Northern snake-necked turtle (Chelodina oblonga)
About Chelodina oblonga
In Australia C. rugosa (previously C.oblonga) is widely distributed throughout the northern wet-dry tropics (Kennett et al 1993) and occurs from the Kimberley region of Western Australia to Cape York Peninsula, Queensland (Cogger 2014). Chelodina rugosa has been recorded from both the Gilbert and Flinders catchments of the Gulf of Carpentaria (University of Canberra 2014; DSITIA 2014). Chelodina rugosa inhabits swamps, billabongs, waterholes and slow-flowing rivers throughout its range (Cogger 2014). C. rugosa could be considered a floodplain specialist as the species displays a preference for ephemeral waters with the highest densities occurring on seasonally inundated, shallow (<2 m), vegetated floodplains (Kennett et al 1993). During the subsequent dry season populations retreat to more permanent waters, however if those refuges continue to dry reproductive activity is curtailed and turtles seek refuge in aestivation sites (Kennett et al 1993). C. rugosa aestivates underground after burying itself in the muddy bottom of receding water bodies (Kennett et al 1993).
Most freshwater turtles leave the water to lay their eggs so turtles inhabiting the tropics often have only a very short period which to find the relatively dry ground suitable for nesting (Georges et al. 1993). Chelodina rugosa however, has developed an unusual nesting strategy whereby it can lay its eggs underwater (Kennett et al., 1993a; Georges et al. 1993). Eggs are deposited in nests dug into the edges of waterholes that it inhabits (Kennett et al., 1993a). This species’ ability to lay its eggs underwater represents an important advantage, giving the embryos a developmental ‘head start’ over other turtle species in the area, helping to ensure that hatchling emergence is timed to coincide with benign flow conditions (Cann 1998). Hatchling growth in C. rugosa is characterized by two phases. There is an initial phase of relatively slow growth under the partial influence of initial egg size and incubation duration, followed by a second phase of relatively rapid growth under the partial influence of water temperature and mass at hatching (Fordham, unpubl. Data).
Chelodina rugosa is highly fecund, laying between 6 and 21 eggs per clutch (Kennett 1999) and potentially up to 3–4 clutches of eggs per season (Fordham, unpubl. data). Eggs are hard shelled and ellipsoid in shape (Kennett et al 1993). Eggs can survive inundation because they can undergo embryonic diapause until the waters recede and the inundation-induced hypoxia ends (Kennett et al 1993a). Eggs that have not begun to develop can survive immersion for up to 12 weeks under experimental conditions but die if immersed after embryonic development has commenced (Kennett et al 1993a; Kennett et al 1993b). Immersion of nest containing partially developed eggs represents a significant risk to C. rugosa (Georges pers. comm.). Late-term embryos may prolong their stay in their egg until favourable conditions return by entering a form of aestivation but the cost of maintaining a fully developed embryo may be high and cannot be maintained indefinitely (Kennett 1999). Post hatching survival is negatively correlated with duration of egg inundation and water temperature. Evidence suggests that inundation of C. rugosa eggs for 6 weeks, incubation of embryos at 28 °C and raising hatchlings in 28 °C water will yield the best overall outcomes (Fordham et al 2007). The combination of a narrow window in time suitable for nesting and extreme unpredictability in when the window is open would make it very difficult for C. rugosa to persist in the floodplains if its nesting requirements were similar to those of other freshwater turtles (Kennett et al 1993). The ability of C. rugosa to nest in flooded ground during the wet season allows it to exploit a longer nesting period, and potentially produce more clutches per year, than would be the case if dry nesting sites were required and females were unable to ‘store’ eggs in the ground (Kennett et al 1993).
The life history strategies that this turtle species has developed have made them tolerant of variable climatic and flow conditions however alterations to hydrology do have the potential to represent risks to their long-term persistence. For example, an unseasonal rise in water level as a result of flow supplementation may inundate northern snake-necked turtle nests and cause egg mortality during periods where nesting sites would normally be drying (Cann 1998). For this reason, C. rugosa has been selected as an ecological asset for the Gulf plan area, reflecting it’s strong dependence on various aspects of the flow regime and vulnerability to infrastructure development that has the potential to disrupt these natural flows.
The purpose of the model is to analyses long times series of modelled flow to determine if the risk to successful breeding events for Northern snake-necked turtles is adversely affected by proposed water resource development (represented in the modelled flow).
This model, describing floodplain connectivity requirements for Chelodina longicollis, has been developed using quantitative information from the literature and expert opinion.
It was developed to support the Queensland Department of Environment and Resource Management’s ecological risk assessment for Water Resource Plan (WRP) reviews.
This model and its default parameters were created for application in the Condamine River, Queensland.
However, the model parameters could be edited to suit other locations where Chelodina longicollis, occurs or to apply it for other turtle species with similar requirements.
For an annual nesting season to occur:
- flows exceeding the specified threshold (the 1.5 year ARI or wetland inundation threshold) must have occurred between November 1 and May 31 to provide suitable breeding conditions.
