The Red-Eared Sliderelegans (Trachemys scripta ) In Singapore
Abigayle Ng Pek Kaye, Ruth M. O’Riordan, Neil F. Ramsay & Loke Ming Chou Red-eared Sliders Trachemys scripta elegans (Wied, 1839)
• Natural range: in south-eastern region of US in the Mississippi Valley from Illinois to the Gulf of Mexico (Ernst, 1990) • Commonly kept as pets and have been released in many places outside of their natural range, including Asia Including Singapore, Hong Kong, Indonesia, Taiwan, Thailand, Vietnam and Japan • Average life span: 40 to 50 years Concerns in the Asian region
2003 survey of turtles Korea in Japan, reported that over 60% are foreign species- majority were red- eared sliders (Turtle & Tortoise Newsletter, Dec. ‘04)
Vietnam Are they a cause for concern?
• Ban on the sale of those smaller than 4cm in U.S. • Ban on the import into New Zealand Possible vector for zoonotic transfer of Salmonella to humans • Import is banned by EU countries impact on native terrapins, e.g. Emys orbicularis (e.g. Cadi & Joly, 2003). • In Singapore: The only species of chelonian allowed to be imported for the pet trade by the Agri-Food and Veterinary Authority of Singapore (AVA) • In Singapore: 11 species of terrapin had been recorded prior to this work, indigenous and introduced Some local species of terrapins
• Both listed as Vulnerable by the IUCN and both are listed in Appendix II (CITES).
Malayan Box Terrapin (Cuora amboinensis )
Black Marsh Terrapin (Siebenrockiella crassicollis ) Imports of T. s. elegans into Singapore from the US
Year Number 1985 15,100 1986 5,000 1988 20,000 2001 284,000 2002 266,604 2003 147,363 2004 (January – June) 86,955
Data from (AVA) Red-eared sliders in Singapore
• Release by irresponsible pet owners
• Releases for religious reasons
• Also sold in markets for human consumption Previous records of the sliders in Singapore
• 1983- Tinsley mentioned the problem of releases of sliders into Singapore Botanic Gardens’ lakes • 1990-Trachemys scripta elegans was included in Lim and Chou’s herpetofauna checklist • 1997- Teo and Rajathurai’s study of the nature reserves of Singapore reported that the sliders were numerous in the Botanic Gardens, but they were not yet established in the forest streams • 2003- Semi-quantitative survey by Goh Ter Yang of 29 localities in Singapore Where these terrapins can be found Visual survey done by Goh Ter Yang, 2003
Highest Lowest density density The potential problems caused by the sliders in Singapore
• Eat pond vegetation • Decrease dissolved oxygen content • Impact on aquatic fauna either by predation or making the habitat unsuitable • Impact on native terrapins due to competition, for e.g. Resources Basking spots • Anecdotal account that they may be predated upon by monitor lizards and large fish The Ecology of Red-eared Sliders in Singapore- AIMS
• Current status of the Trachemys scripta elegans in Singapore Population size and structure • Reproduction • Dietary Studies •Behaviour •Growth rate • Disease status
• Determine if the sliders are having an impact on native terrapins • Management solutions for National Parks Board and if possible, other countries in the region Population size and structure
• Test out various methods and traps
• Visual counts
• Mark and recapture Examples of Study Sites
Central Nature Reserve • MacRitchie Reservoir 700,000m2, 10km • Lower Peirce Reservoir 500,000m2, 6.6km
Singapore Botanic Gardens •Eco-lake 10,590m2, 717m • Swan Lake 10,312m2, 567m Method one: Visual counts
• At least three observers • Each pond was visually divided into sections • Each observer made 5 counts of all the terrapins observed within each section • Quick method for getting quantitative data
Drawbacks • Observers have to be “calibrated” against one another • Only possible for small areas • Possibility of observer bias • Only count data collected Method two: Mark-recapture
• Mark-recapture method • Baited ‘crab’ traps deployed at various locations near to the banks of the water body • Four to six occasions of trappings Mark all terrapins on the first occasion Use same trap effort on subsequent occasions and record all recaptures n m ---1 ≈ ---2 N n2 • Three to six sessions of 25 to 40 minutes each, depending on the location Various methods of trapping were tried Crab traps!
