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Research Article
First population assessment of the Asian Water Dragon (Physignathus cocincinus Cuvier, 1829) in Thua Thien Hue Province, Vietnam
expand article infoTruong Quang Nguyen§, Hai Ngoc Ngo|, Cuong The Pham§, Hoang Nguyen Van, Chung Dac Ngo, Mona van Schingen#¤, Thomas Ziegler«¤
‡ Institute of Ecology and Biological Resources, Vietnam Academy of Science and Technology, Hanoi, Vietnam
§ Graduate University of Science and Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam
| Vietnam National Museum of Nature, Vietnam Academy of Science and Technology, Hanoi, Vietnam
¶ Hue University, Thua Thien Hue, Vietnam
# Federal Agency for Nature Conservation, Bonn, Germany
¤ Cologne Zoo, Cologne, Germany
« University of Cologne, Cologne, Germany
Open Access

Abstract

The Asian Water Dragon, Physignathus cocincinus Cuvier, 1829, was originally described from southern Vietnam. Wild populations of this species are strongly affected by harvesting, particularly in central Vietnam, while potential negative impacts are not yet foreseeable. This study provides the first population size estimation using a capture-recapture approach and an evaluation of threats to and trade levels in P. cocincinus in Thua Thien Hue Province, central Vietnam. Field surveys were conducted at 14 sites of known occurrences of the species in Thua Thien Hue during April and June of 2016 and 2017. The estimated population size of the species amongst these sites ranged from 232 to 250 individuals. The mean abundance varied from 1.98–2.64 individuals/100 m along inhabited streams. This study revealed harvest activities for local food consumption and to supply the pet trade. Accordingly, imports of several thousand living individuals per year from Vietnam into the EU were documented. It was further demonstrated that the level of negative impacts on different sub-populations is in accordance with respective abundances and the incidence of adults. This finding indicates that rising harvest levels in concert with decreasing habitat quality, might seriously affect the wild population of P. cocincinus in the future. Thus, the authors herein emphasise the necessity for increased conservation measures and for monitoring of the species as well as its natural habitat in order to warrant a sustainable use of the species in the future.

Keywords

Population size, capture-recapture method, trade, conservation, risk assessment

Introduction

The Asian Water Dragon, Physignathus cocincinus, was originally described by Cuvier, 1829 from southern Vietnam, subsequently recorded from southern China, Myanmar, Thailand, Laos and Cambodia and reported as introduced in Taiwan (Smith 1935, Taylor 1963, Cox et al. 1998, Nabhitabhata et al. 2000, Stuart and Emmett 2006, Stuart et al. 2006, Grismer et al. 2008 a, b, Suzuki et al. 2015, Uetz et al. 2017). Typical habitats of the species consist of undisturbed rocky streams within evergreen lowland rainforest, but sometimes the species can also be found in densely vegetated streams nearby villages and in agricultural areas (Ziegler 2002, Nguyen et al. 2009). In Vietnam, the Asian Water Dragon is considered as a species with a wide geographic range. However, habitat degradation represents an emerging threat and there is rising evidence for strong local use of the species. In addition, its attractive appearance, as well as its semi-aquatic lifestyle, made the species a highly desirable target species in the international pet trade (UNEP-WCMC 2017). As a consequence, the species was recently listed in the Vietnam Red Data Book (2007) as Vulnerable. Furthermore, the species was listed in Appendix D on the “Council Regulation (EC) No. 338/97 on the protection of species of wild fauna and flora by regulating trade therein” in order to identify potential unsustainable levels of trade in the species in good time. However, basic knowledge on the current population status of P. cocincinus and anthropogenic impacts on the species is still lacking. Population size estimations provide essential baseline information for the assessment of the conservation status of species and populations and are crucial for wildlife management strategies (Reed et al. 2003, Traill et al. 2007, Ngo et al. 2016). The present study provides the first assessment of the population size of P. cocincinus in Vietnam, based on surveys conducted in Thua Thien Hue Province, as well as an evaluation of threats in order to assess its conservation status and as a basis for timely development of adequate conservation management to warrant both the persistence and the sustainable use of the species.

