Pig-nosed Turtle (Carettochelys insculpta)

Advice to the Minister for the Environment and Heritage from the Threatened Species Scientific Committee (TSSC) on Amendments to the list of Threatened Species under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act)
13 July 2005

1. Scientific name, common name (where appropriate), major taxon group

Pig-nosed Turtle (Carettochelys insculpta)

2. Description

The Pig-nosed Turtle is a very large freshwater turtle, with a shell length of 60-70 centimetres. It is grey to greyish brown in colour on the upper surface, and white to yellow in colour on the lower surface. It has broad paddle-like flippers, each with two claws. Its shell is covered by a soft-pitted skin, instead of the hard scutes that cover the shells of most turtles. The Pig-nosed Turtle gets its common name from the placement of its nostrils at the end of a prominent fleshy, trunk-like snout (Wilson and Swan, 2003).

The Pig-nosed Turtle inhabits freshwater river systems and prefers large, still bodies of water and sandy riverbeds (Wilson and Swan, 2003). The Pig-nosed Turtle is largely restricted to an aquatic habitat, as it lacks a hard, protective shell and has paddle-like flippers that are not suited to traversing land (Georges and Wombey, 1993). Males never wholly leave the water and females only leave the water to nest on the riverbanks.

Pig-nosed Turtles eat a wide range of food, including aquatic plants such as ribbon weed, fruits and flowers of fig, bush apple and eucalyptus trees, plus crustaceans, insects and molluscs (Wilson and Swan, 2003).

It is estimated that female Pig-nosed Turtles reach maturity at a minimum of 25 years of age (Heaphy, 1990). Females nest late in the dry season, laying their white, hard-shelled eggs on sandy river banks above the water (Wilson and Swan, 2003).

3. National context

The Pig-nosed Turtle is known from Australia, Papua New Guinea and Irian Jaya (Indonesia) (IUCN, 2004). The Australian Pig-nosed Turtle population is completely isolated from the Papua New Guinean and Irian Jayan populations from which it was apparently derived (Cogger and Heatwole, 1981).

In Papua New Guinea and Irian Jaya, the Pig-nosed Turtle is thought to occur in all of the larger and some of the smaller southern flowing rivers, although the exact boundary of its distribution is not known (Georges and Rose, 1993).

In Australia, the Pig-nosed Turtle is endemic to the Northern Territory. The species is known from four major river drainages in the Northern Territory: the East Alligator, South Alligator, Daly and Victoria Rivers. The species has also been reported to occur in the smaller Goomadeer River, although the extent and size of this population is not well known (Georges and Rose, 1993). A Pig-nosed Turtle was recently reported from the Peter John River, approximately 100 kilometres east of the Goomadeer, which is the eastern extreme of its current confirmed range. Further surveys are required to determine whether there is a resident breeding population in the Peter John River. There have been reports that the species occurs in other rivers in the Northern Territory, including the Roper and Wenlock Rivers, but these reports have not been confirmed (Georges and Rose, 1993).

The Pig-nosed Turtle was listed as Vulnerable on the IUCN Red List of Threatened Species in 2000.

The Pig-nosed Turtle is listed as Near Threatened under the Northern Territory Territory Parks and Wildlife Conservation Act 2000.

4. How judged by TSSC in relation to the EPBC Act criteria.

TSSC judges the species to be ineligible for listing under the EPBC Act. The justification against the criteria is as follows:

Criterion 1 - Decline in numbers

The total size of the Australian Pig-nosed Turtle population is not conclusively known, however it is estimated to contain approximately 3000 individuals (R. Sims, pers comm.). This estimate is based on several studies in the Daly River drainage, which have calculated the density of Pig-nosed Turtles along lengths of the river (Heaphy, 1990; Doody 1996, 1997, 1998 unpublished data). These studies revealed that high density sites contain an average of 7 Pig-nosed Turtles per kilometre of river, and low density sites contain approximately 2 Pig-nosed Turtles per kilometre. Species experts extrapolated from the Daly River density calculations to estimate the total number of Pig-nosed Turtles in other river drainages. The extrapolations were based on the experts' knowledge of the length of river inhabited by Pig-nosed Turtles in each river drainage, and the proportion of high density and low density sites. It is estimated that there are 1256 individuals in the Daly River system, 980 in the South Alligator River, 430 in the Victoria River, 158 in the East Alligator River and 105 in the Goomadeer River, a total of almost 3000 individuals (R. Sims, pers comm.). The total number of individuals could exceed 3000 if further surveys reveal that there are resident populations in other rivers.

