North Dakota Bighorn Sheep Survey Shows Mixed Results

Results from this summer’s bighorn sheep survey indicate North Dakota’s bighorn population has increased from last year, despite the ongoing presence of pneumonia.

North Dakota Game and Fish Department big game biologist Brett Wiedmann said the July-August survey showed a minimum of 304 bighorn sheep, an increase of 6 percent from 2014. Results revealed 87 rams, 159 ewes and 58 lambs. The department’s survey does not include approximately 30 bighorn sheep that live in the North Unit of Theodore Roosevelt National Park. “This year’s slight increase can be attributed to better-than-expected lamb recruitment over the past couple years, prior to the disease outbreak of 2014,” Wiedmann said. However, the pneumonia-related die-off that occurred in 2014 appears to have resurfaced again this summer. “Despite no documented pneumonia-related mortalities since last January, the disease reoccurred last month,” Wiedmann said. “Similar to last year, we were pleased with the initial results of the survey as high lamb numbers were recorded, coupled with no signs of clinical disease among the adult segment of the population. However, significant signs of disease began to occur in mid-August as the survey was nearing its completion.”

Wiedmann noted that the bighorns showing evidence of disease this summer were from the same herds that were most affected last year. “Unfortunately, these are three of our core herds in the northern badlands, so we’ll have to once again closely monitor the impacts of pneumonia, as many adults and lambs showing signs of disease in August likely will not survive the winter,” he said.

Those herds in the northern badlands that were not affected by last summer’s die-off appeared healthy again this summer, and achieved very high female-to-lamb ratios.

Game and Fish wildlife veterinarian Dr. Dan Grove said it is too early to tell the severity. “Since summer 2014, mortalities have not yet been catastrophic, however, the outbreak is ongoing, and we detected a virulent strain of bacteria last year that was collected from both dead and live-sampled bighorns. Consequently, the recurrence of pneumonia this summer may be more apparent when females and lambs are recounted next March.”

The Midwest Chapter of the Wild Sheep Foundation pledged $20,000 to the department to help monitor effects of the die-offs.

Each summer, Game and Fish Department biologists count and classify all bighorn sheep, a process that takes nearly six weeks to complete as biologists locate each bighorn herd in the badlands by tracking radio-marked animals from an airplane, and then hike into each group to record demographic data using a spotting scope and binoculars. Biologists then complete the annual survey by recounting lambs in March to determine lamb recruitment, or lambs that survive the first winter.

October 14, 2015 | Vol. 41 • No. 35  | View PDF