Monitoring Dall's Sheep in Yukon-Charley Rivers

History of aerial sheep surveys

Dall’s sheep aerial surveys were first conducted in the Charley River basin in the 1970s to help identify sheep habitat prior to the creation of the Alaska National Interest Lands Conservation Act (ANILCA) national parks in Alaska. These surveys identified important sheep habitat in the Yukon-Charley Rivers area and its role in the broader sheep population across the Yukon-Tanana Uplands. After the creation of Yukon-Charley Rivers National Preserve, National Park Service (NPS) biologists began monitoring sheep across the Charley River Basin mountains in 1983. In the late-90s, surveys began including areas along the cliffs above the Charley River itself, as it was recognized as an important lambing area. The south facing, 1000-foot cliffs above the Charley River provide a perfect mix of both food and escape terrain. However, since the canyons and cliffs along the Charley were difficult to survey using small fixed-winged aircraft, helicopters have been used to survey sheep along the Charley River since 1994.

 

Aerial Surveys

Aerial surveys provide a way to monitor the population size and composition (number of ewes, lambs, and rams) of Dall’s sheep across a large study area. In Yukon-Charley Rivers, these surveys typically take place over a few days in July. A helicopter, with a trained pilot and one observer, survey all the sheep habitat within each survey unit and record group size and composition of all observed sheep. This provides an abundance estimate based on the minimum number of sheep observed.

 

Population Dynamics

Dall’s sheep population abundance remained stable in Yukon-Charley Rivers in the 1990s to the late 2000s, ranging from 282 to 333 sheep over six surveys conducted between 1997-2009. However, a 48% decline in sheep abundance was observed during the 2015 survey. This decline was likely driven by a cold, late spring in 2013 that contributed to declines in sheep abundance across Alaska. Sheep appeared to be rebounding as of the 2018 survey, when as 27% increase was reported. Unfortunately, the most recent survey in 2023 revealed another large decline, with only 70 total sheep observed, representing a 75% decline from the long-term average. Although no formal analysis has occurred, poor environment conditions, brought on by multiple years of higher-than-normal snow packs and colder-than-normal spring temperatures, are thought to be driving the decline, much like the last major decline in 2013.

Monitoring Implications

Monitoring of Dall’s sheep in Yukon-Charley Rivers informs wildlife management decisions that effects the long-term sustainability of both subsistence and sport hunting. Due to the small size and isolated population, sheep that inhabit Yukon-Charley Rivers are more susceptable to local extirpation. Increased understanding of their population dynamics will ensure future users of the preserve will still enjoy new lambs bounding along the cliffs of the Charley River each summer.

 

Learn about Dall's Sheep in Yukon-Charley Rivers