As humans increasingly exploit natural areas, wildlife populations face a growing number of threats that often result in population decline and isolation. Small, isolated populations are vulnerable to extirpation due to both genetic and demographic factors. Yet, low detectability of many imperiled species often precludes the collection of population-level data important for assessing population viability and implementing successful conservation. The eastern massasauga (Sistrurus catenatus) is a cryptic pitviper that has been extirpated throughout much of its historic range due to agricultural conversion of wetland habitat and other synergistic threats. Consequently, this species is federally listed as threatened in both the United States and Canada, and most remnant populations are believed to be small and isolated. However, most extant populations lack data on population size and long-term survival rates, making effective management, monitoring, and viability assessments difficult. To address these data deficiencies, I estimated the genetic effective population size (Ne) and census population size (Nc) for eastern massasaugas at two sites in southwest Michigan. My results revealed small Nc, with approximately 108 (95% CI = 87–165) and 148 (95% CI = 102–295) adults estimated at the study sites in Cass County and Barry County, respectively. Estimates of Ne were even smaller: approximately 29.5 (95% CI = 22.2–40.5) for Cass County and 44.2 (95% CI =29.7–73.4) for Barry County. Additionally, Ne/Nc ratios were similar across study sites. Secondly, for the Barry County population, I used mark-recapture data spanning 2008–2016 to estimate annual apparent survival rates of adults. Using these estimates and other parameter values obtained from my site and a nearby population, I modeled population viability over the next 100 years. I also performed a sensitivity analysis to assess the relative influence of model parameters on extinction risk. I estimated annual apparent survival rates of 0.79 (95% CI = 0.68–0.87) for adult males and 0.78 (95% CI = 0.68–0.86) for adult females. Results of my sensitivity analysis suggest that actions promoting high survival of adult females should be a management priority, followed by activities that facilitate high reproductive output and neonate survival