There is minimal quantitative data available to determine past trends in the number of Pig-nosed Turtles in Australia. Surveys conducted in the Daly River in 1986 and 1996 indicate that the Pig-nosed Turtle population was stable over this time (Doody et al 2000), however there is a paucity of data available elsewhere in the species' range. It is believed that feral animals and other stock caused a decline in the South Alligator River Pig-nosed Turtle population prior to the declaration of Stage III of Kakadu National Park (1987-1991) (A. Carr in personal communication with Pritchard 1979), but this decline was not quantified, and it is not clear whether the population has subsequently recovered.

A range of threats to the Pig-nosed Turtle have been identified, but the impact of many of these threats has not been quantified. Feral water buffalo have been identified as a threat to the species, through the destruction of riparian vegetation that Pig-nosed Turtles depend on for food (Georges and Kennett, 1989; Georges and Rose, 1993). Feral water buffalo, and pastoral stock that have access to riverine habitat, could also impact on Pig-nosed Turtles through trampling their nests. Pig-nosed Turtle nests are shallow and not able to endure the impacts of hoofed animals (Georges and Kennett, 1989). Agricultural and pastoral activities could also impact on Pig-nosed Turtles, if there is clearance or modification of the riparian vegetation, or if there is increased erosion leading to the siltation of freshwater rivers and billabongs (Georges and Rose, 1993). In northern Australia, Pig-nosed Turtles are regularly eaten by indigenous people, however there is insufficient data on the level of exploitation, and its relationship with total Pig-nosed Turtle numbers, to determine whether indigenous take is sustainable (Georges and Rose, 1993).

Another potential threat to the Pig-nosed Turtle is increased water extraction from rivers inhabited by the species. Georges et al (2003) modelled the impacts of different levels of flow reduction on habitat viability for the Pig-nosed Turtle along a 74 kilometre stretch of the Daly River which is estimated to contain 525 mature individuals. The Daly River undergoes seasonal high and low flows each year, the magnitude of which can vary greatly from year to year. For example, dry season water flow in the Daly River can range from 2.0 to 14.7 cubic metres per second (cumecs). The model predicted the effect of water flow reductions (ranging from 3 to 12 cumecs) on the probability of the occurrence of 'cease-to-flow' years, which do not occur under natural dry season conditions. Under the most conservative flow reduction scenario of 3 cumecs, the model predicted that there would be a 'cease-to-flow' year approximately every 9 years, which would have catastrophic consequences for the Pig-nosed Turtle. Under 'cease-to-flow' conditions, the river would be fragmented into a series of disconnected pools, which would restrict the movement of all female Pig-nosed Turtles across their home range. More than 55% of the pools would not provide adequate access to nesting banks, and over 53% of the pools would not contain beds of ribbon weed (a main food source in the dry season). Under these conditions, the local population of Pig-nosed Turtles could be seriously depleted due to resource limitation and reproductive failure (Georges et al, 2003). Given that 8 'cease-to-flow' events could occur within 75 years, there would be a high potential for the local population to become extinct within three generations.

While the Northern Territory Government has been exploring opportunities for the future development of the Daly Region, there has been no decision to intensify agriculture or increase water extraction from the Daly River. Given that there is no imminent threat of water resource development to the Pig-nosed Turtle, it not possible to conclude that the Pig-nosed Turtle population is likely to decline substantially in the immediate future. Should water resources be developed for agricultural use in the Daly Region, impacts on the Pig-nosed Turtle could potentially be mitigated through either: a) instating a flexible allocation regime, whereby the cap on flow reduction is adjusted for the magnitude of dry season flows; or b) extracting water from aquifers, in order to buffer the river from unacceptable flow reductions in very dry years (Erskine et al, 2003; Georges et al, 2003).

In conclusion, there is insufficient historical data to determine whether there has been a past decline in the number of Pig-nosed Turtles in Australia. A number of threats to the Pig-nosed Turtle have been identified, but the impacts of many of these threats have not been quantified. The impact of flow reduction in the Daly River has been quantified, and it is likely that small reductions in flow in very dry years could reduce the Pig-nosed Turtle population size. However, given that there has been no decision to increase water extraction from the Daly River, it is not possible to conclude that the Pig-nosed Turtle is likely to undergo a substantial reduction in numbers in the immediate future.

Therefore, the species is not eligible for listing under this criterion.

Criterion 2 - Geographic distribution

The Pig-nosed Turtle is known from four major river drainages in the Northern Territory: the East Alligator, South Alligator, Daly and Victoria Rivers. It is also known from the minor Goomadeer River (Georges and Rose, 1993). A Pig-nosed Turtle was recently reported from the Peter John River, approximately 100 kilometres east of the Goomadeer, which is the eastern extreme of its current confirmed range. Further surveys are required to determine whether there is a resident breeding population in the Peter John River. There have been anecdotal reports of the Pig-nosed Turtle occurring in other rivers, including the Roper and Wenlock Rivers, but these reports have not been confirmed (Georges and Rose, 1993).