- Maximum number of clutches per season (default = 4)
- Incubation period (default = 120 days)
- Maximum diapause (period that eggs can be continuously submerged before development commences) (default = 90 days)
- Time between clutches (default=30 days)
- Soil moisture drying (eggs are laid under water surface, this is the period required for the soild near the eggs to dry sufficiently before development commences – default – 42 days)
This model produces binary daily results (daily clutch success). These results are then aggregated to a yearly result, and then further to a temporal result based on the defined assessment parameters.
The temporal results are then analysed across locations to report an overall landscape risk by considering the simultaneous occurrence of failures across the system.
- Daily flow data
- Data – Overall season to partition the data into. Usually annual or water year.
- Flow parameters – Settings to define flow required for clutch attempt. Day must be in season (defined by dates), with flow below a set threshold, or calculated ARI flow.
- Egg criteria – Settings to define clutch success, including max clutch attempts per season, incubation period, max diapause, time between clutches and soil drying time.
- Daily time series of successful clutches, a score of 1 applies when a laid clutch has suitable conditions to fully develop and hatch. This time series also includes intermediate details such as if the day is in the clutch season, if a clutch was attempted and why the attempted clutch failed (if applicable).
- Yearly time series of summary statistics, including the number of successful clutches, the number of attempted clutches and the clutch failure reasons.
- Yearly time series of assessment results
- Temporal time series of assessment results
- Spatial time series of assessment results
This plugin is written in Python and its underlying code is publicly available from the Eco Risk Projector computation repository.
Beynon, F, 1991, Comparative rates of oxygen consumption and development in two species of turtle from the genus Chelodina. Honours thesis, University of Canberra.
Cogger, HG, 2014 Reptiles and amphibians of Australia (7th edition). CSIRO publishing, Victoria, Australia.
Department of Science, Information Technology, Innovation and the Arts (DSITIA) 2014, WetlandInfo species database search. Available at http://wetlandinfo.ehp.qld.gov.au/wetlands/ [Accessed February 2014]
Ernst CH, Altenburg RGM and Barbour RW, 1997, Turtles of the World – Chelodina oblongata. ETI Information Systems Ltd., Netherlands. Available at: http://wbd.etibioinformatics.nl/bis/turtles.php?selected=beschrijving&menuentry=soorten&id=150 [Accessed February 2014]
Fordham DA, Georges A, & Corey B, 2007, Optimal conditions for egg storage, incubation and post-hatching growth for the freshwater turtle, Chelodina rugosa: Science in support of an indigenous enterprise. Aquaculture, vol. 270, pp. 105 – 114.
Georges A, 1988, Sex determination is independent of incubation temperature in another chelid turtle, Chelodina longicollis. Copeia, vol. 1988, pp. 248–254.
Georges A, 1985, BIOLOGY OF AUSTRALASIAN FROGS AND REPTILES ed. by Gordon Grigg, Richard Shine and Harry Ehmann, Royal Zoological Society of New South Wales.
Kennett R, Christian K, Pritchard D, 1993a, Underwater nesting by the tropical freshwater turtle, Chelodina rugosa (Testudinata: Chelidae). Australian Journal of Zoology. vol. 41 pp. 47–52.
Kennett, R, 1999, Reproduction of two species of freshwater turtle, Chelodina rugosa and Elseya dentata, from the wet-dry tropics of northern Australia. Journal of Zoology vol. 247, pp. 457–473.
Kennett, R, Georges, A, Palmer-Allen, M, 1993b, Early developmental arrest during immersion of eggs of a tropical freshwater turtle, Chelodina rugosa, from Northern Australia. Australian Journal of Zoology. vol. 41, pp. 37–45.
McCord, WP, & Thomson, SA, 2002, A new species of Chelodina (Testudines: Pleurodira: Chelidae) from Northern Australia. Journal of Herpetology. vol. 36, pp. 255-267.
Thomson, S, 2003, Long Necks, Flat Heads and the Evolution of Piscivory. World Chelonian Trust, Vacaville, California, and the Applied Ecology Research Group and CRC for Freshwater Ecology, University of Canberra, Australia. Available at: http://www.chelonia.org/articles/longneck_flathead_evolution.htm [Accessed: March 2014]
University of Canberra, 2014, Distribution Maps for Australian Freshwater Turtles. Available at: http://iae.canberra.edu.au/html/staff/georges/georges-turtle-maps.php?utm_source=eLys+–+Australasian+Turtle+Newsletter&utm_campaign=eLys&utm_medium=email [Accessed: March 2014]
Webb, GJW, 1996, Sustainable use of crocodiles by Aboriginal people in the Northern Territory. In: Bomford, M, & Caughley, J, (Eds.), Sustainable Use of Wildlife by Aboriginal Peoples and Torres Straight Islanders. Australian Government Publishing Service, Canberra, pp. 176–185.