An Initial Evaluation of Factors Limiting Bighorn Sheep in the Yarrow-Castle Region of Alberta


Michael Jokinen, Darren Dorge, Carita Bergman


The Yarrow-Castle Bighorn Sheep Study was initiated in 2002 as a collaborative effort between the Alberta Conservation Association (ACA) and Fish and Wildlife Division of Alberta Sustainable Resource Development (ASRD). During the early 1980s, pneumonia had affected southwestern Alberta's Yarrow-Castle bighorn sheep population resulting in a dramatic die-off. The population declined from approximately 400 sheep to fewer than 150 over a 2-year period. The population recovered to approximately 200 individuals by 1995. During the mid 1990s, management changes were implemented in the Yarrow-Castle region to accelerate the recovery of the bighorn sheep. The management changes included: 1) imposing access restrictions into the canyons and ii) altering hunting regulations as requested by Alberta hunters in hopes of re-establishing quality trophy rams. However, despite the management changes, analysis of the 30 years of aerial population data revealed an unexplained decline in the ewe segment of the Yarrow-Castle population since the mid-1990s. The primary objective of the Yarrow-Castle Bighorn Sheep Project was to determine the factors that may be limiting overall ewe numbers. The activities complated to date include the: i) capture and collaring of 45 ewes and ii) monitoring of reproductive success and survival. This data will be used to develop a predictive, age-structured population model that will assist managers to make informed management decisions relating to this sheep population. Other project benefits include detailed monitoring of seasonal habitat use and movement corridors through the use of Global Positioning System technology, ultimately providing key areas for habitat enhancement. These data will establish the foundation for future recommendations that will assist in determining the most effective approach for management of this population. 

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