WAFWA Update - Idaho
SHEEP STATS
JURISDICTIONAL WAFWA SUMMARIES PROVIDING IN-DEPTH, STATISTICAL SHEEP INFORMATION.
In Idaho, bighorn sheep exist in both small, isolated populations and in interconnected metapopulations (Figure 1). For management purposes, these populations and metapopulations have been divided into 21 Population Management Units (PMUs). Currently, an estimated 3,400 bighorn sheep occupy about 15.5% of the state. Historical and recent data indicate most PMUs can sustain higher populations of bighorn sheep, and overall management direction will be to increase population levels where feasible. To attain this goal, IDFG will write an annual action plan to describe in more detail planned activities to address management directions and strategies.
FIGURE 1
Current distribution of bighorn sheep in Idaho, as defined by Population Management Units.
Populations
Rocky Mountain bighorn sheep (approximately 2,950 animals – Figure 2) occur in 16 PMUs in central and southeastern Idaho. Eighteen out-of-state translocations and 17 in-state translocations were conducted between 1975 and 2005 to restore Rocky Mountain bighorn sheep populations to historically-occupied habitat. Translocations have successfully expanded the distribution of bighorn sheep, but most of the largest populations are still native Rocky Mountain bighorn sheep that were never extirpated in the Salmon River drainage. In south-central and southwestern Idaho about 450 California bighorn sheep (Figure 2) occur in five PMUs. Bighorn sheep were completely extirpated from this part of the state, and current populations are the result of 11 translocations from outside Idaho and 18 in-state translocations between 1963 and 2004.
figure 2
Statewide population estimates of California bighorn sheep and Rocky Mountain bighorn sheep in Idaho, 2000-2022. Statewide estimates are based on the most recent survey in each population management unit, which may, or may not, reflect the year of the estimate.
Harvest Management
Over the last 85 years, ram tags and harvest have varied considerably with changes in populations (Figure 3). Disease-related die-offs that have impacted large portions of Idaho’s bighorn sheep populations typically resulted in large reductions in tag levels, followed by slow increases in tags when populations recovered.
FIGURE 3
Bighorn sheep tags (issued) and harvest, 1980-2020, Idaho.
Beginning with the 1991–95 bighorn sheep management plan, hunting was not recommended unless a population was estimated at >100 animals. However, a 100-animal minimum may preclude legitimate ram-only harvest opportunities in some smaller populations where habitat carrying capacity prevents achieving minimum population size or risk of catastrophic, all-age die-off is high.
In Idaho, harvest was restricted to ¾-curl or larger rams from 1970–1983 and ¾-curl or larger rams or rams >4 years old (≥3 annual growth rings on horns) from 1984–2006. In 2007, regulations were changed to allow harvest of any ram. Data gathered since 1994 indicate this change had no effect on the average age of harvested rams in Idaho.
Since 1991, management direction is to set tag levels so that harvest is ≤20% of class III and IV rams (¾-curl or larger) observed during the most recent survey for each hunt area. This conservative harvest strategy ensures adequate mature rams for harvest and biological-behavioral requirements (social dominance hierarchy, genetics, mature male/female ratios, etc.).
Current timing of bighorn sheep seasons avoids hunting during the breeding season. Most bighorn sheep seasons start 30 August and continue until 8 October for California bighorns and until 13 October for Rocky Mountain bighorns. Some late-season hunts exist; for example, some hunts for Rocky Mountain bighorns extend through 31 October.
Reduction of ewe numbers may be necessary when sheep numbers have increased above population objectives, including when habitat degradation is possible due to overpopulation or bighorn sheep move into habitat with an increased risk of contact with domestic sheep and goats. Removal of ewes can be accomplished through capture and translocation (in-state or to other jurisdictions) or regulated harvest. Ewe removal is generally not recommended when populations are below habitat carrying capacity, newly reintroduced, or suppressed by a mortality factor (e.g., disease). There is currently no ewe harvest in Idaho.
FIGURE 4
Resident, nonresident, and total number of applicants for bighorn sheep tags, 1982-2022.
All bighorn sheep hunting in Idaho is allocated via a controlled hunt (random drawing) system. Currently, nonresidents are limited to ≤10% of all bighorn sheep tags and not more than one nonresident tag can be issued for controlled hunts with ≤10 tags (≤10% to nonresidents in hunts with >10 tags). Chances of obtaining a bighorn tag generally declined over time as interest and demand have increased, particularly for nonresidents (Figure 4).
Disease and Herd Health
Population health is an essential component of bighorn sheep restoration and management. Historically, disease was an important factor contributing to declines and extirpation of bighorn sheep in much of their range, and disease continues to limit bighorn sheep numbers today. Not only does disease affect populations directly, risk of disease transmission also affects where and how IDFG manages for bighorn sheep in Idaho.The primary limiting factor for Idaho bighorn sheep populations is disease, although other factors including habitat, genetics, climate change, predation, and hunting can also be important. The disease that has the most widespread and severe impacts on bighorn sheep population abundance is a microbiologically complex pneumonia triggered by the bacterium Mycoplasma ovipneumoniae (Movi) (Figure 5).
FIGURE 5
Mycoplasma ovipneumoniae (Movi) infection status and current distribution of multi-locus sequence strain types in Idaho bighorn sheep, 1980-2020.
The most recent die-off was first detected in the Bruneau-Jarbidge PMU (California bighorn sheep) in 2017. The population has decreased from 150 bighorn sheep counted in 2010 to 26 bighorn sheep counted in 2021.
IDFG is currently conducting test and remove research/adaptive management to clear Movi from multiple infected bighorn sheep populations. This includes testing and removing infected sheep in the Lower Salmon, Lower Panther-Main Salmon, and the South Beaverhead PMUs. We are gathering baseline data in other populations and evaluating the North and South Hells Canyon PMUs post Movi clearance.
— Hollie Miyasaki, Idaho Department of Fish and Game
