A precipitous drop in the bighorn sheep population north of Plains has forced the state to close sheep hunting in the district while state wildlife officials determine the cause — suspected to be another deadly outbreak of pneumonia.

The herd in Hunting District 122 dropped from 112 animals in 2013 to just 18 according to the most recent count two weeks ago. 

Montana Fish, Wildlife and Parks Wildlife Biologist Bruce Sterling said the population had dropped to about 86 sheep last year, but because of the difficulty in obtaining accurate population data in the rugged area, wildlife managers initially were not as concerned.

“It sort of raised a few eyebrows, but we were attributing it to the fact that the Forest Service had done a lot of prescribed burning in the area,” he said. “I thought, with all added helicopter activity, that’s why we missed the sheep.”

Bighorn sheep prefer rugged, bare-rock cliffs that offer the herds protection from predators, but that makes it difficult for wildlife officials to track down animals to test them for disease. The counts are usually conducted via helicopter over the remote, mostly roadless mountains in the district.  

“We may or may not find any. That’s pretty rugged country, and depending on where those sheep die, it’s going to be hard to find them,” he said.

Despite the presence of a number of predators in the area, the population has generally remained robust over the years, and has experienced low rates of mortality from other factors such as train and car collisions. 

Wildlife officials can’t say for sure that pneumonia is the culprit behind the falling numbers until tissue tests positively identify bacterial infection.

Neil Anderson, the regional wildlife manager, has decades of experience studying diseases in local wildlife, and helped developed the health protocols currently in place for bighorn sheep. He agreed that positively confirming the reason for the apparent die-off is not an easy task.

“We'll have to get out there on the ground and walk some of the drainages, gullies and hillsides and see if we can find them,” Anderson said. “It's kind of a needle-in-the-haystack trying to find the carcasses, and even more so trying to find ones in good enough condition where we can actually get samples from them.”

The herd is believed to have come into contact with domestic sheep, which act as carriers for the pathogens that cause the lung infections, within the past three years.

Even if the population stabilizes, some of the sheep could survive while still retaining the infection, passing it on to successive generations of sheep. Anderson said that if officials attempt to augment the population with sheep from other herds, they will need to confirm that the pneumonia outbreak has run its course.

Last month the Fish and Wildlife Commission closed bighorn sheep in Hunting District 305 near Gardiner due to a confirmed pneumonia die-off of 34 sheep.

Prior to this emergency closure in the Plains district, Fish, Wildlife and Parks had offered six either-sex and four ewe licenses there. The approximately 200 applicants for sheep tags will be offered the opportunity to apply elsewhere or be given refunds. Last year about 900 hunters applied for the district's licenses.

Reporter Samuel Wilson can be reached at 758-4407 or by email at swilson@dailyinterlake.com