• Commercially available locally • Relatively cheap • Light weight • Collapsible • Manageable size
Most effective in trapping terrapins Methods
• Animals caught were measured, weighed, sexed and notched for unique Left Right identification • Marking method as described by Cagle (1939) • Marks made using a square file on the marginal scutes • Able to uniquely identify 2516 individuals 2,3-1 Methods continued
• Terrapins placed in holding containers until all trapping sessions for the day are complete. • All are released back into the water
Eco-Lake Eco-Lake at the Botanic Gardens
• Results from one trap session (25 minutes)
0 7 1 1
2 4 3 Trap location Population structure
Lower Peirce Reservoir, n=170 MacRitchie Reservoir, n=99
140 140 120 120 100 100 80 80 60 60 40 40 20 20 0 number of individuals number 0 <10 10.0- 15.0- 20.0- 25.0- of individuals number <10 10.0- 15.0- 20.0- 25.0- 14.9 19.9 24.9 29.9 14.9 19.9 24.9 29.9 size classes size classes
Swan Lake, n=150 Eco-Lake, n=130
140 140 120 120 100 100 80 80 60 60 40 40 20 20 0 number of individuals number 0 <10 10.0- 15.0- 20.0- 25.0- of individuals number <10 10.0- 15.0- 20.0- 25.0- 14.9 19.9 24.9 29.9 14.9 19.9 24.9 29.9 size classes size classes Females Males al
Population estimate using CAPTURE (White, et. ., 1978)
Lower Peirce Reservoir, n=170 MacRitchie Reservoir, n=99
140 140 120 562.0 ± 38.13120 274.0± 23.34 100 100 80 80 60 60 40 40 20 20 0 0 number of individuals number number of individuals number <10 10.0- 15.0- 20.0- 25.0- <10 10.0- 15.0- 20.0- 25.0- 14.9 19.9 24.9 29.9 14.9 19.9 24.9 29.9 size classes size classes Swan Lake, n=150 Eco-Lake, n=130
140 140 120 363 ± 22.99 Count 120 392.0 ± 28.06 Count 100 100 80 =177 80 =213 60 60 40 40 20 20 0 0 number of individuals number number of individuals number <10 10.0- 15.0- 20.0- 25.0- <10 10.0- 15.0- 20.0- 25.0- 14.9 19.9 24.9 29.9 14.9 19.9 24.9 29.9 size classes size classes Females Males Population size and structure
• Test validity of previous results Same method at the same site but traps placed in different locations
• Complete removal experiment (Andersen, 1953) Visual counts + mark and recapture Complete count
• Estimate population size at less accessible locations
• Correlation between public accessibility and slider density Home Range?
