Investigating Cause-Specific Mortality of Sheep to Determine the Impacts of Carnivores on Domestic Livestock

Abstract

Populations of mammalian carnivores are growing in the western United States, leading to increases in conflicts with people because carnivores prey on species that are highly valued by humans, such as domestic livestock. Accurate and updated information on the impact of predators on prey is essential when making management decisions and mitigating domestic livestock losses, yet it is difficult to detect mortalities and determine whether depredation has occurred when livestock graze on public allotments. We sought to accurately quantify the effects of predation on domestic lambs when grazing on public allotments. We placed 934 very-high-frequency (VHF) collars on lambs in seven herds that graze on public allotments at two study sites—one in northern and one in central Utah, United States. We then monitored collared lambs daily throughout the summer grazing season (April−September 2021). Of the 934 collared lambs, there were 51 (5%) lamb mortalities, for which 28 were determined to be from predation, 12 from nonpredatory causes, and 11 from unknown causes because they were too decomposed to accurately determine a cause of death. Predators responsible included coyote (Canis latrans), cougar (Puma concolor), and bobcat (Lynx rufus). Overall, few lambs die in their first 3 mo, but of those that died, we found noticeable differences in the causes and number of mortalities among herds but no differences between study sites or in predation rates within herds over time. Our results suggest lamb survival may primarily be a function of animal husbandry practices or specialization by individual predators within a grazing allotment. This study provides insight into what factors influence lamb mortality and highlights areas ranchers and managers might focus on when seeking to increase lamb survival and reduce depredation.