The Sierra Nevada bighorn sheep (Ovis canadensis californiana) formerly occurred in 20 subpopulations on the eastern slope of California's Sierra Nevada, and at least one subpopulation on the western slope . The historic population size is not well known, but has been estimated at 1,000 or more individuals. It began declining in the mid-1800s due to hunting, disease and the introduction of domestic sheep. About half the subpopulations were gone by the early 1900s. The species continued dwindling to five populations in the 1950s and two in the 1970s .
The population was estimated at 250 in 1978 on Mount Williamson, Mount Baxter, and Sawmill Canyon, and 300 in 1985 (mostly In Mount Baxter and Sawmill Canyon) [1, 3, 5]. Thereafter it plunged to a historic low of 100 in 1995 before increasing to 300 in 2002. It was listed as an endangered species in 1999.
 USFWS. 2003. Draft Recovery Plan for the Sierra Nevada Bighorn Sheep. U.S. Fish and Wildlife Service, Portland, Oregon. xiii + 147 pp.
 USFWS. 2000. Final rule to list the Sierra Nevada Distinct Population Segment of the California Bighorn Sheep as Endangered. U.S. Fish and Wildlife Service, January 3, 2000 (65 FR 20).
 Sierra Nevada Bighorn Sheep Foundation. 2000. Sierra Nevada Bighorn Population Status, Census Work 1995 On. www.sierrabighorn.org/endanger/population_status.htm
 Sierra Nevada Bighorn Sheep Foundation. 2002. Sierra Nevada Bighorn Sheep 2002 Update, Herds Continue to grow. www.sierrabighorn.org/endanger/2002%20update.htm