This species is listed on CITES Appendix I and is legally protected in Viet Nam (Appendix 1b of Decree 32, 2006) and in Lao PDR (Category 1 protected species in accordance with the Wildlife Law (2007) and Decree 81/PM of 2008). In Viet Nam, the species is assessed as EN in the Viet Nam Red Data Book (2007) but has been proposed for uplisting to CR under criteria A1cd (Rawson et al. 2011). In Viet Nam, it has been recorded in six sites including two protected areas; Phong Nha-Ke Bang National Park and Bac Huong Hoa Nature Reserve and two proposed nature reserves, Khe Ve and Giang Man, with the rest of its national distribution outside of the protected area network (Rawson et al. 2011). Protected areas in Lao PDR where the species occurs include Nam Kading, Nakai Nam Theun, Hin Nam No and Phou Hin Poun NPAs, though given the uncertainty of the southerly extent of the species distribution range it may occur in additional areas.
There is currently no dedicated protection or monitoring of N. siki in any site in Viet Nam, where uncontrolled hunting and habitat loss remain the greatest threats to N. siki (Rawson et al. 2011). The species is in decline throughout its range in Viet Nam and there are no clear instances of conservation success to control declines, either directly or as part of other biodiversity initiatives. Key sites for conservation in Viet Nam are two large forest complexes, the Phong Nha-Ke Bang National Park-Truong Son State Forest Enterprise and Khe Giua State Forest Enterprise-Bac Huong Hoa Nature Reserve complexes. The most critical conservation interventions in Viet Nam are to curtail hunting of gibbons in the key sites of Phong Nha-Ke Bang National Park and Bac Huong Hoa Nature Reserve through dedicated and accountable ranger patrols (Rawson et al. 2011). Outside of the protected area network, within state forest enterprises which support N. siki, sustainable logging practices and the inclusion of biodiversity conservation in site logging plans is required (Rawson et al. 2011). Additional survey efforts in Phong Nha-Ke Bang National Park, Bac Huong Hoa Nature Reserve, Giang Man Proposed Nature Reserve and Khe Giua State Forest Enterprise may reveal these populations as higher global priorities than currently recognized (Rawson et al. 2011).
In Lao PDR, support for protected area management and protection of N. siki is ongoing in a number of protected areas including; Nam Kading NPA, supported by Wildlife Conservation Society; Nakai – Nam Theun NPA supported by Project Anoulak in collaboration with the Watershed Management and Protection Authority (WMPA); Phou Hin Poun NPA supported by IUCN; and Hin Nam No NPA supported by GIZ-Lao Program. These sites represent the highest priority populations for conservation (Rawson et al. 2011). Hunting is the main threat to N. siki in Lao PDR and therefore strict enforcement of gun control laws in key areas must remain a priority action for conservation groups. Additional efforts to control forest encroachment are also required to stop declines of N. siki populations. Eco-tourism opportunities in Lao PDR and building capacity of Lao conservationists represent investments which may have increasing impact in coming years (Rawson et al. 2011). Additional, research and field surveys throughout their global range, specifically tape recordings, genetic analysis and photographic recordings will likely help better define the distribution area of the taxon relative to N. annamensis and N. leucogenys (this is an on-going project implemented and coordinated by Project Anoulak http://conservationlaos.com). Knowledge of the species' ecology is also lacking, suggesting additional ecological research is required.
This species occurs in central Lao PDR, east of the Mekong River, and central Viet Nam (Rawson et al. 2011). The limits of distribution of this species have been significantly reduced after the the description of Nomascus annamensis to the south (Thinh et al. 2010), which is now recognized to include much of the formerly recognized range of N. siki. The species distribution limits are not finally settled, both with regards to N. annamensis in the south and N. leucogenys in the north. The northern limit of the species might be the Kading River in Laos and the Rao Nay River in Vietnam (Thinh et al. 2010a, 2010b). The southern limit of the species in Lao PDR is unknown, but the Banghiang River could be a possible natural barrier. In Viet Nam, the species occurs in parts of Ha Tinh, Quang Binh and Quang Tri Provinces (Rawson et al. 2011) with the southern limit as the Thach Han River (Thinh et al. 2010a, 2010b). It has been suggested that there may be some range overlap between N. siki and N. annamensis in Quang Tri province (N.M. Ha pers. comm.). Importantly, these areas are contiguous with forest in Lao PDR where N. siki populations may be more robust.
