Black and brown bear density estimates using modified capture-recapture techniques in Alaska

Miller, Sterling; Becker, Earl; Ballard, Warren

Population density estimates were obtained for sympatric black bear (Ursus americanus) and brown bear (Ursus arctos) populations inhabiting a search area of 1,325 km² in south-central Alaska. Standard capture-recapture population estimation techniques were modified to correct for lack of geograhic closure based on daily locations of radio-marked animals over a 7-day period. Calculated density estimates were based on available habitat in the search area (1,317 km² for brown bears and 531 km² for black bears). Calculated density was 2.79 brown bears/100 km² (2.52-3.30 bears/100 km²) and 8.97 black bears/100 km² (7.74-10.21 km²). Calculated 95% confidence intervals were +- 13.7% of the estimate for black bears and -9.8% to +18.5% of the estimate for brown bears. Probabilities of capture based on calculated sightability indices were not equal in some instances, so confidence intervals should be interpreted cautiously. Increasing the number of marked bears during the study period resulted in altered brown bears estimates and smaller confidence intervals, but because closure was a relatively good assumption for black bears in our study area, had little effect on black bear estimates or confidence intervals. When telemetry data were used to correct input values for lack of geographic closure, the Schnabel estimator and the mean of 7 separate daily estimates yielded estimates close to our results. We recommend the technique for additional testing as a method to objectively compare bear densities between different areas or between different times. These procedures may also be appropriate for use with other species.

Black and brown bear density estimates using modified capture-recapture techniques in Alaska

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