ESTIMATING SUSTAINED YIELDS FOR MOOSE IN CENTRAL BRITISH COLUMBIA USING A PREDATOR-PREY MODEL

Abstract

One of the fundamental principles of wildlife harvesting is that it must result in a sustained yield (SY), a harvest that can be taken year after year without jeopardizing future harvests. Predator-prey models are rarely incorporated into estimates of SYs for moose, despite predation of moose by wolf (Canis lupus), grizzly bear (Ursus arctos), and black bear (U. americanus) throughout much of western North America. A simple predator-prey model was parameterized from a stable moose-wolf-bear system in central British Columbia during 1987–1998. Modelled moose, wolf, and harvest parameters compared favourably with observed parameters when the annual rate of wolf removal (human-caused wolf mortality) was 31%. SY curves were modelled by incrementally increasing wolf removal rates from 0 to 40% while maintaining selective moose harvests of 16% bulls, 2% cows and 9% calves. SYs displayed an S-shape curve with wolf removal rates, a hook-shape curve with wolf densities, and were linearly related to moose density. Optimal harvests included a moderate harvest of bulls (16–21%), a nil-to-very low harvest of cows (0–0.2%), and moderate-to-high harvests of calves (15–43%) when wolf removal rates were ≥ 20%. Higher cow harvest rates (2%) could be accommodated without substantially lowering SYs if calf harvest rates were reduced. Optimal harvest rates did not improve yields over bull-only hunting when wolf removal rates were 0–10% and management constraints were placed on adult sex ratios. This study supports previous findings that the optimal harvest strategy for moose should primarily target bulls and calves, whereas cows should be harvested minimally. However, for low-density, predator-limited moose populations, bull-only harvests may provide equivalent yields while maintaining higher moose and wolf densities.