Methods

Study sites

Survey sites were selected based on previous reports on known occurrences of P. cocincinus in Phong Dien Nature Reserve (NR) in Phong Dien District, Sao La NR in A Luoi District and Bach Ma National Park (NP) in Nam Dong District, Thua Thien Hue Province, Vietnam (Nguyen et al. 2009) (Figure 1). The vegetation throughout the study area is dominated by evergreen broadleaf forest, intermixed with bamboo forest. The region is characterised by a tropical climate and seasonal monsoons, with an annual average temperature of 24.9 ± 0.51 °C, an annual average precipitation of 3243.2 ± 780.6 mm and an annual average humidity of 86.8 ± 1.05% (General Statistics Office of Vietnam 2013, Nguyen et al. 2013).

Figure 1.

Map of Thua Thien Hue Province in central Vietnam showing the survey sites: 1 Phong Dien NR in Phong Dien District; (2) Sao La NR in A Luoi District; and 3 Bach Ma NP in Nam Dong District.

Field surveys

Field surveys were conducted in April and June 2016 and 2017, respectively, during the active season of the Asian Water Dragon. During previous surveys, P. cocincinus was found to be a riparian species, being exclusively recorded in the immediate proximity to or above the water at the stream banks. Thus, the riverine vegetation of selected rocky streams was sampled from downstream to upstream by walking through the water and intensively investigating both sides of the streams equally. The surveyed streams were characterised by a width of about 1–12 m, depths of up to 200 cm and dense vegetation along and above the stream. Since P. cocincinus is a diurnal species, night excursions were conducted between 1900h and 2400h, when animals were expected to rest on branches above the water (Fig. 2A).

Figure 2.

A Habitat of Physignathus cocincinus in Nam Dong District, Thua Thien Hue Province B Resting perch in front of waterfall C Adult male D Adult female. Photos: Hai Ngo.

In total, 14 sections of 14 different streams with lengths varying from 320 to 1400 m (see Table 1) were surveyed. Thereof, two streams were in Phong Dien NR (Phong Dien District), at elevations between 43 and 90 m a.s.l, eight streams were within Sao La NR (A Luoi District) at elevations between 176 and 820 m a.s.l and four streams were within Bach Ma NP (Nam Dong District) at elevations between 105 and 269 m a.s.l. Each stream was repeatedly surveyed in intervals of several days. At each point of investigation, the team consisted of 2–4 persons. Coordinates and elevations for each captured individual were recorded with a GPS Garmin 64. These streams were selected based on information about the presence of P. cocincinus obtained during previous surveys and interviews with local people. Lengths of stream sections were dependent on habitat suitability and accessibility.

Survey sites in Thua Thien Hue Province, central Vietnam.

Stream District Physignathus cocincinus Elevation (m. asl) Length (m) Habitat type
1 Phong Dien Present 44–90 1,160 Primary forest
2 Phong Dien Present 43–75 1,300 Primary forest
3 A Luoi Present 706–780 320 Secondary forest
4 A Luoi Present 720–820 500 Secondary forest
5 A Luoi Present 623–770 420 Secondary forest
6 A Luoi Present 176 – 250 950 Secondary forest
7 A Luoi Present 179–214 700 Secondary forest
8 Nam Dong Present 129–179 1,400 Primary forest
9 Nam Dong Present 173–269 1,300 Primary forest
10 Nam Dong Present 111–145 1,000 Secondary forest
11 Nam Dong Present 105–129 460 Secondary forest
12 A Luoi Absent 550-740 1,000 Secondary forest
13 A Luoi Absent 650-875 450 Secondary forest
14 A Luoi Absent 690 Secondary forest

All encountered individuals were captured, individually marked with a permanent pen (Edding Eraser) and released at the same spot after taking measurements (Fig. 2B, C). This marking technique has the advantage of being non-invasive, not affecting the animals, inexpensive and enables the short-term identification of animals, while marking lasts until the next shedding of the animals (for details see Ngo et al. 2016). Each spot was marked by a label made of waterproof paper to identify exact positions of captured and recaptured animals. It was observed that animals occupied previous perches after marking, indicating that capturing does not affect the behaviour and perch choice of animals. Generally, individuals of all ages and sexes were found in a likewise manner. To estimate population sizes, a non-invasive “Capture-recapture Method” was applied using both the “Lincoln-Peterson Index” in case of one-time mark and recapture event and the “Schnabel Index” in case of several mark and recapture events (e.g. Caughley 1980; Schlüpmann and Kupfer 2009; Smith and Smith 2009). The Petersen-Lincoln estimation requires several assumptions, namely a closed population, the same capture probability for all animals and that markings neither affect animals nor will be lost between sampling. Surveys were repeated along each site within the timeframe of only three days, as migration events were not expected to occur due to the sedentary behaviour of P. cocincinus. Based on the reasons mentioned above, it was also expected that the assumptions on markings and capture probability would be fulfilled. Estimated population sizes only refer to survey sites and might not be representative for the entire population of the species in Central Vietnam.

