Current Population Update
The population of Sierra bighorn experienced about 50% mortality during the extreme winter of 2022-2023. Snow water-equivalent in the southern Sierra Nevada exceeded 400% of normal in some locations. After accounting for lamb recruitment during the summer of 2023, we estimated that the population declined by 40% from the summer of 2022.
Two factors account for the severe mortality in 2022-23. Sierra bighorn who winter at high elevations to avoid predation were impacted by heavy snow that kills bighorn through avalanches and starvation; we really have no way to manage this type of mortality. Bighorn who winter at low elevations as they attempt to escape extreme weather are at much greater risk of predation, and during the past year we documented at least 42 bighorn sheep that were killed by mountain lions.
Sierra bighorn, when protected from excessive predation, have shown a propensity to rebound from severe winters. They exhibit a remarkable ability to persist in an incredibly harsh environment (picture the snowfall that occurred this winter in the alpine).
The Recovery Program intensively monitors mountain lions to determine which lions prey on Sierra bighorn. The mountain lion population in the eastern Sierra Nevada is the largest documented in decades, and in 2023 we estimated there to be 55 lions. We currently use translocation of lions as an alternative to lethal removal.
During 2022-2023, we translocated 11 female and subadult lions; the 91% survival rate of those translocated lions exceeds that typically observed in unmanaged populations. Moving lions that prey on Sierra bighorn, particularly on low elevation winter range, is essential for protecting translocation stock needed to augment small bighorn herds.
Population Growth for Each Herd Unit (Ewes)
In bighorn sheep, the number of adult ewes determines how quickly a population can grow or recover from losses. Because of this, the health of a population is often gauged by the number of ewes present. This graph shows population trajectories for adult and yearling females from 1999-2023 based on a combination of population estimates (marked resight and minimum counts) for 6 herds in the Sierra with annual population data.
Numerical Recovery Goals
The population goal for downlisting to threatened status is 305 females (for >1 year).
The population must be distributed among 12 of 16 herd units within the 4 recovery units. For delisting the required number of bighorn sheep must persist for a minimum of 7 years without management intervention.
The numerical and herd unit / recovery unit occupation is tabularly summarized below. Achieving final recovery goals will require additional translocations and augmentations. These ewes may potentially be supplied by the Mt. Baxter, Sawmill Canyon, and Wheeler Ridge herds. Therefore, surplus animals must be available for removal from these herds (see map of herds (JPG)).
| Recovery Unit
|
Herd Unit
|
Downlisting / Delisting
Criteria
Recovery Unit #Ewes
|
Current Population
Recovery Unit
(2023)
|
Current Population
Herd Unit
(2023)
|
Plan to Achieve
Numerical Goals
|
| Kern
|
Laurel Creek*
|
50
|
0
|
0
|
Translocation
|
| Big Arroyo*
|
0
|
Translocation
|
| Southern
|
Olancha Peak*
|
155
|
136
|
37
|
Natural growth
|
| Mt. Langley*
|
13
|
Natural growth
|
| Mt. Williamson*
|
6
|
Natural growth
|
| Bubbs Creek**
|
9
|
Natural growth
|
| Mt. Baxter*
|
47
|
Natural growth
|
| Sawmill Canyon*
|
24
|
Natural growth
|
| Taboose Creek*
|
0
|
Natural growth
|
| Black Divide
|
0
|
Translocation
|
| Coyote Ridge
|
0
|
Translocation
|
| Central
|
Wheeler Ridge*
|
50
|
27
|
27
|
Natural growth
|
| Convict Creek*
|
1
|
Natural growth
|
| Northern
|
Mt. Gibbs*
|
50
|
11
|
3
|
Natural growth
|
| Mt. Warren*
|
8
|
Natural growth
|
| Green Creek
|
0
|
|
| Twin Lakes
|
0
|
|
| Cathedral Range**
|
0
|
Translocation
|
Key
| Symbol
|
Description
|
| *
|
Required, currently occupied
|
| **
|
Not Required, currently occupied
|