MOUNTAIN GOAT (OREAMNOS AMERICANUS) POPULATION RECOVERY AT POST-ERUPTION MOUNT ST. HELENS (LAWETLAT'ɬ A), WASHINGTON

Abstract The powerful volcanic eruption of Mount St. Helens in 1980 is commonly reported to have killed large numbers of wildlife species, including near-extirpation of the local Mountain Goat (Oreamnos americanus) population. In the decade after 2000 we increasingly received reports of goat sightings around Mount St. Helens, including nannies with kids in 2007. By 2011, we estimated 15 to 25 goats were present on the landscape. In 2014, we initiated 2 lines of research. The first was a review of historic records to document pre-eruption goat populations. The second was the establishment of an annual minimum population-size survey using simple ground-count methods. In 2015, we revised our methods, and our results between 2015 and 2020 show a general growth trend of minimum population size from 152 goats in 2015 to 200 in 2020, with a peak of 240 goats observed in 2019. In 2017 and 2020, we paired ground counts with aerial surveys (with sighting correction) to better estimate population size, and observed 246 goats (245.9, 90% CI = 232.3–260.4) in 2017 and 252 (251.7, 90% CI = 233.2–270.1) in 2020. Our 2020 aerial estimate of ∼252 goats is a population 5 times (and possibly 10 times) the size of the 20–50 Mountain Goats we speculate were present in the same landscape prior to the 1980 eruption landscape, and reveals a substantial local recovery of the species.