To assess the population structure of P. cocincinus, lizards were categorised into three age classes based on the snout-vent length (adult with SVL > 140 mm, sub-adult with 100 mm < SVL ≤ 140 mm and juvenile with SVL ≤ 100 mm). As sexual dimorphism is only expressed in adult P. cocincinus (e.g. presence of well-developed nuchal and dorsal crests in adult males; see Fig. 2C, D), accurate sex determination of living animals thus could only be performed in mature individuals. To test for differences in population structure and frequency of snout-vent length between April and June, 2016 and 2017, a c2 test with α = 0.05 was applied. Statistical analysis was carried out with the SPSS software, version 16.0 (SPSS Inc., Chicago).

Threat evaluation

In order to assess levels of harvesting and use of the Asian Water Dragon in Thua Thien Hue Province, 21 hunting groups were interviewed between February and October 2016, including six groups from Phong Dien District, seven groups from A Luoi District and eight groups from Nam Dong District. Each group contained two or three lizard hunters. Some rapid surveys were also conducted in local markets and restaurants in district towns of Phong Dien, A Luoi, Nam Dong and in Hue City to obtain evidence of local trade and consumption of lizards. Single impacts on local sub-populations were ranked according to severity as “low” defined as being observed only once or less, “medium” defined as being observed several times or “high” defined as being recorded frequently. Furthermore, several internet platforms were screened to obtain an overview of the availability, demand, prices and evidence of trade in the species. Since the species is listed on Annex D of the Council Regulation (EC) No 338/97, which requires an import notification at EU border customs, retrospective data were obtained for the recorded imports of P. cocincinus from Vietnam into the EU from 2010 to 2016 from the UNEP-WCMC CITES trade database (UNEP-WCMC 2017). Taking into account that CITES trade database records might be incomplete (D’Cruze and Macdonald 2017) and considering this dataset only represents imports into the EU and does not encompass imports into other states, this data however provides a rough impression on current trade levels.

Results

Population status

During the present study, P. cocincinus was found along 11 of 14 investigated sites within three districts of Thua Thien Hue Province. In 2016, the authors only surveyed A Luoi District and recorded a similar number of individuals of P. cocincinus in April and June along all five sites, counting 97 and 95 individuals, respectively. In 2017, a total of 167 animals (thereof 31 adults) were captured in April in Phong Dien and Nam Dong districts and 172 individuals were observed in June in all three districts, of which 28 were adults. Higher fluctuations in population sizes were observed between the two consecutive surveys in April and June 2017 in Phong Dien and Nam Dong than those in the year before in A Luoi. The estimated total population size at the survey area was about 250 animals in April 2017. Even though two additional sites in A Luoi District (viz. five sites in total) were included in the estimation, the estimated population size decreased to 233 individuals in June 2017 (Table 2). With respect to the sub-population in Phong Dien District, site 2, the estimated population size was distinctly higher in April than in June 2017 (124 vs. 87 individuals, respectively). In Nam Dong District, transect 9, the number of observed individuals also significantly decreased from 25 individuals in April to 2 individuals in June and only 3 individuals were recorded along transect 11 in June 2016. The authors failed to observe any adult specimens of P. cocincinus in the transect 8 in Nam Dong District in June, whereas the species had been earlier recorded in April 2017 (Table 2).

Summary of observed individuals as well as densities and population size estimates (N^) of Physignathus cocincinus in Thua Thien Hue Province, central Vietnam in April and June, 2016 and 2017. D: Density; ind: individuals; Asterisks indicate missing data.

