HARVESTING BIGHORN EWES - CONSEQUENCES FOR POPULATION-SIZE AND TROPHYRAM PRODUCTION

We wanted to test whether ewe hunting would cause a decline in populat ion size or in trophy ram production, and whether a reduction in ewe d ensity would increase the size of ram horns. Thus, we examined the con sequences of a bighorn sheep (Ovis canadensis) ewe hunting season thro ugh an experimental manipulation of an isolated population in Alberta, 1972-91. The number of ewes remained stable during 9 years despite ye arly removals of 12-24% of the total ewe population. The removals did not affect (P > 0.5) ewe mortality due to other causes, lamb productio n by adult ewes, or lamb survival. The prevalence of lactation among 2 -year-old ewes was higher (P < 0.001) during the removal than afterwar ds. 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 hu nters were independent (P > 0.5) of ewe numbers A threefold increase i n 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 year s 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 seaso ns have the potential to limit population increase and can increase tr ophy ram size. In the absence of significant predation, about 12% of t he ewes could be harvested annually, based upon conservative estimates of herd size in summer. We caution against ewe removals in population s with a history of pneumonia, because in these herds, population grow th following die-offs appears slow and density-independent, and huntin g mortality would likely be additive.