Adapting sampling plans to caribou distribution on calving grounds

Abstract

Between 1984 and 1988, the size of the two caribou herds in northern Québec was derived by combining estimates of female numbers on calving grounds in June and composition counts during rut in autumn. Sampling with aerial photos was conducted on calving grounds to determine the number of animals per km2, telemetry served to estimate the proportion of females in the census area at the time of photography in addition to summer survival rate, and helicopter or ground observations were used for composition counts. Observers were able to detect on black and white negatives over 95 percent of caribou counted from a helicopter flying at low altitude over the same area; photo scale varied between = 1:3 600 and 1:6 000. Sampling units covering less than 15-20 ha were the best for sampling caribou distribution on calving grounds, where density generally averaged » 10 individuals-km"2. Around 90 percent of caribou on calving grounds were females; others were mostly yearling males. During the 1-2 day photographic census, 64 to 77 percent of the females were present on the calving areas. Summer survival exceeded 95 percent in three summers. In autumn, females composed between 45 and 54 percent of each herd. The Rivière George herd was estimated at 682 000 individuals (± 36%; alpha = 0.10) in 1988. This estimate was imprecise due to insufficiens sample size for measuring animal density on the calving ground and for determining proportion of females on the calving ground at the time of the photo census. To improve precision and reduce cost, it is proposed to estimate herd size of tundra caribou in one step, using only aerial photos in early June without telemetry

Long-term fluctuations of a caribou population revealed by tree-ring data

“…Historical documents suggest that the population reached a peak in the second part of the 19th century (Low 1896), declined rapidly between 1905 and 1916 (Elton 1942), and remained scarce until the 1960s (Banfield and Tener 1958;Bergerud 1967). Recent dynamics of the RGCH were characterized by steady growth from the mid-1950s to the mid-1980s (Messier et al 1988) and relative stabilization since then (Couturier et al 1990;Hearn et al 1990;Crête et al 1991;Couturier et al 1996;Crête et al 1996). Description of population changes before the 1970s, however, is based on fragmentary information and superficial scientific surveys.…”

Post-calving photo-census of the Rivière George caribou herd in July 1993

“…However, various other para- Crête et al (1991) do not present directly the number of adults in the population. We computed the adult numbers from their data (total population size) and from the proportion of calves observed in the fall (see Table 12 in Crête et al, 1989).…”

“…Fall population estimates (including calves) ranged from 644 000 in 1984 to 283 000 in 1986 and to 682 000 caribou in 1988 (Crête et al, 1991). However, the 1986 and 1988 censuses were relatively imprecise, with 90% confidence intervals of 36% and 39%, respectively.…”

“…Since 1984, population estimates of the Rivière George caribou herd have been derived by photographing caribou in calving areas (Crête et al, 1991). To obtain a more robust estimate of the size of the herd, two independent photographic censuses were carried out in 1993: a calving ground census in June (Couturier et al, 1994) and a post-calving census in July (this study).…”

“…Unfortunately, recent population estimates, determined by photographing caribou on the calving grounds, have suffered from technical problems and lack of precision (Messier et al, 1988;Crête et al, 1991). Due to historic fluctuations in population size and recent evidence of a decline in growth rate, it is important to obtain reliable population estimates at regular intervals to establish population trends.…”