COLORADO BIGHORN SHEEP MANAGEMENT PLAN 2009−2019

COLORADO BIGHORN SHEEP MANAGEMENT PLAN
EXECUTIVE SUMMARY
Bighorn sheep (Ovis canadensis) are likely the most iconic of Colorado’s wildlife species. Bighorn sheep are
the Colorado state mammal and are also the symbol of the Division of Wildlife (DOW). Today, Rocky Mountain
bighorn sheep (O. canadensis 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. Given this high level of interest and status it is imperative that the DOW develop policies,
guidelines and procedures that are designed to maintain and, to the extent possible, increase Colorado’s bighorn
populations.
HISTORY
It is difficult to estimate how many wild sheep were present in Colorado in pre-settlement times. Journals of
explorers indicate great numbers of sheep in both the mountainous areas and along the Front Range. Since the
late 1800s the general trend of wild sheep populations in Colorado and throughout the west has been downward.
Historical statewide estimates of 7,230 bighorn sheep in 1915, 3,200 in 1958 and 2,200 in 1970 reflect this trend;
there were an estimated 6,045 bighorns in Colorado in 1988, and in 2007 there were an estimated 7,040 bighorn
sheep statewide.
One reason for the apparent increase in Colorado’s bighorn populations is a longstanding effort to trap and
translocate wild sheep to establish new populations or supplement existing populations. From 1945–2007, there
were 147 releases of bighorn sheep in Colorado resulting in the translocation of 2,424 animals (excluding bighorns
moved to research facilities). The majority of these transplants occurred during the 1980s. In 2007, translocated
herds accounted for 54% of the total herds in Colorado and 48% of the total statewide bighorn population. Most
transplant herds (78%) had less than 100 sheep in 2007 and relatively few of these herds have shown the sustained
growth needed for long-term viability. Extant herds that have been supplemented with translocated sheep
accounted for 24% of the total herds and 30% of the total statewide bighorn population in 2007.
Disease has often been implicated in periodic “all-age” die-offs and sustained bouts of poor lamb survival
in Colorado bighorns. In the late 1800s, die-offs were reported in bighorn sheep in the Tarryall Mountains and
elsewhere, and in 1933 a die-off extirpated bighorns in what is now Dinosaur National Monument. In 1953, the
state’s largest bighorn population residing in the Tarryall and Kenosha Mountains experienced a die-off caused
by pneumonia that reduced the population from an estimated 1,000 animals (some observers have said 2,000) to
30 within two years; the Tarryall-Kenosha epidemic likely extended from a 1952 outbreak on Pikes Peak. The
causes of these early die-offs are hard to verify retrospectively, but contact with domestic livestock that led to the
introduction of exotic diseases and parasites seems the most logical explanation. Agents of disease suspected to be
responsible for historical epidemics have included “scabies” (also called “scab” or “mange”, and caused by mite
infestations), “nasal bots” (parasitic fly larvae), “hemorrhagic septicemia” (later termed “pasteurellosis”, a bacterial
infection), and lungworms (a natural parasite of bighorns).
Other problems such as unregulated harvest, overgrazing, competition with other livestock, plant community
succession and forestation of native ranges, and increasing human development of winter ranges have been
identified as contributing to bighorn sheep declines either historically or presently.
In the 1970s, the DOW embarked on a series of research and management programs to reduce lungworm
in the state’s bighorn herds to see if this could have a positive effect on populations by increasing lamb survival.
About 20% of the state’s herds were treated with various drugs; some herds were treated annually and others
more sporadically. In some cases the treatments were just administered as part of trap and transplant operations.
Comparisons of treated versus untreated herds from the 1970s and 1980s and found no difference in population
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