CHELAN — Wild bighorn sheep appear to be thriving in North Central Washington, including three herds in Chelan County, the state Department of Fish and Wildlife says.

Recent surveys indicate those herds are growing at a rate of about 10 percent a year, said Jon Gallie, the agency’s assistant district biologist.

“We have not seen any die-offs, like we did in Yakima” a few years ago, he said.

In late 2009 and early 2010, a pneumonia outbreak killed several bighorn sheep in the Umtanum herd in the Yakima River Valley, and as many as 60 percent showed signs of pneumonia, the agency’s website said.

But herds in Chelan County have had more trouble with highways than domestic diseases.

Gallie said the herds here include the Swakane herd, which frequents hillsides north of Wenatchee from the Rocky Reach Dam almost to Entiat. The state Department of Transportation installed a fence on Highway 97A to help prevent vehicle collisions with the big sheep, and that fence is reducing the death rate in that herd, now believed to be about 120 strong, Gallie said.

The Chelan Butte herd spends most of its time on Chelan Butte, just south of Chelan. Its range extends into Howard Flats near the Chelan Airport, and over to Chelan Falls and Beebe Springs, he said.

That herd is estimated to include about 135 sheep.

“Both of those herds are showing a modest growth each year, and we expect that to continue as long as highway mortality stays low,” he said.

Gallie said the Chelan Butte herd includes from 10 to 20 sheep seen regularly in the northern part of the range, in the Beebe Springs area, and does experience two or three highway collisions each year. “I wouldn’t say it’s a major problem yet,” he said.

Since the sheep aren’t fitted with radio collars, biologists don’t know whether one group has expanded into that area and is now regularly occupying a part of its range with the growth of the entire herd, or if some sheep travel back and forth to the main herd on the butte. “I would say each month of the year we have sightings of sheep up there,” he said.

Gallie said the state conducts surveys to count the sheep twice a year — in the spring and in the fall. “Our estimate is pretty much what we’ve seen on the ground,” he said.

A third herd, on the north shore of Lake Chelan known as the Manson herd, is seen mostly by boaters, and biologists have no estimate of its size because it would be too difficult to count.

But all three herds appear to be healthy, he said. “We have no knowledge of any disease problems in these herds, and based on the fact that their populations increase each year, we think there are not any,” he added.

K.C. Mehaffey: 997-2512

mehaffey@wenatcheeworld.com