COLORADO BIGHORN SHEEP MANAGEMENT PLAN 2009−2019

Chapter 3
INVENTORY & POPULATION ESTIMATION
Reliable data on bighorn sheep (Ovis canadensis)
herd composition, recruitment and population numbers
are needed to develop and evaluate population
management goals and objectives and to make
informed management decisions. Estimates of these
population parameters should be based on rigorously
collected data with known levels of precision whenever
possible. However, many of Colorado’s bighorn sheep
populations occur in remote, rugged areas with limited
human access making this information expensive and
time consuming to collect. Because it will not be
possible to acquire precise estimates of these parameters
for all herds, inventory methods need to be efficient and
populations (and herds within populations) should be
prioritized to best use available resources.
ManageMent goals & strategies
Management Goal
All bighorn sheep Data Analysis Units should
be regularly surveyed with frequency and intensity
dependent on their prioritization.
Strategy: At a minimum, all bighorn sheep populations
should be surveyed every two years by either helicopter
or by coordinated ground surveys to obtain herd
composition, minimum population numbers, status of
individual herds, and population trends. Primary and
secondary core (i.e., Tier 1 and Tier 2) (see Ch. 1, pp.
13 for definitions). DAUs and other populations used
as source stock for translocations should be surveyed
or modeled annually. Surveys of Rocky Mountain
bighorn herds should be conducted during winter (Dec-
Mar) whenever possible, with December being the
preferred month in most DAUs. When summer surveys
are necessary, efforts to collect winter lamb:ewe ratios
of at least a subsample of the DAU are recommended, if
possible. Surveys should not be conducted immediately
prior to or during a bighorn sheep hunting season to
avoid potential impacts to hunter experience.
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Strategy: Population estimates using mark-resight
or other statistically rigorous techniques should be
conducted periodically (e.g., once every 10-15 years)
in Tier 1 and Tier 2 DAUs (or representative herds
within those DAUs) and in other populations of
special interest dependent on funding, feasibility of
marking an adequate sample, and access. During
these studies, marked animals also should be used
to estimate survival rates and document seasonal
ranges and movements. Alternatively, more frequent
inventory and monitoring of select herds within
DAUs may be considered as a means of tracking
bighorn population performance.
Strategy: Population modeling should be used to
develop annual population projections and population
trends in Tier 1 and 2 DAUs and in other populations
or herds where mark-resight surveys have been
conducted.
BaCkground & literature review
Inventory
Bighorn sheep populations have been inventoried
via ground, fixed wing and helicopter surveys. Data
from these surveys have been used to estimate the
size of bighorn populations using various methods
including using count data without adjustments for
sightability (Cook et al. 1990, Bodie et al. 1990,
Karasek et al. 1992), with standard upward adjustments
(15-40%) (Skjonsberg 1988, George et al. 1996, Utah
Division of Wildlife unpubl. data 2007), in sightability
models (Bodie et al. 1995, Bernatas and Nelson
2004), with double-count methods (Magnusson et
al. 1978, Graham and Bell 1989), and with markresight
methods (Neal et al. 1993, George et al.
1996, McClintock and White 2007). In some cases,
population estimates have been used along with other
parameter estimates in population models to project
annual population numbers and trends.