Evaluating Survival and Demography of a Bighorn Sheep (Ovis canadensis) Population


Mike E. Jokinen, Paul F. Jones, and Darren Dorge


Biennial Symposium of the Northern Wild Sheep and Goat Council 16:138-159; 2008


Having an understanding of how animal populations interact with their natural community is fundamental to wildlife management. In 1982 and 1983, pneumonia in southwestern Alberta's Yarrow-Castle bighorn sheep population resulted in a dramatic die-off, in which the population declined from approximately 400 sheep to fewer than 150. The population recovered to approximately 200 individuals by 1995, but a decline was observed in the proportion of ewes throughout the mid-1990s. We assessed the survival and demography of this bighorn sheep population using data from 46 radio-collared ewes from 2003 to 2005. Annual adult ewe (≥2 years of age) survival estimates ranged from 0.83 ±0.07 to 0.90 ±0.06, and ewe survival did not differ significantly among years or core habitat areas, nor among seasons, or between probable causes of mortality. Annual lamb survival to ten months ranged from 0.41 ±0.01 to 0.54 ±0.02 over three years. The estimated reproductive rate among years (2003-2005) was 0.40 (95% CI: 0.29-0.55), with a recruitment (female lamb survival to 10 months) estimate that averaged 0.18 (95% CI: 0.12-0.27). Population growth rates fluctuate near 1.0, although recruitment appears low in comparison with other populations. We discuss possible factors influencing this bighorn sheep population and compare results to demographic patterns observed in other ungulate populations.