Manuscript Report 164

Gunn A., J. Nishi, J. Boulanger, and J. Williams. An Estimate of Breeding Females in the Bathurst Herd of Barren-Ground Caribou, June 2003. 75 pp

ABSTRACT

Prior to 2003, the most recent survey of the number of breeding females in the Bathurst herd of barren-ground caribou Rangifer tarandus groenlandicus was conducted in June 1996. At that time, the estimate of breeding females was 151 000 + 35 200 (Standard Error). To determine the trend in the number of breeding females – a key indicator for herd health - we followed the standardized method for an aerial photographic survey to determine relative abundance and distribution of breeding cows in June 2003. We flew systematic visual reconnaissance surveys in a fixed-wing aircraft on 4 and 5 and again on 7 June
2003 to delineate the annual calving ground and determine relative caribou densities. We used those observations to delineate high density and moderate density strata for the photographic survey. A blizzard delayed the photography for 5 days and we flew another systematic reconnaissance survey on 13 June to
re-align stratum boundaries. The photography of the high and moderate density strata was completed 14 and 15 June and we also completed a visual survey of the low density strata on 13 and 14 June. To estimate sex and age composition of caribou on the annual calving ground, we used a helicopter to position observers on the ground to classify caribou. We estimated the proportion of caribou that were breeding cows to be 6%, 47% and 82% in low, medium, and high density stratum. The spatial extent of the annual calving ground we observed in spring 2003, was similar to 2002. The distribution of pre and post
parturient caribou occurred south of the Hood River. Based on the combined estimates from the low density visual stratum, and two photo strata, we estimated that there were 109 983 + 15 990 (SE) 1+ year old caribou on the calving ground. After adjusting this overall estimate by the proportion of breeding females observed in each stratum during composition surveys we estimated that there were a total of 80 756 + 13 167 (SE) breeding females. The 2003 estimate is relatively precise and reveals a significant decline in the number of breeding females since 1996.