A PRELIMINARY STUDY ON POPULATION ESTIMATION TECHNIQUE OF SIAMANG (Sympalangus syndactilus) in WAY CANGUK RESEARCH STATION, BUKIT BARISAN SELATAN NATIONAL PARK Nafila Izazaya Idrus 1 , Ryan Setiono 2, Elly L. Rustiati 3 , Fahrudin Surahmat 4 1,3 Biology Department, Faculty of Mathematics and Natural Science, University of Lampung 2,4 Wildlife Conservation Society – Indonesia Program (WCS-IP) 1 nafilaizazaya@gmail.com Abstract In collaboration with WCS-IP, visual, audio and audiovisual line transects were carried out during the month of July – August 2016 in Way Canguk Research Station, Bukit Barisan Selatan National Park to estimate population of siamang (Sympalangus syndactilus), an endangered primate in Indonesia. A 2.5 x 2.5 km 2 plot was set up in its natural habitat. Siamang were observed from 07AM- 11.PM, individuals and group were recorded using visual, audio or both audiovisual technique. Audio technique was not applicable for young or infant individuals as vocal communication was recorded for adult siamang. Visual and audiovisual were applicable, but audiovisual seems better suited technique as it gives information about location and the number of individuals. Visual technique looks more effective for smaller area. Keywords: siamang, visual, audio, population estimation, Way Canguk 1. Introduction Siamang (Sympalangus syndactylus) is known as one of endemic gibbon in Indonesia, and Malaysia. In Indonesia, siamang can be found in Sumatran tropical rainforest, including in Bukit Barisan Selatan National Park. S. syndactylus is protected by Indonesian government and was categorized in Appendix I by Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) (O’Brien et al., 2003). Since the Dutch colonial era, siamangs are protected and listed as one of the endangered primate (Director General of Forest Protection and Nature Conservation, 1995 on Bashari 1999). In 2009, International Union for Conservation of Nature (IUCN) Red List classified siamang as an endangered animal (Nijman & Geissman, 2008). Siamang remains in the conservation and protected areas. Researcher and conservationist pay a lot of attention and concern on declining number of siamang population in their natural habitat. Natural siamang habitat have been threatened by human activities such as deforestations, forest degradation (Kwatrina et al., 2013), illegal trading and poaching. Therefore, strict surveillance of siamang should be carried out in accordance with government regulation. In-situ conservation of siamang will prevent declining number of siamang population in their natural habitat. An important tool in the conservation of wildlife in nature is the information about the siamang population density (Kwatrina et al., 2013). However, The Indonesian Gibbon Workshop in 2008 stated that total population size of siamang in Sumatra is less than 200.000 (MacKinnon, 1987). More information about the actual status of this species could lead IUCN Red List to reevaluate siamang as critically endangered (Geissman, 2007). Wildlife Conservation Society (WCS) is a conservation organization from the United States. Their main goal is to save the wildlife around the world through research, environmental action, education and community approach. In Indonesia, the Wildlife Conservation Society-Indonesia Program