In Viet Nam, the species has been surveyed at some level for all major areas of occurrence since 2000, with no recorded extirpations since that time, although populations appear to be small and in continuing decline because of hunting and habitat loss (Rawson et al. 2011). Rawson et al. (2011) identified key sites for conservation including two large forest complexes, the Phong Nha-Ke Bang National Park-Truong Son State Forest Enterprise and Khe Giua State Forest Enterprise-Bac Huong Hoa Nature Reserve complexes. Surveys across the species' range in Viet Nam suggest that these areas support most of the known N. siki population in the country. Importantly, these areas are contiguous with forest in Laos where N. siki populations may be more robust. Sites that have recorded N. siki since 2000, along with minimum confirmed numbers of groups, were summarized by Rawson et al. (2011) and include: Khe Ve Proposed Nature Reserve - ≥ 7 groups; Giang Man Proposed Nature Reserve - ≥ 5 groups; Phong Nha-Ke Bang National Park - ≥ 50 groups; Truong Son State Forest Enterprise - ≥ 10 groups; Khe Giua State Forest Enterprise ≥ 4 groups; Bac Huong Hoa Nature Reserve - ≥ 23 groups. While these surveys do not represent full counts of the gibbon populations at these sites, it is clear that many populations are small and isolated.
In Lao PDR, population estimates are generally lacking, but the species is more common than in Viet Nam, and represents the majority of the global population (Duckworth et al. 1999; Duckworth 2008). In Nakai-Nam Theun NPA, population density has been estimated at 1 to 3 groups/km2 in a 120 km2 area in the central part of the NPA (Coudrat and Nanthavong 2014). In Hin Nam No NPA, Phiapalath (2009) documented a relative abundance of 0.03 groups/km of reconnaissance trails walked with an average group size of 3.8 individuals, while, in Phou Hin Poun NPA, Phiapalath et al. (2012) estimated an abundance of 0.37 groups/km and an average group size of 3.6. The species is known from Nam Kading, Nakai Nam Theun, Hin Nam No and Phou Hinpoun National Protected Areas, although given the uncertainty of its distribution range to the south this may be an incomplete list.
Hunting, in particular, is a major threat in both Viet Nam and Lao PDR where they are used both in traditional “medicine,” for food, and in the pet trade (Duckworth 2008; Duckworth et al. 1995; Geissmann et al. 2000; Rawson et al. 2011). Hunting of gibbons with guns is often opportunistic rather than specifically targeted at gibbons (Duckworth 2008; Duckworth et al. 1995; Geissmann et al. 2000; Rawson et al. 2011). Unlike in Lao PDR, the forest habitat of Nomascus siki in Viet Nam is heavily fragmented due to the human pressures of deforestation and agricultural encroachment which, when coupled with hunting, are resulting in populations becoming small, isolated and potentially unviable in many locations (Rawson et al. 2011). Hunting continues to be a significant threat to gibbons in Lao PDR, with increased access through infrastructure development opening up areas with previously relatively secure gibbon populations (Duckworth 2008). Currently, large areas of available habitat and availability of higher value species for exploitation mean that N. siki is relatively secure in Lao PDR, however it is expected that declines will continue at high rates due to lack of genuine protection activities in most areas (Duckworth 2008).
Habitat loss has negatively impacted past gibbon generations and could become a more significant threat in future years. According to Global Forest Watch statistics, Viet Nam lost approximately 15% of its forest cover in the last 15 years (one generation) and, should current rates of deforestation continue, could conceivably lose an additional 45% or more in the next 30 years (two generations). The situation in Lao PDR is similar, a 15% loss of forest cover from 2004-2018 and a potential 50% overall loss of forest cover (or greater) as we approach the year 2050.