DENVER - A 20-year government effort to restore the population of an endangered native trout in Colorado has made little progress because biologists have been stocking some of the waterways with the wrong fish, a new study says.

Advances in genetic testing helped biologist discover the error, which was called a potential black eye, but they said there is still hope for restoring the greenback cutthroat trout.

The three-year study, led by University of Colorado researchers and published online in Molecular Ecology on Aug. 28, said that five of the nine populations believed to be descendants of the endangered trout were actually the more common Colorado River cutthroat trout, which look similar.

The study said the results imply that the effort has "failed to improve the species' status."

Lead author, Jessica Metcalf, who recently completed her doctorate in ecology and evolutionary biology at the university, was optimistic about the ongoing restoration program because four populations have been identified as "pure greenback cutthroat trout."

Bruce Rosenlund of the U.S. Fish and Wildlife Service, which is leading the recovery effort, said the agency is reviewing the study.

"The report is just a continuation of different expert input provided to the team for consideration for restoration," Rosenlund said.

Colorado and federal biologists have a goal of 20 self-sustaining populations of at least 500 fish each. The cost of the program was not available.

Greenback cutthroat trout were historically found in the drainages of the Arkansas and South Platte rivers in Colorado and a small part of Wyoming. They were declared extinct in 1937 because of overfishing, pollution from mines and competition from nonnative fish.