The Sierra Nevada Bighorn Sheep Recovery Program is specifically charged with implementing the recovery of Sierra bighorn, one of the rarest large mammals in North America. Additionally, CDFW is involved in the conservation and recovery of desert bighorn sheep in the Peninsular Ranges, another population listed under the Endangered Species Act. CDFW also manages the remaining desert bighorn sheep in California, allowing hunting where populations are healthy and can tolerate some harvest.
Historically, bighorn were distributed along the crest of the Sierra Nevada in California from Sonora Pass in the north to Olancha Peak in the south. Prior to European settlement, it has been estimated that there were more than 1,000 bighorn in the Sierra Nevada. However, the Sierra bighorn population was severely reduced during the 19th and 20th centuries because of diseases contracted from domestic sheep, forage competition with domestic livestock, and market hunting. By the late 1970s, there were only two geographic areas where bighorn were found (near Mt. Baxter and Mt. Williamson), with a combined population of 250. Between 1979 and 1988, CDFW translocated bighorn from the Mt. Baxter area to Wheeler Ridge, Mt. Langley, and the Mono Basin to reestablish herds in historic ranges. These recovery efforts were thwarted by another major population decline during the 1990s, attributed primarily to mountain lion predation and drought. By 1995 only about 100 bighorn remained in the Sierra Nevada. Sierra bighorn have been listed under the California Endangered Species Act (CESA) since 1974, but were uplisted from threatened to endangered in 1999 by the California Fish and Game Commission. In the same year, the United States Fish and Wildlife Service (USFWS) temporarily listed the subspecies as endangered on emergency basis under the U.S. Endangered Species Act (ESA) (Emergency ESA Listing Sierra Bighorn Sheep 1999 (PDF)(opens in new tab)). Final listing as endangered occurred early in 2000. CDFW was identified as the lead agency for Sierra bighorn recovery implementation. A team of stakeholders was then assembled and drafted a recovery plan.
Federal endangered status was sought for these bighorn because of a dangerously low population size and the inadequacy of existing regulatory mechanisms. Two concerns were identified: the negative effects of mountain lion predation and the threat of a major respiratory disease epizootic that could result from contact with domestic sheep grazed on public lands adjacent to bighorn ranges. The Sierra Nevada Bighorn Sheep Recovery Plan (PDF)(opens in new tab) identified 16 historic herd units grouped into 4 recovery units. The plan specified herd units that needed to be occupied as well as minimum numbers of females by recovery unit necessary for downlisting and delisting. Issues identified for management actions included losses to predation, factors limiting use of low elevation winter ranges, domestic sheep grazing, and unoccupied herd units. The plan also called for the development of regular demographic data on bighorn herds and identified areas of desired research. Recovery goals required to downlist to threatened are a minimum of 305 females distributed among the recovery units and the occupancy of 12 herd units; these conditions must persist unaided for seven years in order for the subspecies to be removed from the ESA.
To meet delisting requirements, recovery activities focus on understanding and managing factors influencing population performance and expansion of the distribution of these bighorn. Factors known or suspected to limit Sierra bighorn are risk of disease from domestic sheep, predation by mountain lions, forest succession, genetic diversity, severe weather, climate change, and reduced geographic distribution.
Currently the CDFW Sierra Nevada Bighorn Sheep Recovery Program conducts winter and summer population surveys of all bighorn herds, evaluates nutritional status, monitors survival and habitat use patterns of more than 100 individuals, collects data from more than 80 global positioning system (GPS) collars, and identifies resource selection patterns across the Sierra Nevada. The program also determines patterns of genetic variation across the subspecies, models risk of disease transmission from domestic sheep and goats, quantifies the effects of natural and prescribed fire on bighorn forage and habitat use, monitors mountain lion movements, predation rates and population numbers, implements and monitors translocations, and models bighorn response to potential management actions. Information acquired during these investigations is used to direct recovery activities.
Since its listing under the ESA, the Sierra Nevada bighorn sheep population grew from about 125 individuals in 1999 to over 600 in 2015. Subsequently, the Sierra experienced several years of heavy snowfall combined with heightened levels of mountain lion predation, and the Sierra bighorn population dropped to below 300 in early 2023 (numbers for 2023 are still being tabulated). Of the 16 herd units identified as suitable habitat for Sierra bighorn, 13 are believed occupied, but we are currently determining if enough (or any) ewes remain in some of these herds to sustain them. Additionally, a new herd unit not identified in the Recovery Plan, Cathedral Range, is also now occupied, but it is unclear if enough ewes remain there to consider it a viable herd.