Capture methods in five subspecies of free-ranging bighorn sheep: an evaluation of drop-net, drive-net, chemical immobilization and the net-gun

Abstract

Six hundred thirty-four bighorn sheep (Ovis canadensis) were captured in the western United States between 1980 and 1986, using four different methods: drop-net (n = 158), drive-net (n = 249), chemical immobilization (n = 90) and net-gun (n = 137). The net-gun was found to have considerable advantages over the use of ground nets and chemical immobilization methods for capturing bighorn sheep. Evaluation of specific outcome categories for individual sheep, including normal, compromised (stress-induced), mortality from capture myopathy (CM), and accidental mortality, revealed significant differences in these rates between capture groups (P less than 0.05). The use of the net-gun resulted in the lowest proportion of compromised sheep at 11% (15/137), had no CM mortality, and resulted in a 2% (2/137) accidental mortality. The use of drop-nets resulted in 15% compromised sheep (24/158), a CM mortality rate of 2% (3/158), and an accidental mortality rate of 1% (2/158). A similar proportion of sheep were compromised with the drive-nets (16%, 39/249). This method also had the highest CM mortality rate at 3% (7/249), and an accidental mortality rate of less than 1% (2/249). Chemical immobilization resulted in the most compromised sheep at 19% (17/90), had a CM mortality rate of 2% (2/90), and caused the most accidental deaths at 6% (5/90). Drop-nets and drive-nets were comparable when combining total mortality with rates for compromised bighorn sheep, 18% and 19%, respectively (29/158 and 48/249). Chemical immobilization had the highest combined measure of risk at 27% (24/90) and net-gun lowest at 12% (17/137).(ABSTRACT TRUNCATED AT 250 WORDS)