For the first time since 1997, the Oregon Department of Fish and Wildlife will not be transplanting any California bighorn sheep this winter.

”As a group we sat down and, based on the data, there were no sheep to move,” said Don Whittaker, program coordinator with the ODFW for bighorn sheep and Rocky Mountain goats. ”We let the biology drive that.”

However, biologists are planning to capture 47 sheep for study purposes.

In recent years, the ODFW has been capturing sheep mostly from the John Day River and Deschutes River canyons – two of the largest herds in the state – and transplanting them to other areas. The goal of the transplants has been to start new herds and improve genetic diversity in existing herds.

”We’ve been working on the John Day River and Deschutes River herds, taking animals out of there the last few years fairly hard,” said Craig Foster, an ODFW biologist based in Lakeview and the ”capture boss” for the program. ”We wanted to give those herds a break.”

And no other herds in the state had large enough or stable enough populations that biologists felt comfortable taking sheep out.

The ODFW estimates Oregon is home to about 3,700 California bighorn sheep located in 32 herds around the state. Herds are the result of transplanted California bighorns, which became extinct in Oregon in 1915 due to unregulated hunting and disease brought in by domestic sheep.

Money to pay for the transplants comes mostly from hunting groups, and an auction tag that since 1985 has provided stable funding for the transplant program. That auction tag has sold for as much as $110,000.

Though the ODFW won’t transplant California bighorns this year, biologists hope to put radio collars on 40 sheep – 20 rams and 20 ewes – in January to begin a four-year study called the Hart Mountain Research Project. The purpose of the project is to determine why bighorn numbers on the Hart Mountain National Antelope Refuge appear to be declining.

Also, seven sheep will be captured from Coglan Butte near Paisley to have blood taken and tested to determine why pregnancy rates in that herd are so low.

”Most of the ewes we caught last year were not pregnant,” said Foster. ”We want to go back in and figure out if that was a disease problem or a one-year lack of nutrition problem.”

All the sheep will be captured using a net gun fired from a low-flying helicopter.

The Hart Mountain Refuge was the first area in Oregon to receive California bighorns, getting 20 from the Canadian province of British Columbia in 1954. The Hart Mountain herd eventually grew to the largest in the state with an estimated 600 animals in the early 1990s, Foster said. But the herd dropped to about 200 to 250 individuals in 2000 and since has rebounded to about 300 animals.

Recent surveys have shown that the herd has been producing lambs at the rate of 40 to 55 per 100 ewes each year.

”You can’t ask for better than that, really,” said Foster. ”However, the population hasn’t really responded. What we’re looking at is the herd is not responding at the level that we would expect it to respond based on the lamb recruitment we’ve seen the last three years. The question is: Why is (the population) not increasing? What has changed out there?”

The decline in that herd is one reason that California bighorn hunting tags – some of the most coveted in the state – fell from a high of 98 in 1995 to a low of 44 in 2002. More than 10,000 hunters applied for those 44 tags.

Since 1965 when the first California bighorn hunt was authorized following the reintroduction, Oregon hunters have taken 1,125 California bighorns rams. Of hunters who managed to get a tag, 85 percent then were successful at getting a bighorn. The largest California bighorn ram taken since the reintroduction came from Alvord Peaks in 1997 and scored 184 7/8 points.

There are two possible causes for the apparent drop in herd numbers at Hart Mountain Refuge, Foster said, and biologists hope to discover the cause with the research project.

The first possibility is that the herd is experiencing an adult mortality rate much higher than the 10 to 12 percent observed in the late 1980s and early 1990s.

The other possibility is that the survey methods the ODFW currently use are not accurately estimating the herd size. Aerial surveys are used to count sheep, first with a fixed-wing aircraft aided by radio tracking. That is followed by a helicopter census in which radio tracking is not used.

The numbers are compared and, using a mathematical formula, an estimate of the total population is made.

However, that method was adopted from the Idaho Fish and Game, and Oregon biologists suspect that the habitat for which it was developed might mean it’s not a suitable model for the Hart Mountain Refuge.

Specifically, the number of sheep might be underestimated because juniper trees have grown and expanded enough since the first introduction of sheep to Hart Mountain in 1954 to make it difficult to count sheep.

”In a 50-year period, absolutely,” said Foster. ”At what point in the Juniper encroachment cover does it get thick enough to preclude your ability to try to count sheep?”

Besides the apparent problem with the Hart Mountain herd, four other herds in Oregon are listed as declining, including a herd in Central Oregon that lives in the Fort Rock area.

That herd has been bolstered with three releases totaling 29 sheep dating back to the early 1990s. But the current population is estimated at only 40 animals.

”It’s not the population level we would really like to see in there,” said Steven George, a Bend-based biologist with the ODFW. ”The main thing is we’re getting very poor kid recruitment into the population.”

In an effort to increase the number of lambs born into the population, the ODFW in 2001 released seven bighorn ewes into the area. However, biologists didn’t observe a significant increase in lambs.

”They did not have good production but that was their first year there,” George said.

Coyotes and cougar could also be playing a role, George noted.

”We do know that predation is an issue out there,” he said. ”But to what extent it’s playing on the population, we don’t know.”

Keith Ridler can be contacted at 383-0393 or kridler@bendbulletin.com.