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Bighorn herd struggling along Rocky Mountain Front

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Bighorn decline

Bighorn reproduction has stalled along the Rocky Mountain Front and biologists are uncertain how to remedy the decline.

With the Sun River bighorn sheep herd declining by more than half in five years, state wildlife biologists are trying to figure out how that will affect hunting licenses, what it means for the animals’ long-term future and what’s causing the drop.

“That’s the million dollar question,” said Brent Lonner, Fish, Wildlife and Parks wildlife biologist. ”We’ve got a pretty good idea that pneumonia is the ultimate culprit. What’s causing it is the big question.”

In April 2010, Lonner counted 933 bighorn sheep in the Sun River herd on the Rocky Mountain Front. The Sun River herd is roughly defined as bighorn sheep from Ear Mountain south to the Benchmark area.

This year he counted 329 animals. Wildlife counts are considered a minimum of the population.

“The number was probably closer to 400 because the spring weather was mild and the animals were not as concentrated,” he said.

The most significant drop in numbers has hit each year’s lamb crop.

“Ideally I should see 30 to 40 lambs per 100 ewes on early spring surveys,” Lonner said. “Now, I’m seeing 10 lambs per 100 ewes, though that is up slightly from 2011 when I saw five lambs per 100 ewes.

“Based on summer surveys, the ewes are producing lambs, but many of the lambs are not surviving to late summer or early fall. This phenomenon is similar to what has been observed in other wild sheep herds post disease and die-off. It’s caused by the lingering effects of the disease.”

Pneumonia in wild sheep often comes from bacteria carried and tolerated by domestic sheep. If enough of the bacteria get into a bighorn’s lungs, it starts a cascade of events that triggers an autoimmune response, where the body attacks its own defenses, leading to pneumonia and death.

Because there are no domestic sheep near the Front, it’s unclear how the wild sheep picked up the bacteria or even the type of bacteria causing the problem. What is clear is that this herd has not added many young animals to its ranks for five years.

Lonner is also positive the significant increase in deaths of each year’s lambs is not tied to predators or weather: “That’s not the cause of the big downturn in sheep.”

Although young sheep are not entering the population, the herd has a solid number of older rams in some areas.

“For example, in Hunting District 422, which includes Castle Reef,” Lonner said, “I counted 49 rams out of 119 total sheep this spring, and of those rams about 75 percent were three-quarter curl or better. Those are 4- to 5-year-old sheep or older.”

This year FWP has proposed four either-sex sheep licenses for HD 422; there have been no ewe tags for two years. Lonner hasn’t decided whether to recommend upping the either-sex licenses, perhaps, by one or two.

“Do we maintain four tags for a few years in hopes we can make the most of the older-age rams we have now for as long as we can?” Lonner asked. “Or bump that number up by a tag or two for the next couple of years to add opportunity now?”

The FWP Commission will finalize the number of 2015 bighorn sheep licenses at its June 11 meeting.

Anyone with questions or comments can contact Lonner at his Fairfield office at 406-467-2488.

Whatever the decision, how to improve current sheep health is not certain.

“Until lamb recruitment improves, I don’t expect the numbers to rebound any time soon,” Lonner said. “How to stop it? That’s what we are trying to figure out.”

Last year, Montana State University and FWP began a statewide bighorn sheep research project, looking at the animals’ biology and ecology. The study includes several herds throughout the state, including the Sun River bunch.


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