District Phong Dien A Luoi Nam Dong Total
Stream 1 2 3 4 5 6 7 8 9 10 11
April-2016
Total (observed) * * 4 4 6 25 21 * * * * 60
Mature (obs.) * * 1 4 0 7 2 * * * * 14
D[ind/100 m of stream] * * 1.25 0.8 1.43 2.63 3.0 * * * * 1.98
N^ * * 6 6 12 35 38 * * * * 97
June-2016
Total (observed) * * 4 5 5 22 22 * * * * 58
Mature (obs.) * * 3 2 1 5 0 * * * * 11
D[ind/100 m of stream] * * 1.25 1.0 1.19 2.32 3.14 * * * * 1.98
N^ * * 5 9 6 42 33 * * * * 95
April-2017
Total (observed) 13 86 * * * * * 26 25 17 * 167
Mature (obs.) 0 18 * * * * * 5 6 0 * 29
D[ind/100 m of stream] 1.12 6.62 * * * * * 1.86 1.92 1.7 * 2.64
N^ * 124 * * * * * 74 52 * * 250
June-2017
Total (observed) 33 67 * * * 25 22 * 2 20 3 172
Mature (obs.) 8 17 * * * 2 1 * 0 0 0 28
D[ind/100 m of stream] 3.84 5.15 * * * 2.63 3.14 * 0.15 2.0 0.65 2.51
N^ 56 87 * * * 35 24 * * 30 * 232

In total, most individuals were encountered in Phong Dien District, where the highest incidence of adult individuals was also recorded compared to the other two sub-populations. The mean density of P. cocincinus along suitable stream transects was estimated to be around 25–26 individuals per km of transect. By comparing different habitat sites, the highest abundance of P. cocincinus was found in April 2017 at site 2 in Phong Dien District (about 6.6 individuals per 100 m stream length). In A Luoi District, being situated at elevations of 623–820 m, densities ranged from 0.8 to 1.4 individuals per 100 m stream length along sites 3, 4 and 5. The authors failed to observe any lizards along the other three sites, namely 12, 13 and 14, in April and June 2017. In comparison, densities along transects of lower elevations (from 49–269 m), even though in a slightly disturbed habitat, were about or higher than two individuals per 100 m stream length during at least two surveys (sites 6 and 7).

Comparing the data achieved in the different months, the investigated population of P. cocincinus in Thua Thien Hue Province consisted mainly of juveniles (139 individuals) in April, followed by sub-adults, adult males and adult females (60 %, 19 %, 11 %, 10%, respectively; Fig. 3A). In June, the population structure significantly differed (c2 = 50.401; df = 3; a < 0.001), with sub-adults (113 individuals) constituting the major proportion, followed by juveniles, adult females and adult males (50 %, 33 %, 10 %, 7 %, respectively; see Fig. 3A). Frequency histograms of SVL (Min–Max = 42.4–260 mm) showed a tendency for a monthly variation in the presence of age classes of P. cocincinus from Thua Thien Hue Province (c2 = 77.87; df = 6; a < 0.001). Individuals with the shortest SVL (40–60 mm) were mostly found in June, while juveniles with higher SVL (60–100 mm) and adults accounted for a larger number in April. Sub-adults with SVL larger than 100 mm were observed more frequently in June (Fig. 3B).

Figure 3.

Population structure of Physignathus cocincinus from Thua Thien Hue Province, central Vietnam (April vs. June, respectively); B. Frequency histogram of snout-vent length of P. cocincinus for the months April and June, 2016 and 2017.

