Lifetime Sex‐Specific Moose Mortality During an Intentional Population Reduction
Authors: Boertje Rodney D. , Frye Graham G. , Young Donald D. 
Published: November 2019

Where elevated harvest of ungulates is a priority, managers benefit by understanding how various sources of mortality affect the age and sex structure and trend of ungulate populations. Prior studies reported a long period (1997–2014) of moose (Alces alces gigas) nutritional stress from overabundance in our study area, an intentional 31% reduction in moose numbers using liberal harvests of females (2004–2012), and low bear (Ursus spp.) predation and high moose harvest densities relative to other largely roadless systems with moose, bears, and wolves (Canis lupus). In this paper, we detailed management findings after describing causes and rates of mortality from 226 female and 164 male moose radio‐collared at 9 months of age (1997–2008) and followed through life (1997–2019) and throughout the population reduction. We listened for mortality signals on radio‐collars 1–2 times/month when snow cover was complete and 2–4 times/month when snow cover was incomplete. Upon hearing a mortality signal, we investigated mortality sites usually within 24 hours via helicopter. Excluding hunter‐caused mortality, we estimated 28% annual mortality for male yearlings versus 17% for female yearlings, then low annual mortality rates (0–4%) to 84 months of age for males and 96 months of age for females, and gradually increasing annual mortality rates thereafter. Most (83%) male moose ≥24 months of age died from hunters; minor causes included wolves (8%), malnutrition or disease (5%), grizzly bears (U. arctos; 2%), and accidents (2%). Most female moose ≥24 months of age died from wolves (37%) or hunters (33%); minor causes included malnutrition or disease (15%), grizzly bears (10%), and accidents (5%). The proportion of radio‐collared females killed by hunters varied depending on numbers of permits issued to hunters; the kill rate of females ≥24 months of age was 58% during the initial 4 years of the 9‐year reduction, moderated at 29% during the final 5 years of the reduction, and was only 7% for all other study years. We attributed 32% of hunter kills to illegal harvest and unrecovered hunter kills. Hunters played a key role in the intentional population reduction by harvesting prime‐age and near prime‐age male and female moose that rarely died from other sources of mortality compared with calf, yearling, and older moose. Restricting general season hunters to primarily harvesting prime‐age and older male moose with antler spreads ≥127 cm did not appreciably reduce harvest of adult males. Male moose 2.0–5.3 years of age rarely died from non‐hunter causes and were largely harvested at older, prime ages (5.3–8.3 yr of age). Yearling moose of both sexes died primarily from wolves, with wolves selecting more for males. By using liberal harvests of female moose to reduce the population, managers improved moose nutrition and reproduction, met mandates for elevated harvests, and may have avoided a reoccurrence of a previous precipitous decline in moose numbers that was initiated by overabundance and extreme snow depths. © 2019 The Wildlife Society.