Comparing Release Methods of Desert Bighorn: Survival and Cause-Specific Mortality

Taylor S. Daily, Carlos E. Gonzalez, Louis A. Harveson, Warren C. Conway (Texas Tech University), and Froylan Hernandez (TPWD)

Historically, desert bighorn sheep (Ovis canadensis mexicana) were prevalent throughout the Trans-Pecos region of Texas. However, they were extirpated by the 1960s due to unregulated hunting, habitat loss, predation, and disease transmission from livestock. Restoration efforts have been successfully conducted by Texas Parks and Wildlife Department to increase population numbers of resident (i.e., animals that currently populate a region of interest) desert bighorn at Black Gap Wildlife Management Area (BGWMA) through the use of translocations.

In winter 2017-18, we radio-collared and released 30 resident (8 male, 22 female) and 70 within-state translocated desert bighorn (36 male, 34 female) to BGWMA. Of the 70 translocated, 28 (12 male, 16 female) were hard-released (i.e., translocated animals were immediately released onto the landscape) and 42 (24 male, 18 female) were soft-released (i.e., released into an enclosure temporarily before being released onto the landscape). Survival and cause-specific mortality has been monitored throughout the duration of the study. Resident desert bighorn had the greatest overall survival (S = 0.87), followed by hard-released (S = 0.78), and then soft-released individuals (S = 0.62). To date, 26 mortalities (13 male, 13 female) were recorded. Of those mortalities, 4 were residents (15%), 6 were hard-released (23%), and 16 were soft-released (62%).

Soft release is thought to be a better strategy for translocating large mammals; however, for this study it did not improve survival. This is potentially influenced by acclimation time and individual exit strategy from the soft release pen, which should be managed for future restoration efforts.

Funding sources: Texas Parks and Wildlife Department, West Texas Chapter of the Safari Club International, San Antonio Livestock Exposition, and the Borderlands Research Institute