Swan Eco-lake Lower MacRitchie Lake Peirce Reservoir Reservoir
Total area (m2) 10,312 10,590 500,000 700,000 Recaptures 19 6 14 7 Recaught at same 3 1 11 4 location Proportion 15.79% 16.67% 78.57% 57.14% Distance between ~ 100m 660m to 1000m traps
• Moll and Legler (1971) suggested the home range of Trachemys scripta to be 32,977m2, with an average length of 278m. Many terrapins with wounds and/or shell deformities
Symphony Lake, SBG
Lower Peirce Reservoir
Symphony Lake, SBG Eco-lake, SBG
East Coast Park Pond
Swan Lake, SBG Other chelonians recorded
East Lower Mac Swan Eco- Coast Pierce Ritchie Lake Lake Park Malayan Box Cuora amboinensis 5 1 Black Marsh Siebenrockiella 20 crassicollis Chinese Striped-neck Ocadia sinensis 1 3 1 American Cooter Pseudemys sp. 1 Chinese Softshell Pelodiscus sinensis 3 Asiatic Softshell Amyda cartilaginea 3 Forest Softshell Dogania subplana 1 1 Limitations
• Size of reservoirs Limit to the number of traps that can be deployed (travel and processing time)
• Limit on the frequency of visits Help and authorisation from Public Utilities Board and National Parks Board
• Fieldwork is labour-intensive Need a lot of field-help Logistics Where the sliders can be found Goh (2003) and present survey
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Highest Lowest density density Reproduction of the sliders
• Reason: Indicate whether successful establishment of sliders in Singapore water bodies
• If they are breeding, it exacerbates problem caused by releases
• Courtship behaviour has been observed in almost all water bodies surveyed
• Nesting behaviour has been observed in a few places (anecdotal accounts) Singapore Botanic Gardens, Lower Peirce Reservoir, MacRitchie Reservoir, Sungei Buloh Wetlands Reserve, Singapore Zoological Gardens Nests and hatchling found at Eco- lake
1cm Reproductive study
• Monthly samples of 5 female terrapins from each of 3 size classes (10-14.9, 15-19.9 and >20cm long) and 5 males taken from Eco-lake at Botanic Gardens • Sampling from June 2004 to May 2005 • Terrapins dissected and the gonads staged • Similar sampling at Bedok Reservoir from November 2004 • Continuous temperature loggers have been deployed
• Full results will be presented next month at the 5th World Congress of Herpetology in South Africa The presence of developing follicles and/or eggs within female sliders (June-November 2004) f 60
50
40 Eggs IV 30 III II each stage 20
10
0 Abundance o follicles/eggs of 3 0 0 8 4 7 5. 8. 9. 1. 11.9 13.1 13. 1 17.5 1 18.0 18.8 19.3 1 20.5 21. 2 22.8 23.2 Carapace length (cm) Reproduction
• Females not synchronous • No follicles/eggs found in smallest size class • Larger females carried more and better developed eggs • Only the largest terrapins contained shelled eggs • No seasonal trend in testes and epididymis development (carapace size : 10-17cm long) • Suggests potential for year-round reproduction in this species in Singapore Reproduction-further work
• Locate and monitor potential nesting grounds
• Collect slider hatchlings Compare sex ratio of hatchlings in Singapore (and relationship with temperature) and those sold in pet shops
• Use of X-ray photography to assess reproductive status of native terrapins (seasonality) Dietary studies
• Reason: To assess the impact of the sliders on local flora and fauna
• Examine the gut contents of the sliders to assess their diet in the wild
• Compare the diet of sliders with that of native terrapins
• Laboratory controlled experiments Gut clearance time of native versus sliders Compare appetite between native and sliders Behaviour studies
• Document the daily activity patterns of the sliders Attachment of temperature loggers and radio-tracking
• Use of artificial solaria to assess basking behaviour of native terrapins and competition with the sliders Cadi and Joly (2003) Lower Peirce Reservoir
• External collaborator - Prof. John Davenport (UCC) Biomechanics of the feeding behaviour of native species and sliders Growth Rate
• Reason: Growth rate in the equatorial region has not been documented. The rate of growth might be significantly higher here
• Growth of marked terrapins in Eco-Lake to be monitored over two years Disease status
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• Parasites of native terrapins Cuora amboinensis nd Siebenrockiella crassicollis have been documented by Lowe (1979)
• Compare the ecto- and endo-parasites of sliders with native terrapins
• Assess the prevalence of Salmonella in wild caught as well as store bought terrapins Conclusions
• Very dense populations of red-eared sliders in the highly accessible water bodies, such as in the Singapore Botanic Gardens
• Other species of terrapin recorded in the same habitats as the sliders
• There seems to be a strong bias towards females in the sliders
• Red-eared sliders likely to be reproducing in Singapore Acknowledgements
• National Parks Board and Public Utilities Board of Singapore
• Student helpers: Yvonne and Weisong and Lab mates at the Marine Biology Lab
• National University of Singapore for funding