Threat evaluation

According to interviews with local hunters and the authors’ own observations, wild P. cocincinus populations are affected by negative anthropogenic impacts in all investigated districts (Table 3). In two of three districts, namely A Luoi and Nam Dong, the total impact is even considered to be high. Harvesting of the Asian Water Dragon has been frequently recorded by the team in all surveyed districts of Thua Thien Hue Province. Interviews with 21 local hunter groups in the remote villages within the species distribution range revealed that adult lizards are heavily collected by hand and/or with traps (Fig. 4A) from February to October, while the number of captured animals is usually highest in May and June. With respect to quantities, a total of about 1,000 kilograms of animals collected by hunters was recorded from the three study sites in Thua Thien Hue Province in 2016, which corresponds to more than 2,000 individuals considering a mean mass of 450 g. Eggs of P. cocincinus were also collected and soaked in rice wine for traditional medicine (Fig. 4D). It was regularly recorded that captured animals were sold in local markets and then finally were transferred to local restaurants as food (Figs 4B and C). The price of living animals was approximately 250,000 Vietnam dong (about $12) per kilogram in remote districts and fetching up to 450,000 Vietnam dong (about $20) per kilogram in restaurants in Hue City. In addition, the Asian Water Dragon became very popular in local pet markets due to its striking appearance. P. cocincinus was frequently observed being offered for sale for prices ranging from 150,000 (about $7) to 450,000 Vietnam dong (about $20) / individual, depending on size and/or colouration in local pet shops and on several Vietnamese internet platforms, such as Facebook from Hanoi and Ho Chi Minh City.

Impacts on Physignathus cocincinus at study sites based on interview data and own observations.

District Trapping/ hand collection Use for traditional medicine Consumption in restaurants Incidence on local markets Habitat degradation Total impact
Phong Dien High Low Medium High Low Medium
A Luoi High Medium High High High
Nam Dong High Low High High High High
Figure 4.

Threats to Physignathus cocincinus in Thua Thien Hue Province: A A trapped individual in Nam Dong District B Specimens collected for food in Hue City C The Asian Water Dragon served as food in a restaurant in Hue City D Eggs of P. cocincinus preserved in alcohol in Nam Dong District.

With respect to the international trade in the species, a total of 43,100 specimens have been officially exported by Vietnam into the EU between 2010 and 2016 according to the UNEP-WCMC trade database. Thereof, most specimens had an unknown origin and some were stated to be “wild caught”, while the purpose was “for commercial use” in any case. Germany was the country with by far the most imports. It can be assumed that the majority of exported specimens with unknown origin are actually wild individuals, since there is no knowledge about breeding facilities. According to interviews with locals, there have been some people keeping and rearing P. cocincinus to supply local restaurants for a short period. However, these facilities were recently closed, because it is allegedly more convenient to collect wild animals than to spend time and efforts in breeding them, as long as animals can be found in the wild.

It was found that habitat loss and habitat alterations, such as road building and illegal timber logging (Figs 5A and B), are further critical threats to P. cocincinus in Thua Thien Hue Province. For the building of highways from Nam Dong to A Luoi District, huge parts of the forest have been cleared, which resulted in the fragmentation of forest streams, thus affecting stream-adapted species like P. cocincinus. The construction of roads furthermore has facilitated illegal logging, because accessibility of forested regions has significantly increased and now almost every area within Bach Ma NP and Saola NR can be easily reached. Phong Dien District is the only one of the three investigated sites, which is hardly affected by habitat degradation.

Figure 5.

Threats to Physignathus cocincinus in Thua Thien Hue Province: A. Illegal timber logging in Nam Dong District; B. Building a highway from Nam Dong to A Luoi District.

Discussion

In order to manage the wild populations of the Asian Water Dragon properly and to implement conservation measures accordingly, the assessment of its population status, abundance and distribution is important. In total, the estimated population size probably does not encompass all occurrences of P. cocincinus in Thua Thien Hue Province, however due to the strong association of the species with small freshwater streams, which only represent small areas of the Province, it can be assumed that the total population is not disproportionately higher. A detailed further assessment on the availability of suitable habitats would be helpful.

Generally, P. cocincinus was found in monthly densities of 2.6 and 2.5 individuals per 100 m in April and June, 2017, respectively. However, three stream transects (3, 4 and 5) in A Luoi District, located at higher elevations (623–820 m), revealed a constantly lower density of P. cocincinus than found in other streams, although the habitat was not disturbed. It seems that not only hunting activities, but also elevation and habitat quality of each stream, might influence the abundance of the species. Recently, van Schingen et al. (2015) explained the abundance of the Crocodile Lizard (Shinisaurus crocodilurus), another semi-aquatic lizard occurring in northern Vietnam, to mainly depend on the stream properties and water quality that varied along elevation gradients. Thus, these hypotheses explaining abundances of P. cocincinus need to be tested in further studies.

