginning to implement the annual marking of all hatchery salmon and trout, about 30 million fish.

It would cost roughly $5.5 million to establish a program to mark and tag all hatchery salmon in the Central Valley, and about $4.5 million per year to run it. Yet this price tag seems small compared with the value gained, particularly if the alternative is to keep the fishery shut down indefinitely.

Marking all salmon released from hatcheries is an important tool for salmon conservation in the Pacific Northwest. We are not using it enough in California. We should.

Peter Moyle is a professor of fisheries at the University of California, Davis; Richard Sitts is an ecologist with the Metropolitan Water District of Southern California.