Calf mortality and population growth in the Delta caribou herd after wolf control

Document Type

Article

Publication Date

9-1-2004

Abstract

A program to control wolves (Canis lupus) in interior Alaska in 1993 and 1994 did not result in expected increases in calf survival in the Delta caribou (Rangifer tarandus) herd (DCH). Therefore, the Alaska Department of Fish and Game conducted a study to determine causes of calf mortality during 1995-1997 and monitored recruitment, mortality, and population size annually in the DCH for 6 years after wolf control ended. Despite removal of 60-62% of the autumn 1993 wolf population, wolves still killed 25% of 166 radiocollared calves between birth in mid- to late May and 30 September during 1995-1997. Although autumn calf:cow ratios in the DCH increased after wolf control, similar increases in calf:cow ratios occurred in the adjacent Denali Herd, where wolves were not controlled. Calf:cow ratios following wolf control in 1993 and 1994 were lower than ratios obtained in the same area after wolf control from 1976-1982. We identified 4 factors that contributed to continued low calf:cow ratios in the DCH following the 1993-1994 wolf control program: 1) other predators in combination (i.e., golden eagles [Aquila chrysaetos] and grizzly bears [Ursus arctos]) were the most significant mortality source for caribou calves, 2) the temporal and spatial extent for wolf removal was inadequate to effectively reduce wolf predation, 3) in 1987 the DCH shifted its main calving area, a move that may have increased predation by golden eagles and grizzly bears, and 4) natality rates and nutritional condition of caribou declined during the 5 years before wolf control coincident with a density-dependent population decline. We conclude that wolf control within the range of the DCH failed because the wolf trapping program did not remove enough wolves and was not conducted long enough to substantially reduce predation by wolves on caribou calves. In addition, wolves that lived outside the control area were responsible for about 40% of the wolf-caused mortality to collared caribou calves, and significant numbers of calves died from unknown, neonatal causes.

Publication Title

Wildlife Society Bulletin

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