Management Responses to Pneumonia Outbreaks in three Washington State Bighorn Herds: Lessons Learned and Questions yet Unanswered

Pneumonia induced die-offs and subsequent periods of low lamb survival are the greatest impediments to restoration of historic bighorn sheep abundance in North America, and developing effective responses to disease outbreaks in bighorn sheep has been frustrating for wildlife managers. A difficulty in understanding the phenomena is that no 2 situations seem identical. Thus, careful documentation of individual events is needed to understand common patterns and processes. We provide an update on 3 bighorn herds in Washington State that recently experienced pneumonia-related declines (Yakima Canyon, Asotin, and Tieton); management responses and outcomes differed in each case. The Yakima Canyon herd experienced an all-age die-off during winter 2009-2010, during which we culled animals showing signs of respiratory disease. In 2011 and 2012, the herd briefly rebounded, but then suffered 2 consecutive years of recruitment less than 10 lambs:100 ewes, accompanied by pneumonia (cohorts born in 2013- 2014). The Yakima Canyon herd is characterized by considerable spatial structuring that was reflected in intra-herd patterns of disease. The Asotin herd suffered an all-age die-off in 2012, and we took no management actions during the outbreak. Similar to Yakima Canyon, after 1 year of high lamb mortality, survival returned to normal and we failed to detect evidence of disease. In 2015 we removed 3 ewes (~10% of survivors) that tested positive for Mycoplasma ovipneumoniae and recruitment has been 31- 54 lambs:100 ewes with no pneumonia or M. ovipneumoniae detected through the 2015 cohort. The Tieton herd suffered a catastrophic all-age die-off in winter 2013. Due to its proximity to the adjacent Cleman Mountain herd, we lethally removed every Tieton individual that did not die of pneumonia during the outbreak. To date, the Cleman Mountain herd has shown no evidence of pneumonia. While we do not assert that our management response was the only reason for the differing outcomes, we hope that follow-up monitoring and replication will help us identify which actions are effective at controlling the impacts of disease in bighorn sheep.

KEY WORDS: bighorn sheep, culling, disease, mortality, Mycoplasma ovipneumoniae, pneumonia, Washington