Population growth, fecundity, and survivorship in recovering populations of bighorn sheep

The single greatest obstacle to the restoration of large, healthy, populati ons of bighorn sheep (Ovis canadensis) in the western United States is epiz ootic outbreaks of bronchopneumonia that may kill 20-100% of the animals in populations. Although the species is capable of rapid initial growth rates following restoration into new habitat (lambda = 1.23-1.30 have been obser ved), these rates of increase are typical only a few years following the re lease of a population, and then most populations either decline to extirpat ion or remnant status (<30 animals) or remain at <100 individuals. We studi ed the fecundity and survivorship of three increasing, and three declining and suspected diseased, populations of bighorn sheep (the latter were subje cted to outbreaks of bronchopneumonia) located in or near several large nat ional parks in the western United States from 1991 to 1996. Titers verified both population categories were exposed to the bacteria Pasteurella haemol ytica serotypes 3; 4; and 3, 4, 10; Moraxella sp., and parainfluenza-3 and bluetongue (BT) viruses. Pregnancy rates of adult ewes were not different i n increasing or decreasing populations (pooled rate = 0.93; p = 0.57), but pregnancy rates of yearlings were lower (0.00 for decreasing vs. 0.33 for i ncreasing populations), initial production of lambs and annual recruitment of lambs was lower (0.14, decreasing vs. 0.66, p < 0.05). Adult survival wa s lower during: the first year of an epizootic, 0.62, in one population, bu t recovered to 0.85 by the second and subsequent years. Survival of adult r ams was variable in diseased populations; in two populations rams appeared to be disproportionately impacted, but in a third population rams survived better during the epizootic. In all the increasing park (unhunted) populati ons, adult ram survival (0.94 +/- 0.01) was higher than adult ewe survival (0.89 +/- 0.02) (p = 0.10), in contrast to published information from hunte d populations where ram survival was lower. Removal of about 20% of one pop ulation for restorations severely impacted one declining population. Remova ls of 12-20% appeared to be excessive and were not readily compensated for in the Canyonlands National Park desert bighorn population. Disease was a s ignificant limiting factor to restoration of bighorn sheep in the study are as; six of 11 total recovering populations we monitored closely were negati vely influenced by apparent disease at some time during our observations.