COLORADO BIGHORN SHEEP MANAGEMENT PLAN 2009−2019

Chapter 1
HISTORICAL TRENDS, STATUS, & LIMITING FACTORS: AN OVERVIEW
Bighorn sheep (Ovis canadensis) are likely the
most iconic of Colorado’s wildlife species. Bighorn
sheep are the Colorado state mammal and also are
the symbol of the Colorado Division of Wildlife
(DOW). Today, Rocky Mountain bighorn sheep
(O. c. canadensis) and desert bighorn sheep (O. c.
nelsoni or O. c. mexicana) provide hunting recreation
for approximately 300 hunters annually and are
among the most sought after watchable wildlife
species in the state. Colorado is fortunate to have the
largest estimated number of Rocky Mountain bighorn
sheep in the United States (Beecham and Reynolds
2007). The estimated 2007 statewide, posthunt Rocky
Mountain and desert bighorn populations were 7,040
in 79 herds and 325 in 4 herds, respectively (Table
1). In 2008, there were 66 Rocky Mountain bighorn
sheep units and 4 desert bighorn sheep units in
Colorado (Fig. 1). Given the high level of interest
in bighorn sheep and their status, it is imperative that
the DOW develop policies, guidelines and procedures
that are designed to maintain and, to the extent
possible, increase bighorn populations in Colorado.
History
Based on early accounts by trappers and explorers,
Rocky Mountain bighorn sheep were common in
Colorado prior to settlement in the mid-1800s (Moser
1962). Available evidence indicates Rocky Mountain
bighorns were widely distributed and occupied suitable
habitat across a range of elevations throughout the state.
With increased settlement and mining booms, bighorn
numbers declined rapidly in the late 1800s, likely as
a result of subsistence and market hunting, habitat
fragmentation and conversion, and the introduction of
domestic livestock and their diseases. Concerns about
declining bighorn populations resulted in bighorn
sheep becoming a protected species in Colorado
in 1885 (Barrows and Holmes 1990). By the early
1900s, bighorn sheep in Colorado only existed in
isolated, remnant populations.
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There is no documented evidence that desert bighorn
sheep occurred in Colorado when European settlers first
arrived. However, archeological evidence, the close
proximity of historic desert bighorn populations in Utah,
and suitable desert bighorn habitat in southwestern
Colorado make it likely that desert bighorns (likely O. c.
nelsoni) did historically occur in southwestern Colorado
in at least small numbers (Bureau of Land Management
[BLM] and DOW 1989).
Disease has been a major limiting factor for Rocky
Mountain bighorn sheep in Colorado since at least the late
1800s, when novel pathogens and parasites apparently
were introduced by domestic livestock (Warren 1910).
Major disease-related die-offs of bighorn sheep were
reported in some locations (e.g., Tarryall Mountains,
Sapinero Creek, Green River, northern Front Range near
Estes Park) in the late 1800s and early 1900s, and during
the winter of 1923–24 (Warren 1910, Moser 1962).
Undoubtedly, many early disease-related die-offs went
undetected or unreported. The first well-documented
all-age, disease-related die-offs occurred in 1952–1953
in the Pikes Peak (S6) and Tarryall (S27)/Kenosha
Mountains (S23) bighorn populations (Moser 1962,
Bear and Jones 1973). Subsequent major all-age die
offs have been reported in Waterton Canyon, Alamosa
Canyon (S29), Big Thompson Canyon (S57), Trickle
Mountain (S10), and several other areas; most recently,
die-offs were documented in the Greenland (S72) and
Fossil Ridge (S70) herds in 2008. Prior to the 1970s,
disease related die-offs were attributed to causes such
as “hemorrhagic septicemia” (a form of pasteurellosis),
scabies, and verminous pneumonia caused by lungworms
(Warren 1910, Moser 1962). In more recent years, it has
become increasingly evident that pasteurellosis probably
has been the ultimate cause of most all-age diseaserelated
die-offs in Colorado, with other factors such as
other bacteria, viruses, lungworm, and environmental
stressors including weather and nutrition being possible
contributing factors in some cases. In addition to initial
all-age die-offs, pasteurellosis can result in reduced lamb
survival and recruitment for many years after a herd is
infected; in some situations, lamb pneumonia also can