Received: 12 December 2017 | Revised: 4 July 2018 | Accepted: 9 July 2018 DOI: 10.1002/ajp.22903 RESEARCH ARTICLE A distance sampling approach to estimate density and abundance of gibbon groups Thinh T. Vu 1 | Long M. Tran 2 | Manh D. Nguyen 1 | Dung Van Tran 1 | Paul F. Doherty Jr. 3 | Toan T. Giang 1 | Hai T. Dong 1 1 Department of Wildlife, Faculty of Forest Resource and Environment Management, Vietnam National University of Forestry, Hanoi, Vietnam 2 Forest Protection Department, Ministry of Agriculture and Rural Development, Ha Noi, Vietnam 3 Department of Fish, Wildlife, and Conservation Biology, Colorado State University, Fort Collins, Colorado Correspondence Thinh T. Vu, Faculty of Forest Resource and Environment Management, Department of Wildlife, Vietnam Forestry University, Xuan Mai, Chuong My, Hanoi 100000, Vietnam. Email: vutienthinh@hotmail.com Funding information Vietnam National Foundation for Science and Technology Development (NAFOSTED), Grant number: 106-NN.06-2015.37 All 18 species of gibbons are considered threatened with extinction and listed on the IUCN Red List of Threatened Species. Because gibbons (Hylobatidae) are one of the most threatened primate families, a great need exists to determine and monitor their status effectively. To meet this need, we employed distance sampling methods to estimate the density and number of gibbon groups. We focused on southern yellow-cheeked crested gibbon in the Nam Cat Tien sector of Cat Tien National Park, Vietnam, from July to October, 2016. We used an auditory point count method at 48 listening posts to detect gibbon groups. We estimated our detection probability of calling groups of gibbons within 1,200 m of a listening post was 0.63 (95% CI: 0.54–0.74). In addition, we estimated the daily calling probability of a gibbon group to be 0.46 (95% CI: 0.33–0.59) and interpreted this as the probability that a group was available to be detected. We adjusted our group density and abundance estimates by both detection and availability probabilities. Ultimately we estimated 325 (95% CI: 232–455) gibbon groups in the Nam Cat Tien sector. Our results suggest that Cat Tien National Park contains one of the largest populations of southern yellow-cheeked crested gibbon in Vietnam. Our methods are one way of avoiding underestimation of gibbon group density and abundance by incorporating uncertainty in gibbon group availability and detection. KEYWORDS distance sampling, gibbon, hylobatidae, Nomascus, yellow-cheeked crested gibbon 1 | INTRODUCTION All 18 species of gibbons are considered threatened with extinction and recognized on the IUCN Red List of Threatened Species, with four species considered Critically Endangered and one listed as Vulnerable (IUCN, 2017). Because gibbons (Hylobatidae) are one of the most threatened families of primate, we need to determine and monitor their status so that conservation interventions can be effective. However, methods used for surveying gibbons have not always employed the best ways of minimizing bias caused by low “detection,” “availability” or both (as defined below). Resulting estimates might not be accurate for population monitoring and conservation purposes. Because of their loud and long song bouts, gibbons are usually surveyed by using auditory point count methods (Brockelman & Ali, 1987). However, gibbon groups do not call daily; therefore, several (3–5) days are needed to detect most gibbon groups (Brockelman & Ali, 1987; Brockelman & Srikosamatara, 1993) with an important assumption that all gibbon groups in the listening area emit calls in that period. An important advance in estimating gibbon group density and abundance is the application of a probability of calling adjustment factor (Jiang, Luo, & Zhao, 2006), which requires estimating the daily calling probability. The calling probability has been estimated with two methods (Vu & Rawson, 2011): using long-term observation of known groups (Brockelman & Srikosamatara, 1993; O’Brien, Kinnaird, Nurcahyo, Iqbal, & Rusmanto, Am J Primatol. 2018;e22903. wileyonlinelibrary.com/journal/ajp © 2018 Wiley Periodicals, Inc. | 1 of 7 https://doi.org/10.1002/ajp.22903