Harvesting Bighorn Ewes: Consequences for Population Size and Trophy Ram Production

We wanted to test whether ewe hunting would cause a decline in population size or in trophy ram production, and whether a reduction in ewe density would increase the size of ram horns. Thus, we examined the consequences of a bighorn sheep (Ovis canadensis) ewe hunting season through an experimental manipulation of an isolated population in Alberta, 1972-91. The number of ewes remained stable during 9 years despite yearly removals of 12-24% of the total ewe population. The removals did not affect (P > 0.5) ewe mortality due to other causes, lamb production by adult ewes, or lamb survival. The prevalence of lactation among 2-year-old ewes was higher (P < 0.001) during the removal than afterwards. The survival of orphan and non-orphan lambs was similar (P > 0.1). The number of trophy rams in the population and the number shot by hunters were independent (P > 0.5) of ewe numbers. A threefold increase in ewe numbers over the 10-year post-removal period did not affect the number of trophy rams (P > 0.5), but rams born during the removal years had larger horns at 4 and 5 years of age than rams born in the post-removal years (P < 0.05). Our study illustrates that ewe hunting seasons have the potential to limit population increase and can increase trophy ram size. In the absence of significant predation, about 12% of the ewes could be harvested annually, based upon conservative estimates of herd size in summer. We caution against ewe removals in populations with a history of pneumonia, because in these herds, population growth following die-offs appears slow and density-independent, and hunting mortality would likely be additive.