The total length of river occupied by Pig-nosed Turtles has been estimated for the 5 river drainages known to support resident breeding populations. It is estimated that the Pig-nosed Turtle inhabits 397 km of the Daly River system, 265 km of the South Alligator River, 215 km of the Victoria River, 79 km of the East Alligator River and 15 km of the Goomadeer River (R. Sims, pers comm.). The Pig-nosed Turtle's morphology precludes it from moving across land and thus, its distribution is restricted to the river channels of each drainage (Georges and Wombey, 1993). By multiplying the total length of river by the average river width (approximately 50 metres), the total area of occupancy of the Pig-nosed Turtle is estimated to be 48.6 km2 (R. Sims, pers comm.) The total area of occupancy could exceed the current estimate, if further surveys reveal that there are resident populations in other rivers.

As mentioned previously, the Pig-nosed Turtle's morphology restricts it to river channels and consequently, the species cannot disperse between river systems via land. In Australia, Pig-nosed Turtles are not found in estuarine areas and thus cannot disperse between river systems via the mouths of rivers and the oceans (Press, 1986; Georges and Kennett, 1989). Hence, if the Pig-nosed Turtle is threatened by a process operating in one river system, a decline in the population cannot be mitigated via the movement of individuals between river systems.

A number of threats to the Pig-nosed Turtle have been identified, but the extent to which these threats are impacting on habitat quality, population size or geographic distribution is not known. Threats that have been identified include the trampling of nests by feral water buffalo and pastoral stock, the modification of riparian vegetation by feral water buffalo, indigenous harvest, and the clearance or modification of riparian vegetation, and the siltation of rivers, resulting from agricultural and pastoral activities (Georges and Rose, 1993).

Increased water extraction from rivers, to support an intensification of agriculture, has also been identified as a potential threat to the Pig-nosed Turtle. Georges et al (2003) modelled the impacts of flow reduction on Pig-nosed Turtles along a 74 km stretch of the Daly River. The model predicted that a small reduction in water flow of 3 cumecs would increase the probability of 'cease-to-flow' years, which do not occur under natural conditions. 'Cease-to-flow' years would have catastrophic consequences for the viability of Pig-nosed Turtle habitat, and potentially lead to substantial depletion of the local population (see criterion 1 for more detail.) While water resource development in the Daly Region could potentially threaten the Pig-nosed Turtle, no decision has been made to increase water extraction from the Daly River. Therefore, it is not possible to project that there will be a continuing decline in habitat quality, population size or geographic distribution at this location as a consequence of water resource development. There is also insufficient evidence to demonstrate that threats, such as feral water buffalo and current pastoral and agricultural activities, are causing a continuing decline in Pig-nosed Turtle habitat quality, population size or geographic distribution.

In conclusion, while the Pig-nosed Turtle's geographic distribution is restricted to an area of occupancy of approximately 48.6 km2, it is not considered to be precarious for its survival.

Therefore, the species is not eligible for listing under this criterion.

Criterion 3 - Population size and decline in numbers or distribution

The total number of Pig-nosed Turtles in Australia is not conclusively known. The Australian population is estimated to contain almost 3000 individuals, with the largest sub-population consisting of approximately 1250 individuals (R. Sims, pers comm.). This population estimate is based on density measurements of Pig-nosed Turtles in the Daly River drainage, which have been extrapolated to calculate the number of individuals in all 5 river drainages occupied by the species. A more detailed explanation of how the total population size was calculated is provided under criterion 1.

While the total population size has not been measured via surveys at all known locations, and is therefore only an estimate, current available evidence indicates that the total number of individuals is limited. However, existing evidence does not indicate that the number will continue to decline, or that the species' geographic distribution is precarious for its survival.

Therefore, the species is not eligible for listing under this criterion.

Criterion 4 - Population size

The total size of the Australian Pig-nosed Turtle population is not conclusively known, but it is estimated to consist of almost 3000 individuals. This population estimate is based on density measurements of Pig-nosed Turtles in the Daly River drainage, which have been extrapolated to calculate the number of individuals in all 5 river drainages occupied by the species. A more detailed explanation of how the total population was calculated is provided under criterion 1. While the total population size has not been measured via surveys at all known locations, and is therefore only an estimate, current available evidence indicates that the total number of individuals could not be considered extremely low, very low or low.

Therefore, the species is not eligible for listing under this criterion.

