Bighorn Sheep Survival and Demography in the Yarrow-Castle Region of Alberta, Canada


Michael Jokinen, Darren Dorge and Paul Jones


During the early 1980s, pneumonia infected southwestern Alberta's Yarrow-Castle bighorn sheep population resulting in a dramatic die-off in which the population declined over a two-year period from approximately 400 sheep to fewer than 150. The population recovered to approximately 200 individuals by 1995. A general decline in the number of bighorn ewes through the mid-1990s observed through biannual aerial surveys has slowly begun to stabilize more recently.

The Yarrow-Castle Bighorn Sheep Study was initiated in 2002 as a collaborative effort between the Alberta Conservation Association (ACA) and Alberta Fish and Wildlife Division of Alberta Sustainable Resource Development (ASRD) to collect demographic data on the Yarrow-Castle bighorn sheep population. Prior to this demographic study, the population had never been examined beyond regular aerial population surveys, a winter range assessment, and an impact assessment for industrial developments. This report provides bighorn sheep population demographic results for the Yarrow-Castle area using data from 46 radio-marked ewes over a three year period (December 2002 to April 2006).

Annual adult ewe survival estimates (those females ≥ 2 y) during 2003, 2004, and 2005 were 0.83 (95% CI = 0.70 - 0.97), 0.90 (95% CI = 0.79 - 1.00), and 0.89 (95% CI = 0.77 - 1.00), respectively. There was no significant difference in adult ewe survival among years (2003 to 2005), among seasons (winter, summer and fall), among core habitat areas (60% fixed kernel polygon), or between mortality types (predator vs. nonpredator). Annual lamb survival averaged 45% (male and female). Marked adult ewes produced lambs on an average of every 1.3 y and those lambs survived to ten months every 1.7 y. Estimated annual reproductive rates (female lambs produced per adult ewe) were 0.47 (95% CI = 0.27 - 0.82), 0.32 (95% CI = 0.19 - 0.56), and 0.44 (95% CI = 0.24 - 0.81) for 2003, 2004, and 2005, respectively. The estimated reproductive rate over all years was 0.40 (95% CI = 0.29 - 0.55). Yarrow-Castle recruitment averaged 0.18 (95% CI = 0.12 - 0.27). Yarrow-Castle adult ewe survival, lamb survival and reproductive rates are considered low and recruitment rates matched those of declining bighorn populations.

Population growth based on Leslie matrices indicate a female population increasing at approximately 5% (λ 1.051) per year, but such an increase was not observed in aerial census counts between 2003 and 2005. Incorporating stochasticity into population projections depicts a bighorn population that is fluctuating around 1.0 (slight decreases and increases over time).

We compare these results to demographic patterns observed in other ungulate populations, discuss possible factors influencing this bighorn sheep population and propose potential methods on how to effectively manage the Yarrow-Castle population into the future based on these findings.

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