BEND, Ore. — The spotted frog, a candidate for the federal threatened and endangered species list since 1991, appears to be making a comeback in parts of Oregon.

Biologists Christopher Pearl and Jay Bowerman are already counting down until mid-March, when the frogs are expected to emerge from their winter resting spots and migrate toward a shallow pond to mate.

Pearl and Bowerman have spent the past five years studying a population of Oregon spotted frogs that was transplanted downstream on the Deschutes River from its former home at Wickiup Dam.

The spotted frog is considered a "critical sensitive" species by the Oregon Department of Fish and Wildlife. But the frog is not yet considered a high enough priority for the agency to spend the time and effort going through the listing process, according to Phil Carroll, a spokesman for the U.S. Fish and Wildlife Service in Portland.

Scientists estimate that about 25 or 30 populations of the frogs remain in Oregon, and a handful remain in eastern Washington and southern British Columbia.

Pearl, a wildlife biologist for the U.S. Geological Survey, said he doesn't know the number of individual frogs.

But he and Bowerman, semi-retired biologist who volunteers at the Sunriver Nature Center, are tracking the handful of spotted frog populations around the state.

And they pay particularly close attention to a local clan of frogs, which so far represent an unlikely success story.

In 2001, the frogs were trapped and moved away from Wickiup Dam, which was about to undergo a three-year, $44 million renovation.

Scientists moved 20 adult frogs and about 18 juveniles. They also transported nine egg masses.

When the scientists count the frogs during their mating season this spring, they expect to find about three times as many frogs as they initially moved from Wickiup.

"That can only be viewed as a compliment by the frogs that they decided that this was OK and that they didn't need to leave," Pearl said.

Despite these promising results, Pearl said it will be years before he is certain the population is settled and healthy.

Scientists have already identified specific threats that could cause a sudden decline in the population, such as vegetation in the man-made ponds growing more quickly than experts expected.

Pearl said he worries that it is "just a matter of time" before predatory bull frogs discover the comfortable bog where the spotted frogs have settled downsteram from the dam.

"That would be a real source of concern, if they show up," Pearl said.