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Desert bighorn sheep in the White Mountains

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This story was originally published in the Fall/Winter 2021–22 edition of the White Mountain Research Center Newsletter, which features introductions to new staff, goodbyes to former researchers and colleagues, summaries of studies on rock art research, a new flora of the area, GLORIA alpine summit survey updates, and more.

desert bighorn
A desert bighorn ewe with three lambs, on the plateau north of White Mountain Peak. The NRS’s White Mountain Research Center provides road access to CDFW scientists seeking to study these sheep at high elevations. Image: Rick Ianniello

By Rick Ianniello, Environmental Scientist, California Department of Fish and Wildlife

The California Department of Fish and Wildlife (CDFW) has monitored desert bighorn sheep in the White Mountains for decades. The White Mountains are a vast range, with bighorn sheep often inhabiting the barely navigable terrain off the west side. Fortunately, every summer, green meadows along the crest spring out in wake of the melting snow. These meadows bring the sheep up to more accessible habitat and allow us to conduct a survey, from White Mountain to Boundary Peak.

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Minimum counts and mark-resight estimates (with 95% confidence intervals) from desert bighorn surveys in the northern White Mountains from 2015–2020. Image: courtesy Rick Ianniello

With these surveys, we only see part of the total population. In 2015, CDFW counted 78 adult rams on the survey, but in 2016, only counted 16 adult rams. Were there fewer rams in the population, or fewer rams spotted? This is the major limitation of a minimum count survey (a survey in which we only know there are at least the number of sheep seen in the population).

In the fall of 2017, CDFW captured eight rams and 18 ewes in the White Mountains. This allowed us to conduct a mark-resight survey: We marked each sheep with colored, identifiable ear tag combinations so that when surveyed, we would know how many of the collared sheep we spotted. Using the proportion of collared sheep seen to collared sheep not seen, we estimated the population of bighorn in the northern White Mountains alone at around 350 adults. While the number of lambs and yearlings is more variable, we could expect the total population of bighorn between White Mountain and Boundary peak to range between about 400–500 sheep!

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GPS locations of three adult ewes on Montgomery Peak in January 2019. Image: courtesy Rick Ianniello

We also equipped the sheep with GPS collars, which let us track their location year-round. This data has confirmed three distinct ewe populations (or demes). One around Pellisier Flats and another around White Mountain Peak are separated by Birch Creek. The third is located around Silver Canyon, a herd that was reintroduced by CDFW in 1988.

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Approximate habitat occupied by three known and one suspected ewe demes (blue) and approximate habitat utilized by adult rams (green) in the White Mountains. Image: courtesy Rick Ianniello

While ewes occupied these distinct areas, collared rams covered a wider range of the mountains, occupying the same habitat as the demes, and much of the habitat in between. One ram even ranged south into Black Canyon, crossing Highway 168 into the Inyo Mountains. Occasional observations of ewes and lambs in Black Canyon suggest there may be a fourth deme in Black Canyon, which that ram could have been visiting during the rut.

Sierra Nevada Bighorn sheep are a separate subspecies that live on the other side of the Owens Valley. These sheep are well known as alpine specialists who can inhabit craggy, windswept ridges over 12,000 feet in elevation throughout the winter. Desert bighorns, by contrast, are, more known for occupying harsh desert habitat with summer temperatures over 100°F. So you might expect desert bighorn to descend to warmer climates in the winter. However, our GPS collars showed that while some desert bighorn in the White Mountains do just that, others remained between 10,000 and 13,000 feet throughout the winter atop high elevations like Montgomery Peak.

CDFW continues to monitor desert bighorn sheep in the White Mountains, which hold one of the largest and most unique populations of this animal in the state.