From the 1890s to the mid-1960s, the park's bighorn sheep population fluctuated between 100 and 400. Given the vagaries of weather and disease, bighorn sheep populations of at least 300 are desirable to increase the probability of long-term persistence with minimal loss of genetic diversity. The count reached a high of 487 in 1981, but a keratoconjunctivitis (pinkeye) epidemic caused by Chlamydia reduced the population by 60% the following winter and the population has been slow to recover. Although the temporary vision impairment caused by the infection is rarely fatal for domestic sheep that are fenced and fed, it can result in death for a sheep that must find its forage in steep places.
After dropping to a low of 134 sheep following the severe winter of 1996–97, the overall trend has been upward. The 2014 count of the northern yellowstone population by Montana Fish Wildlife, and Parks was 421. Counts identified 77 lambs, 203 ewes, 28 rams, and 13 unclassified sheep. The current ratio of 38 lambs per 100 ewes, indicates a population that is relatively stable to slightly increasing.
Competition with Other Species
Bighorn sheep populations that winter at high elevations are often small, slow growing, and low in productivity. Competition with elk as a result of dietary and habitat overlaps may have hindered the recovery of this relatively isolated population after the pinkeye epidemic. Rams may be hunted north of the park, but the State of Montana has granted few permits in recent years because of the small population size.
Although wolves occasionally prey on bighorn sheep, the population has increased since wolf reintroduction began in 1995. Longer-term data are needed to show whether sheep abundance may be inversely related to elk abundance on the northern range. The Wyoming Game and Fish Department, Montana Fish, Wildlife and Parks, the Idaho Department of Fish and Game, Montana State University, the US Forest Service, and several non-governmental organizations are cooperating with the National Park Service to study how competition with nonnative mountain goats, which were introduced in the Absaroka Mountains in the 1950s, could affect bighorn sheep there.