Ak. Neal et al., EVALUATION OF MARK-RESIGHT MODEL ASSUMPTIONS FOR ESTIMATING MOUNTAIN SHEEP NUMBERS, The Journal of wildlife management, 57(3), 1993, pp. 436-450
Mark-resight methods have been used to estimate numbers of mountain sh
eep (Ovis canadensis canadensis), but no evaluation has been made of p
otential bias or precision of estimates. We simulated the mark-resight
procedure, including simulations with violations of several assumptio
ns, using the joint maximum likelihood estimator from a hypergeometric
distribution (JHE). We also conducted a field study with a mountain s
heep herd near Trickle Mountain, Colorado. These field data, particula
rly estimates of heterogeneity of individual sighting probabilities an
d group sizes, were used to provide realistic simulation distributions
. Aggregation of animals did not bias the estimate but did decrease pr
ecision and confidence interval (CI) coverage. The potential for overe
stimation increased with aggregation. From the Trickle Mountain field
study, mean group size was 6.3 (SD = 6.0), and animals that were seen
together spent about 19.2% of the time together. Violating the assumpt
ion of equal sighting probabilities among individuals may have biased
the estimates, especially with smaller populations (e.g., 50), but the
bias was not large (approx 8% max.). Heterogeneity of individual sigh
ting probabilities lowered coverage and decreased precision. Individua
l sighting probabilities of the Trickle Mountain herd ranged from 0.33
to 0.86, with a mean of 0.58 (SD = 0.2). A modified equation was used
to account for immigration/emigration in the population. The Minta-Ma
ngel estimator, which could be used when heterogeneity of individual s
ighting probabilities exists, did not perform as well as JHE. We recom
mend mark-resight and the JHE as a population estimation method for mo
untain sheep where demographic closure exists within a well-defined st
udy area.