North Dakota Bighorn Sheep Survey Shows Mixed Results
North Dakota's bighorn sheep population was primed for expansion after record numbers in 2008, but two consecutive severe winters have at least temporarily halted progress.
Brett Wiedmann, big game biologist for the North Dakota Game and Fish Department in Dickinson, said exceptionally deep snow last winter affected lamb recruitment significantly more than adult mortality.
"It seems bighorns can handle frigid temperatures, but because of their short legs deep snow takes a heavy toll on them, especially lambs," Wiedmann said. "They expend a tremendous amount of energy foraging, and become even more susceptible to predators and harsh weather conditions when they are trapped on a ridge with no place to go."
The 2009 annual bighorn sheep survey, which covers a period from April 2009 to March 2010, revealed 296 bighorn sheep in western North Dakota, a 5 percent reduction from 2008 but still 8 percent higher than the five-year average. The northern badlands’ population was down 2 percent from last year, while the southern badlands' population saw a 15 percent decrease.
In total, biologists counted 100 rams, 166 ewes and 30 lambs.
Each summer, typically in August, Game and Fish Department biologists count and classify all bighorns. Biologists then recount lambs in March to determine lamb recruitment.
Lamb recruitment was low at only 20 percent, Wiedmann said, which is well below average in North Dakota. Only 60 percent of the lambs counted last summer survived the winter, while 80 percent survived and 50 lambs were recruited into the population in 2008. "Lower lamb recruitment in 2009 is the primary factor contributing to this year’s reduced count," he added.
While lamb recruitment was low, the adult population remained stable and the ram-to-ewe ratio increased to 60 rams per 100 ewes.
Wiedmann said the adult segment of the population more than held its own. "Overall, I am encouraged with the survey results even after these two brutal winters," he added. "Really, it is surprising that any lambs were able to survive all winter long in such deep snow."