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Factors Affecting the Photographic Detection Rate of Grizzly Bears in the Swan Mountains, Montana

Factors Affecting the Photographic Detection Rate of Grizzly Bears in the Swan Mountains, Montana

Seven seasonal population estimates were derived from 1989 to 1991 for grizzly bears (Ursus arctos) in the Swan Mountains of western Montana using a capture-recapture model for closed populations. Mace et al. (1994) discussed in detail the study design and population modeling procedures, which used systematic snaring for initial capture and self-activating cameras for recapture. Twenty-one marked grizzly bears within the camera grids formed the foundation for recapture analysis. We generated 27 photographic records of detections and 71 nondetections in 7 photo sessions. On average, the marked population we sampled was dominated by adult female (29.5%) and adult male (22.6%) grizzly bears. Nearly an equal number of male and female bears were detected on film, but the nondetection sample was skewed towards females (73.3%). Adult males were more frequently detected than adult females or females with young. Most detections (66.6%) were of bears with a prior history of detection. Adult males moved greater distances and consequently encountered more camera stations than other age-gender classes. Generally, movements for all age-gender classes were greatest during the spring and decreased thereafter. When the 7 photo sessions were pooled, it was shown that grizzly bears were not exposed to many camera stations (x̄ = 1.6, SD = 1.8) and bears we successfully detected were confronted with significantly more stations (x̄ = 2.30, SD = 2.3) than those we failed to detect (x̄ = 1.30, SD = 1.44). Logistic regression showed that detections decreased over time. We concluded that differential movement patterns among age-gender classes played an exceedingly important role in photographic detection. The precision of population estimates could be improved by grid densities >5-8 camera stations/100 km2 for grizzly bear populations with similar age-gender structure. Recommendations to increase and sustain precision of population estimates using cameras are provided.

  • Author(s) Richard D. Mace and Timothy L. Manley and Keith E. Aune
  • Volume 9
  • Issue
  • Pages 245-251
  • Publication Date 1 January 1994
  • DOI 10.2307/3872708
  • File Size 370.17 KB
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