Document Type

Thesis - Open Access

Award Date

2023

Degree Name

Master of Science (MS)

Department / School

Natural Resource Management

First Advisor

Jonathan Jenks

Abstract

Reintroduced bighorn sheep (Ovis canadensis) in Nebraska faced a devastating pneumonia epizootic resulting in variable ewe reproductive success and poor lamb recruitment in many of the herds. From 2015-2017, no lamb recruitment was observed in the northern Pine Ridge subpopulation and as many as 60% of adults died. This was in stark contrast to the healthier Wildcat Hills herds to the south, where lamb recruitment was estimated to be between 50-70% during the same years. From 2018-2020, we radio-collared and recaptured 76 adult female bighorn sheep from 2 of 3 Pine Ridge herds and 1 of 2 Wildcat Hills herds. Captured sheep were aged, fitted with a GPS collar, tested for pathogens, and checked for pregnancy. Any ewes from the Pine Ridge that tested positive for a bacterium associated with pneumonia, Mycoplasma ovipneumoniae, at two consecutive annual captures were removed with the objective of eliminating the pathogen and testing the efficacy of chronic carrier removal as a potential management option for increased lamb survival. We designated the sheep within the Pine Ridge once removal began as the treatment cohort, and held both the sheep within the Wildcat Hills and those in the year prior to removals within the Pine Ridge as the control group. We evaluated the survival rates of adults (n=76) and neonate lambs (n=138) between the 3 herds and before and after chronic carrier removal. We also compared cause-specific mortality of adults (n=23) between the 2 subpopulations and treatment cohort, and we examined the cause-specific mortality of lambs (n=81) between 3 herds and treatment cohorts. We established baseline survival rates for both ewes and lambs and determined that predation was a significantly more likely cause of mortality than pneumonia throughout all herds, age class, and treatment cohort. We also found that chronic carrier removals did not eliminate M. ovipneumoniae or significantly affect lamb survival or the probability of mortality due to pneumonia for bighorn sheep in the 2 treated Pine Ridge herds. Throughout the course of the study, we observed an unexpectedly high rate of capture-induced abandonment, and we analyzed the fate of 97 collared lambs to further explore factors associated with abandonment. We examined 10 different predictors that may have had an effect on a dam’s likelihood to abandon her lamb and found that for a given year of the study, lambs born later in the season may have an increased risk for capture-induced abandonment, and lambs born to mothers with a history of M. ovipneumoniae infection were less likely to be abandoned, as were lambs born in the latter years of our study. We also found that individual variation was important as well and some ewes may be consistently prone to abandoning their lambs following capture; the probability of a ewe abandoning a lamb once she has already done so is 67±10%. Our findings provide insight that may help mitigate capture-induced abandonment risk.

Library of Congress Subject Headings

Bighorn sheep -- Mortality -- Nebraska.
Bighorn sheep -- Reintroduction -- Nebraska.

Publisher

South Dakota State University

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Rights Statement

In Copyright