Woodland caribou population dynamics in Northeastern Alberta

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  • Studies of woodland caribou (Rangifer tarandus caribou) in the Birch Mountains of northeastern Alberta were conducted from January 1976 through June 1978. Twenty-nine caribou were radio collared and repeatedly located from fixed wing aircraft. Eight capture-related deaths were associated with increased stress (hazing and handling time) and slow or incomplete absorption of the immobilizing drug. Young bulls (1.0 to 3.0 years old) and adult, cows (>3.0 years old) were sometimes indistinguishable from the air due to similar body size and antler morphology. Time of antler drop among bulls was related to age. A population survey combining fixed-wing transect flights and helicopter tracking over 1400 km2 yielded a late-winter density of 1 caribou/24km2. Radio-tracking data indicated that adult bulls concentrated in this area in winter; adjusted resident density was 1/33 km2. The total caribou population on the 25 000 km2 Alberta Oil Sands Environmental Research Program study area was estimated at 433. Bulls comprised 42% of animals older than calves. Calves made up 12% of the total fall and winter population. Yearlings comprised 14% of captured caribou, but individuals born from 1972 to 1974 comprised only 15%. This apparently reflected low survival of animals born following winters of deep snow and high food-short lynx (Lynx canadensis) populations. Calving occurred from 7 May to 3 June. The pregnancy rate of adult cows (>3.0 years) was 88%. Calf survival was 42% in the first 2 months of life and 17% annually. Annual survival of radio collared adults averaged 85%. At least two of four radio collared adults which died were killed by wolves (Canis lupus). The calculated finite rate of population growth (A = 0.85) indicated a declining population in years with normal snowfall. Radio collared adult bulls remained solitary in summer, as did cows with calves. Both adult bulls and cows formed mixed groups during the rut in September. Groups of adult bulls remained separated from mixed groups of cows and young bulls in winter. Caribou group sizes were smallest in summer (mean: 1.2) and largest in late fall after the rut (mean: 5.4). Continuous associations of radio collared caribou were longest in late winter. Seasonal ranges and movement patterns varied greatly between individuals, but seemed traditional among adult bulls. The latter made long-distance movements (>11 km) up to five times per year to distinct seasonal ranges. Most cows moved two and sometimes three times per year. The time of such movements by either sex was similar in all years. Seasonal range sizes were similar for bulls and cows, though individual cows were much more sedentary and their seasonal ranges overlapped much more. More annual range size of adult bulls was 1196 km2 and that of cows was 539 km2. Seasonal changes in relative use of habitat types seemed related to availability of food resources, snow depths, and social behaviour. Most locations (69%) were in lowland cover, predominantly black spruce (Picea mariana) muskegs. Caribou used upland deciduous cover types very little in any season. The great variability in winter habitat use reflected habitat availability within individual winter ranges.

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