Assessing Effects of Removals for Transplanting on a High‐Elevation Bighorn Sheep Population

Abstract

Abstract: Transplanting Rocky Mountain bighorn sheep (Ovis canadensis canadensis) from established herds to initiate new populations or augment small herds is a common management practice. While transplant herds have frequently been studied, data on effects of removals on source herds are lacking. Assessment of these effects is necessary when removals are large relative to the size of the source population and when maintenance of a viable source population is a management goal. The Colorado Division of wildlife removed 53 bighorn sheep in three trapping operations during 1979–1984 from a bighorn herd estimated (in 1976) at 120 total sheep that ranges largely within Rocky Mountain National Park. The park, in cooperation with the Division of Wildlife initiated this study in 1986 to assess size and trend of the source population and to provide a basis for recommendations on future removals. We found that removals were made from a female‐juvenile subpopulation (the South Never Summer subpopulation) that used only part of the known bighhorn range. We estimated its size at 55–75 sheep and found it was increasing slowly. Recruitment levels indicated that seven years were required to replace ewes removed during one trapping operation. Bighorn ewes of this subpopulation raised lambs on an average of every other year, and maternal care of yearlings was common. Density‐independent factors such as the short growing season and severe winters experienced by bighorn sheep on this high elevation range limit their capability to respond to removals. Because female juveniles normally recruit into their mother's subpopulations and because interchange is rare among female‐juvenile subpopulations, removals may severely affect individual bighorn subpopulations. We recommend that removals be based on productivities of source herds rather than on assumed compensatory responses.