Application of a double-count line transect method to estimate density of brown bears in arctic Alaska
Reynolds HV, Becker EF, Strauch RA, Burch JW. 2012. Application of a double-count line transect method to estimate density of brown bears in arctic Alaska. Fairbanks, AK
We used a double-count aerial line transect method to estimate grizzly bear (Ursus arctos) densities in the mountains of the Brooks Range in northern Alaska. The technique used tandem 2-place aircraft flying transects at about 100 m above ground level with the pilot and passenger as observers. A pilot study in 1999 showed that observational efficiency declined when observers searched both sides of transects. As a result, pilots and passengers recorded bear observations from the same side of the aircraft, on one side of the transect only. We truncated the northern edge of the study area at 300 m elevation as bear densities were very low on the arctic coastal plain. Transect searches also did not include habitat above 1220 m. In 2000 and 2001, we flew a total of 821 35-km transect lines and observed 143 bear groups. The most distant groups were eliminated from the data set and 137 bear groups were used in the analysis. Line transect models that produced detection functions estimated the probability of observers detecting a bear at various distances for a given set of covariates (group size, type, activity, percent cover). The sum af group size divided by the probability of detection gave an estimate of population size. We estimated grizzly bear densities to be 17.6 bears/1000km2 (95% CI = 13.7) in the 20,720 km2 area surveyed in 2000 and 2001. In 2003 we completed another 134 transects and observed an additional 29 bear-groups that was used to increase the precision of the estimate. Using data from all 3 years, we estimated grizzly bear densities to be 18.3 bears/1000 km2 (95% CI = 12.3). Observations of at least 160 bear groups are desirable for this technique to be effective. This technique does not require marking or radiocollaring bears. It is useful for surveying remote rugged areas and can be spread over more than one year te reduce annual costs and logistics.
- Type
- Unpublished Report
- Authors
- Reynolds, Harry; Becker, Earl; Strauch, Rebecca; Burch, John
- Date of Issue
- 2012
- Units
- GAAR
- Keywords
- Alaska, Bear, Brooks Range, Brown Bear, Density, Gates of the Arctic, observability, Observation, Population, Survey, Transect, Ursos arctos
- Subjects
- Ecological Framework: Biological Integrity | Focal Species or Communities | Mammals