More than 500 bighorn sheep in six populations across western Montana died as a result of a pneumonia outbreak that began in November 2009, and now Montana Fish, Wildlife & Parks is assessing the population numbers and lamb survival rates as the bighorns work to rebuild.
The disease was responsible for the death of 56% of the bighorns in the Lower Rock Creek, Upper Rock Creek, Bonner and East Fork Bitterroot populations in the fall and winter of 2009-2010. The speed of population recovery now hinges on how soon lambs in the affected populations begin surviving their first year.
Lambs are often born in normal numbers in the first spring after a pneumonia outbreak, FWP says. But, most lambs in recently affected populations die within a few months of birth, as immunities in the mother’s milk decline. This pattern of high lamb mortality can continue for several years following a pneumonia outbreak, and the return to normal rates may be gradual.
Two lambs born in the aftermath of a bighorn sheep die-off in Lower Rock Creek last year have overcome the odds by surviving to be counted as yearlings earlier this month, according to FWP officials.
“They were in a group with 38 adult ewes, about eight miles up Rock Creek,” said Mike Thompson, Regional Wildlife Manager. “We couldn’t see any other yearlings or newborn lambs in this group, but later we saw another group of 12 ewes and 10 newborns nearby.”
As far as anyone knows, the two yearlings are the last survivors of the lambs born in Lower Rock Creek in 2010, Thompson says.
“We were only able to document five lambs born in Lower Rock Creek, as of June 2010. There were probably more that we didn’t see. But to have some survive their first year—even if only two—is a small success in the wake of such a recent pneumonia outbreak.”
FWP attempted to avoid similar deaths of lamb crops in the Bonner and East Fork Bitterroot herds by killing sick animals during the outbreak, in hopes of protecting healthy animals from exposure. FWP biologists and wardens killed 179 bighorns from November 2009 through February 2010 in a race with the disease through these two herds.
Sixteen months later, FWP helicopter surveys show that killing these diseased sheep had no greater or lesser effect on total population size than not culling. Bighorn counts in March-April 2011 for the Bonner and East Fork Bitterroot herds were 58% below numbers counted before the die-off. In Lower and Upper Rock Creek, where pneumonia was allowed to run its course, counts in May 2011 were 59% below numbers counted before the die-off.
But, the benefit of culling is starting to show in lamb survival through their first year, according to Thompson.
“In the spring before the die-off across Bonner and the East Fork, we observed yearlings at the rate of 31 per 100 ewes. This spring, we’re again seeing 31 yearlings per 100 ewes across these two populations, where we tried to cull out the pneumonia before the lambs were born.”
In the Lower and Upper Rock Creek herds, where intensive killing of diseased sheep was not feasible, numbers of yearlings per 100 ewes declined by 96% since the die-off. Thompson hopes that the coming year will be different.
“We’ve got more newborns on the ground this year than we knew about in 2010, so that’s a good sign. They still face long odds. Sometimes this pattern of high lamb mortality lasts for more than one year, or only gradually improves over several years. All we can do is monitor, and learn as we go.”
The Lost Creek (Anaconda) and Garrison bighorn herds were most recently stricken by pneumonia. In his first complete survey since these latest die-offs, FWP biologist, Ray Vinkey, counted 104 sheep in the Lost Creek herd from a helicopter in May 2011, where 297 stood one year earlier. Vinkey also counted 57 sheep near Garrison, where 118 sheep were counted in 2005. Next spring’s surveys will begin to tell the story of lamb survival in these two populations.
The West Fork, Skalkaho and Petty Creek bighorn herds remain unaffected by the pneumonia outbreaks in neighboring populations. FWP biologists counted normal numbers of bighorns in these populations during the latest helicopter surveys. FWP culled sick sheep in the Bonner and East Fork populations in part to buffer the West Fork, Skalkaho and Petty Creek herds from exposure.