Nishi J.S., B. Croft, J. Williams, J. Boulanger and D. Johnson. 2008. An Estimate of Breeding Females in the Bathurst Herd of Barren-ground Caribou, June 2006. 177 pp

ABSTRACT

We used the calving ground photo-census technique to estimate abundance and distribution of breeding females in the Bathurst herd of barren-ground caribou (Rangifer tarandus groenlandicus) in June 2006. In late May 2006, we started monitoring movements and locations of satellite-collared Bathurst cows (n= 8-14). We used Lupin Mine at Contwoyto Lake as our base of operations and started systematic aerial surveys on the 6 and 7 June. The distribution of satellite-collared cows was the means of centering survey effort during the initial systematic surveys. Then, we used observations of relative caribou density and composition (presence of hard antlered cows and/or newborn calves) to define our survey extents. Due to concerns regarding the declining trend of the Bathurst herd, we ensured that our systematic coverage was extensive so that we did not miss any breeding females. We conducted another systematic aerial survey on the 8 June, and delineated the annual calving ground based on the systematic surveys. We initially stratified the calving ground into one high density (photographic) stratum, two medium density (photographic) strata and two low density (visual) strata. Although we initiated the photo-census of the high density stratum on the 9 June, poor weather prevented completion of the photography on the 10 June. We flew the boundaries of the high density stratum on the afternoon of the 10 June, and re-aligned the boundaries of the high and medium density strata to reflect changes in caribou distribution. We added additional low density strata to ensure complete coverage of the calving ground. The aerial photography of one high and two medium density strata was completed on the 11 and 12 June. Visual surveys of six low density strata were flown with a fixed wing aircraft on the 9 and 11 June. We used a helicopter to complete composition surveys in high, medium and low density strata from the 11-15 June. Based on the combined results of visual surveys in the low density strata and photographs of transects in the medium and high density strata, we estimated that there were 67,246 ± 9904 (SE) 1⁺-year-old caribou on the annual calving ground. After adjusting this estimate by the proportion of breeding females observed during the composition surveys, we estimated that there were 55,593 ± 8813 (SE) breeding females in the survey area. The high density stratum contributed 92% of the estimated number of total caribou and 98% of the breeding females. The proportion of breeding females in the high density strata was 88% ± 3% (SE). The estimate of breeding females in June 2006 was relatively precise (CV = 16%), and substantiates the results of the June 2003 Bathurst caribou survey. The June 2006 survey confirms that the abundance of breeding females in the Bathurst herd of barren-ground-caribou has significantly declined since 1986.