GRANTS PASS, Ore. — Salmon raised at a Canadian fish farm rapidly evolved to produce smaller eggs, according to a study that heightens doubts about whether hatchery-bred fish can be successfully released into the wild to rebuild endangered species.

Smaller eggs generally produce smaller young fish. And smaller fish do not compete for food in the wild as effectively as larger ones.

"It's sort of a cautionary tale for salmon enhancement efforts," said Daniel Heath, an expert on conservation genetics at the University of Windsor in Canada and author of the study in Friday's issue of the journal Science.

Researchers examined eggs produced by four generations of chinook salmon over the past 12 years at Yellow Island Aquaculture in British Columbia, Canada. They found that the fish produced more eggs, but the size of the eggs declined by 25 percent as wild fish interbred with hatchery fish.

Hatchery fish develop a genetic tendency to produce smaller eggs because in hatcheries, there is no competition for food the way there is in the wild. The lack of competition means smaller fish can more easily survive.

As a result, the genetic trait for small eggs "just swept through the population," Heath said.

The study complicates the debate over how best to rebuild the 26 populations of Pacific salmon and steelhead that are classified as threatened or endangered species.

Conservationists said the Canadian study shows that hatchery fish do not help to rebuild declining runs.

"Some researchers suggest that the only thing wild fish and hatchery fish have in common is water," said Bill Bakke of the Native Fish Society. "What usually comes out of that is some intermediate level of survival."

About 5 billion young fish are released from hatcheries each year around the Pacific Rim.

Hatchery fish are not generally used to supplement wild populations, but to provide fish for sport and commercial fisheries. The two groups do occasionally breed, however.