Gulf snapping turtle
Common name: Gulf snapping turtle
Scientific name: Elseya lavarackorum
Conservation status: The Gulf snapping turtle is Vulnerable in Queensland (Nature Conservation Act 1992) and is Endangered nationally (Environment Protection and Biodiversity Conservation Act 1999).
The Gulf snapping turtle is a medium to large sized short-necked turtle. Fossil material indicates it once obtained a size of over 40 cm and as much as 50 cm straight carapace (shell) length (SCL) (Thomson et. al., 1997). However, in the modern era no Gulf snapping turtle has been measured with a SCL greater than 34 cm (Freeman, 2010). Generally adult females are larger than males with an average SCL and weight of 30 cm and 2.75 kg respectively compared to males whose average is 22 cm and 1.08 kg (Freeman, 2010).
In the wild, they are coloured brown to almost black above while the underside is generally a light brown to pale yellow. In the Lawn Hill Gorge area colouration can be masked by the accumulation of a brown residue on the carapace and plastron (part of a turtle shell forming its underside) presumably from the mineral rich waters present in the area. Algae growth is also present on the carapaces of some individuals. Large females can get the pink nose and pale head and throat colouration seen in other snapping turtles in northern Australia. Juveniles often have a strong pink colouration of the feet that fades with age. Many individuals have one barbell in the centre of the throat. In parts of the Lawn Hill Creek catchment the plastron is often covered in a layer of calcium carbonate sediment which gives the plastron a muddy brown colour.
Habitat and distribution
The Gulf snapping turtle occurs in deep water pools in the upper catchment of permanently flowing spring fed rivers. They reach their highest densities adjacent to areas of good quality riparian vegetation. Recent studies have indicated that certain key plant species may play a significant role in the distribution of the species (Freeman, 2010). For example, there is a strong association of Gulf snapping turtle with cluster figs Ficus racemosa. Cluster figs can be prolific fruiters and will fruit on a spasmodic basis all year round. The fruit and leaves of this species feature prominently in the diet of the Gulf snapping turtle (White, 1999; Freeman, 2010). There is also a similarly strong association with the plant species Pandanus aquaticus. This species appears to fulfil multiple roles in the ecology of the Gulf snapping turtle. As well as a food source, the dense foliage, submerged branches and curtains of submerged roots of P. aquaticus provide structural complexity to bank side habitat (Freeman, 2010). Densities of the Gulf snapping turtle are invariably lower adjacent to riparian vegetation that is degraded either by weed invasion and / or fire.
The riparian ecosystem that supports the food trees used by the Gulf snapping turtle is a fire sensitive system. In some parts of its range fires that are too hot kill vegetation and open up the canopy creating ideal conditions for weed invasion as a result of the deposition of weed seeds from upstream during seasonal floods (Freeman, 2010).
The Gulf snapping turtle is known from parts of the upper and middle reaches of the Gregory and Nicholson drainages of north-western Queensland and north-eastern Northern Territory (Thomson, et. al., 1997; Cann, 1998; White, 1999; Georges and Thomson, 2010). It has also been recorded in the upper reaches of the Calvert River in the Northern Territory near the border with Queensland (Woinarski, et. al., 2007). West of the Calvert River the Garawa, traditional owners of this country, have confirmed that the white headed turtle that occurs in the Calvert also occurs in the upper reaches of the Robinson River.
Life history and behaviour
This species is almost exclusively a vegetarian, eating fruit, leaves and algae which were found to make up 98% of their diet in the Middle Gorge area of Boodjamulla National Park (Freeman, 2010).
A dry season nester (between May and July) little is known of their reproductive behaviour. The few nests that have been found indicate that clutch size can vary between six and nine oblong hard shelled eggs (Freeman, 2010). It is currently not known how long eggs take to hatch in the wild but it is assumed to be similar to the closely related northern snapping turtle E. dentata, at around 150-188 days (Kennett, 1999).
While evidence is limited, major threats to survival are thought to be nest predation by feral pigs Sus scrofa and degradation of riparian vegetation (Freeman, 2010). Predation by feral pigs has been identified as a major threat to nesting turtles, both marine and freshwater (Cann, 1978; Environment Australia, 2003; Fordham et. al., 2006). It is thought that in the northern and southern Gulf region (including areas with Gulf snapping turtle) there are very high densities of feral pigs. During the 2010 nesting season in a part of the upper Gregory Catchment four out of a total of seven nests were destroyed by feral pigs shortly after laying. The three nests that were not predated were in an area that was largely inaccessible to pigs (Freeman, 2010). Pigs were also recorded traversing and digging up suitable nesting banks in the area.
The Gulf snapping turtle is a species that prefers waterways adjacent to riparian vegetation which is relatively intact, areas where the density of weed species is low and the proportion of native vegetation is high. In contrast, sites with low densities or no Gulf snapping turtles are characterised by degraded riparian vegetation which has often resulted from hot fires killing native riparian species, opening up the canopy and facilitating weed invasion. Weeds of concern include wild passionfruit Passiflora foetida, noogoora burr Xanthium occidentale and butterfly pea Clitoria ternatea (Freeman, 2010; Freeman and Ezzy, 2011).
Freshwater turtles including Gulf snapping turtles are considered to be an important food item by local indigenous hunters in north-west Queensland. In this area they are mainly caught at a few accessible localities; the vast majority of turtle habitat is not accessed for hunting. The impact from traditional hunting has also been reduced by the current ban on hunting in the Middle Gorge area of Boodjamulla National Park. This ban provides protection for the most important, accessible and highest density population of Gulf snapping turtles currently known. This species is also harvested in the Northern Territory (Freeman, 2010).
Mortality associated with drowning in traps used by recreational fishers to catch red claw and the freshwater prawn has been identified as a possible threat (Cann, 2008). As well as traps, set nets may also be a threat to Gulf snapping turtles. Scientists carrying out fish surveys in the Lawn Hill Creek and Gregory River observed evidence of the illegal use of gillnets in these freshwater catchments (Davis and Dowe, 2005). What impact recreational and illegal fishing practices may be having on the Gulf snapping turtle is unknown.
A further potentially significant threat to the Gulf snapping turtle may come from climate change. Some preliminary observations from the 2010 nesting season suggest that most embryos in nests on unshaded nesting banks do not survive the high temperatures associated with the late dry season (October-November). While in 2010 this is thought to have been a natural event, it may well be indicative of the potential impacts of increased temperatures on nesting success.
The most significant population currently known is protected within Lawn Hill Gorge, Boodjamulla (Lawn Hill) National Park while the Australian Wildlife Conservancy protects significant areas of Gulf snapping turtle habitat in the Northern Territory. Each of the threats are being managed with special attention to significant places like Lawn Hill Gorge. The Middle Gorge area in Boodjamulla National Park has been identified as containing the most important population of Gulf snapping turtles anywhere. After work by threatened species staff and park rangers indicated that feral pigs may be a significant threat to this population by nest predation, park rangers continue to implement a feral pig control program in this area to minimise their impact.
How can you help?
When travelling in the north western parts of Queensland keep an eye out for freshwater turtles on rivers and watercourses in the area. Report any sightings of unusual species to Threatened.Species@des.qld.gov.au.