Criterion 5 - Probability of extinction in the wild

There is no quantitative data available against this criterion.

Therefore the species is not eligible for listing under this criterion.

5. Conclusion

The total number of Pig-nosed Turtles in Australia is not conclusively known, however it is estimated that there are almost 3000 individuals inhabiting 5 freshwater river drainages in the Northern Territory. There is insufficient historical survey data to determine whether there has been a past decline in the number of Pig-nosed Turtles in Australia.

A number of threats to the Pig-nosed Turtle have been identified, including the trampling of nests by feral water buffalo and pastoral stock, the modification of riparian vegetation by feral water buffalo, indigenous harvest, and the clearance or modification of riparian vegetation, and the siltation of rivers, resulting from agricultural and pastoral activities. However, the extent to which these threats are impacting on the Australian Pig-nosed Turtle population is not known.

Increased water extraction from rivers, to support an intensification of agriculture, has also been identified as a potential threat to the Pig-nosed Turtle. The impact of flow reduction in the Daly River on the viability of Pig-nosed Turtle habitat has been modelled, and it is likely that small reductions in flow in very dry years could reduce the Pig-nosed Turtle population size. However, given that there has been no decision to increase water extraction from the Daly River, it not possible to conclude that the Pig-nosed Turtle population size or distribution is likely to decline in the near future as a consequence of water resource development.

The Pig-nosed Turtle is known from 5 river drainages in the Northern Territory and, given that its area of occupancy is estimated to be 48.6 km2, its geographic distribution is considered to be restricted. However, based on current knowledge of the threats to the species, its geographic distribution is not considered to be precarious for its survival.

The species is not eligible for listing under any of the EPBC Act criteria.

6. Recommendation

TSSC recommends that the species Carettochelys insculpta is not eligible for inclusion in the list referred to in section 178 of the EPBC Act.

Publications used to assess the nomination

Cogger, H. & H. Heatwole (1981). The Australian reptiles: origins, biogeography, distribution patterns and island evolution, pp 1331-1373 in Ecological Biogeography of Australia, A. Keast (ed.) Mongraphia Biologicae Vol. 41, Dr W. Junk, The Hague

Doody, J.S., A. Georges & J.E. Young (2000). Monitoring Plan for the Pig-nosed Turtle in the Daly River, Northern Territory. Unpublished report to the Parks and Wildlife Commission of the Northern Territory, June 2000. Accessed from: http://aerg.canberra.edu.au/reprints/2000_Doody_etal_monitoring_plan.pdf

Erskine, W. D., G. W. Begg, P. Jolley, A. Georges, A. O'Grady, D. Eamus, N. Rea, P. Dostine, S. Townsend & A. Padovan (2003). Recommended environmental water requirements for the Daly River, Northern Territory, based on ecological, hydrological, and biological principals. Supervising Scientist Report 175 (National River Health Program, Environmental Flows Initiative, Technical Report 4), Supervising Scientist, Darwin NT.

Georges, A. & R. Kennett (1989). Dry-season distribution and ecology of Carettochelys insculpta (Chelonia: Carettochelydidae) in Kakadu National Park, Northern Australia in Australian Wildlife Research 16:323-325.

Georges, A. & M. Rose (1993). Conservation biology of the pig-nosed turtle, Carettochelys insculpta in Chelonian Conservation and Biology 1:3-12.

Georges, A., I. Webster, E. Guarino, M. Thoms, P. Jolley & S. Doody (2003). Modelling dry season flows and predicting the impact of water extraction on a flagship species. Final Report to the Northern Territory Department of Infrastructure, Planning and Environment. CRC for Freshwater Ecology and the Applied Ecology Research Group, University of Canberra.

Georges, A. & J. Wombey (2003). Family Carettochelydidae. In: Fauna of Australia. Vol 2A. Amphibia and Reptilia, Australian Government Publishing Service, Canberra.

Heaphy, L. J (1990). The ecology of the pig nosed turtle, Carettochelys insculpta, in Northern Australia. University of New South Wales, Sydney.

IUCN (2004) IUCN Red List of Threatened Species: Carettochelys insculpta, Accessed from: http://www.redlist.org/search/details.php?species=3898 on 31 May 2005.

Press, A. J. (1986). The Gagudju species survey. Unpublished Report to the Australian National Parks and Wildlife Service, Canberra.

Pritchard, P.C.H. (1979). Taxonomy, evolution and zoogeography, pp 1-42 in Turtles. Perspectives and Research, M. Harless and H. Morlock (eds), John Wiley and Sons, New York.

Wilson, S. & G. Swan (2003). A Complete Guide to the Reptiles of Australia, New Holland Publishers Pty Ltd, Australia.