Regarding the sex ratio of the species, investigations in 2016 and 2017 showed that the number of adult males was slightly higher than that of adult females in April. However, adult females were observed more frequently in June, which might be an indication for targeted harvest. Most of the local hunters, who have been interviewed, preferred collecting adult males of P. cocincinus, because of their bigger size compared to adult females, an important aspect in terms of food consumption. There was a significant difference in age structure in P. cocincinus between April and June, with juveniles accounting for the dominant percentage in April and sub-adults being predominant in June. This trend can be explained by the fact that juveniles of P. cocincinus reach the sub-adult stage relatively fast (after only two months), indicating that the survival rate is relatively high in the species. Overall estimates of the population size of P. cocincinus in the survey areas within Thua Thien Hue Province, central Vietnam, revealed that the population remained relatively stable between April and June with 250 to 232 individuals respectively in 2017. This trend was constantly observable at each survey site, except for the sub-population in Nam Dong District with a distinct decline in encountered individuals from April to June. This might be explained by the fact that the population in Nam Dong District is heavily affected by hunting activities. Reed et al. (2003) suggested that a population size of at least 3,000–7,000 mature individuals is required to maintain stable populations over a long time period. Thus, an ongoing decline in the number of mature individuals could negatively impact the genetic diversity and reproductive capacity of P. cocincinus in Thua Thien Hue Province. The compilation of impacts on P. cocincinus at different sites indicates that the total impact is “high” in A Luoi and in Nam Dong and only “medium” in Phong Dien. Accordingly, sizes of sub-populations were smaller and lower numbers of adult individuals were recorded in A Luoi and Nam Dong than those in Phong Dien. These findings indicate that anthropogenic impacts negatively affect population sizes. Furthermore, Phong Dien was the only site with hardly any habitat degradation, which indicates the potential and value of intact habitats to buffer harvesting activities.

Conclusions

In total, several critical threats to the population and habitat of P. cocincinus were recorded in Thua Thien Hue Province, namely harvesting for food consumptions and to supply trade, as well as habitat fragmentation through road construction and timber logging. It is assumed that similar impacts are certainly to be observed in other areas in Vietnam. Besides local use of the species, the recorded imports of more than 43,000 animals from Vietnam into the EU for commercial use during recent years indicate an increasing international interest in the species. Most of those specimens had an “unknown” origin. According to interviews with local people involved in the trade in P. cocincinus, there are no attempts at breeding of the species in captivity, since harvesting from the wild is currently the most economic way. Thus, it is expected that most of the reported imports will represent wild individuals. In addition, this data represents only the tip of the iceberg of actual international trade levels in the species, since only imports into the EU are recorded. In addition to the international trade, the species is also harvested in large amounts for national use. According to these high levels of exploitation, it is recommended that continuous monitoring of wild populations be undertaken in order to warrant the sustainable use of the species. It is also recommended to extend monitoring activities on further populations in the North and South of Vietnam. Moreover, captive breeding could be an alternative, as currently undertaken at the Me Linh Station for Biodiversity in northern Vietnam and in Phong Dien District as well as in Hue City in Central Vietnam. Such breeding programmes could later be transferred to local communities to reduce the pressure on wild populations and supply the high demand for the species. The listing of the species in CITES Appendix III and the establishment of a sustainable export quota could support a shift from the use of wild harvested specimens to captive bred ones. Furthermore, it is herewith recommended to improve conservation measures in the natural habitats in collaboration with the local authorities to enable the long-term persistence and sustainable use of the species.

Acknowledgements

We are grateful to the directorates and staff of the Bach Ma National Park, Phong Dien and Sao La Nature Reserves (Thua Thien Hue) for support of our field work and issuing relevant permits. We thank S. V. Nguyen (IEBR), T. Pagel and C. Landsberg (Cologne Zoo) for support of our research. We thank T. V. Nguyen, T. Q. Le, T. V. Nguyen (Hanoi), B. V. Ngo and L. C. Nguyen (Thua Thien Hue) for their assistance in the field. We thank G. Benyr (Vienna) and U. Schepp (Bonn), as well as the subject editor K. Henle (Leipzig) for their constructive comments to improve an earlier version of the manuscript. This research was funded by the Vietnam Academy of Science and Technology and the People’s Committee of Thua Thien Hue Province (grant number: VAST.NĐP.01/17-18) as well as Cologne Zoo.

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