Causes and Rate of Moose Mortality in the Southwest Yukon

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Abstract
We examined the ways grizzly bears (Ursus arctos), wolves (Canis lupus), black bears (Ursus americanus), and humans affected moose (Alces alces) numbers in a 6,310-km2 area in the southwest Yukon during 1983-85. We documented pregnancy rates, birth rates, and causes and rates of natural mortality for female moose. Pregnancy, birth, and twinning rates were estimated as 84%, 114 calves: 100 females, and 28%, respectively. The mean annual survival rate of adult females was 91%; most deaths occurred from May to October. Grizzly bears and wolves were the primary and secondary cause of adult moose mortality, respectively. Grizzly bears and wolves were responsible for 58 and 25% of all calf deaths, respectively, from known causes. Grizzly bears killed more moose calves than did wolves between birth and 20 June. After 20 June, wolves killed proportionately more calves than did grizzly bears. Grizzly bears, wolves, and licensed hunters accounted for 50, 26, and 9%, respectively, of all adult and calf mortalities over 1 year. We suggest that predation, primarily by grizzly bears followed by wolves, limited the growth of this moose population. This study demonstrates the relative significance of grizzly bear predation to other sources of moose mortality (natural and hunting) and the absolute significance of grizzly bear predation on a northern moose population. Moose calves should be radiocollared prior to 1 week of age to determine rates and causes of early neonatal mortality.

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