The Fraser Island Short-necked Turtle

Photo: Arthur Georges

Table of Contents

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Taxonomy

External Morphology

Distribution

Population Status

Habitat

Diet

Reproduction

Activity and Growth

Population Dynamics

Threats to Survival

Conservation Measures Taken

Conservation Measures Proposed

References

Sources

Taxonomy

The Fraser Island Short-necked Turtle is an undescribed chelid turtle in the genus Emydura, with close affinities to Krefft's river Turtle, Emydura krefftii. Whether it should be accorded specific or sub-specific status is difficult to determine, as with most insular forms. The Fraser Island turtles differ from those of the mainland in colouration (they are melanistic), in body size (they are dwarfed) and in several key ecological attributes (size at maturity, dimorphism, reproductive parameters) (Georges, 1982a; 1983; 1984). Whether these differences are phenotypic responses to differing environmental conditions or proximally determined by genetic factors is not known. Even if they are principally genetic, the differences between the island and mainland forms may be no greater than would be expected of allopatric populations of a polytypic species. A decision on the status of the Fraser Island Short-necked Turtle awaits detailed morphological and possibly molecular comparisons with its mainland relatives.

External Morphology

The Fraser Island Short-necked Turtle has only a moderately long neck in comparison with some other Australian chelids -- the head and neck combined is much shorter than the shell. The head lacks the horny casque typical of Elseya and the temporal region is smooth. There are five claws on each of the webbed forelimbs and four claws on each of the webbed hindlimbs. The gular shields of the plastron are entirely separated by the intergular, and the latter does not come into contact with the pectorals. A cervical shield is typically present. The above suite of characters places the form firmly within the genus Emydura (Cogger, 1988).

The carapace, limbs and other extremities are typically very dark, almost black. The plastron is white, cream or grey. Most specimens lack the cream or yellow stripe behind the eye present in many mainland Emydura, though there is usually a creamy yellow stripe extending from along the lower jaw to the side of the neck to just below the tympanum. In some populations on Fraser island, a proportion of turtles have mottled carapaces with dark blotches on a deep brown background (e.g. Lake Garrawongera, Lake Birrabeen). In Lake McKenzie, some have the light brown carapaces and juveniles often have a distinct yellow stripe behind the eye, both features more typical of the mainland Emydura krefftii.

Once the shell is unfolded, the hatchling is nearly circular in outline (carapace length to width ratio of 89.4% +/- 0.4, n = 17) and its shell is relatively deep (shell depth to carapace length ratio of 49.1% +/- 0.3). Hatchlings weigh 4.6 +/- 0.1g on average (n = 5) and have a carapace length of 29.5 +/- 0.3 mm (n = 15). As a turtle grows, its carapace becomes broadly oval, moderately expanded posteriorly; turtles longer than 200 mm have a carapace width to length ratio of only 68% +/- 0.3 (n = 70). The relative depth of the shell declines steadily as the juvenile turtle grows. Turtles in the size range 100 - 150 mm CL have a mean shell depth to carapace length ratio of 35.4% +/- 0.1 (n = 135) which is maintained at this value by males throughout their later life. However females deepen in the shell once more as they age, with the ratio rising to 39.3% +/- 0.3 (n = 67) for females longer than 200 mm.

Males and females are sexually dimorphic with the most obvious differences in the tail. The tails of females and of juveniles of both sexes are relatively small, extending over 2.5 to 3.0 marginal scutes when withdrawn beneath the marginals to lie against the body. When the tail is extended, the anterior margin of the cloacal opening lies well within the margin of the carapace. The tails of mature males are longer much more muscular, necessary for successful copulation. When the tail is extended, the cloacal aperture lies well outside the margin of the carapace, a feature useful in identifying juvenile males. When withdrawn, the tail of mature males extends over four or more marginal scutes. There are differences in the shell also. Females become much deeper in the shell and broader in the head as they grow, than do males. Maximum carapace length recorded for a male is 196.5 mm whereas the largest recorded female is 246.3 mm long.

Distribution

The Fraser Island short-necked turtle is restricted in range to Fraser Island. Fraser Island is the largest sand dune island in the world with a length of 124 km and a maximum width of 24 km. At its highest point, it is 240 m above sea level (Whitehouse, 1968). It lies off the Queensland coast between latitudes 24|o|40' and 25|o|50' South and longitudes 152|o|55' and 153|o|20' East. Fraser Island is well endowed with freshwater lakes, and is one of the few areas in Australia, outside the central plateau of Tasmania and the western district of Victoria, that might be considered a lakes district.

The turtles have been found in Lakes Allom, Benaroon, Birrabeen, Boemingen, Boomerang, Bowarrady, Coomboo, Freshwater, Garawongera, Hidden, Jennings, MacKenzie, Ocean and Wabby on Fraser Island. They probably inhabit all of the permanent dune lakes of the island. A single shell closely matching those from Fraser Island in shape and colour was found in Cooloola National Park on the adjacent mainland. The species does not occur in Freshwater Lake at Cooloola.

Population Status

Lake Coomboo, where the turtles have been intensively studied (Georges, 1982b) had an estimated population size of 718 +/- 14 turtles of which 668 were captured and marked. Lake Coomboo has a surface area of 16.7 ha of which 8.8 ha are accessible to the turtles, yielding a population density of 87 turtles per ha. total biomass was 253.5 +/- 5.7 kg, equivalent to 28.8 +/- 0.7 kg/ha. Biomass production in 1977/78 was 18.5 +/- 3.4 kg per ha with nett production (including reproductive output) of between 1.85 and 2.15 g/m|2|/yr. These figures compare favourably with those calculated for chelid turtles in other Australian environments (Chessman, 1978) and indicate that the Fraser Island short-neck is not currently endangered by low population sizes. However, it should be considered rare in the sense of an extremely limited distribution and narrow habitat requirements.

Habitat

The Fraser Island short-necked turtle inhabits only permanent dune lakes and is not found in the numerous permanent streams or ephemeral ponds and swamps of the island. The dune lakes form in sand dune depressions and so are closed with surface inflows but generally no outflows. They are effectively isolated from the ocean and other waterbodies. They are dystrophic with dilute acidic waters (pH 4.0-6.0) and high accumulations of organic material of terrestrial origin (Bayly 1964; Bayly et al., 1975). The dominance of humic acids among this organic material and the relatively low pH are not conducive to bacterial degradation, so that particulate and dissolved humic compounds are metabolized only very slowly (Wetzel, 1975). The brown colour of the water severely limits penetration of light (Bayly, 1975) which, together with low concentrations of inorganic ions, restricts photosynthetic activity. Phytoplankton is poorly developed in lakes on Fraser Island (Bayly, 1964) and chlorophyll-a concentrations (Miller, 1975) fall within the range of concentrations reported for dystrophic and ultra-oligotrophic lakes elsewhere (Wetzel, 1975). Limited photosynthesis and slow bacterial degradation of the abundant humic materials presumably result in low secondary production. Low production, virtual isolation from other waterbodies, and possible direct effects of the organic acids and other secondary compounds on potential inhabitants (Janzen, 1974), explain the low biotic diversity of perched dune lakes. Highest turtle population densities occur in humified tea-coloured lakes with relatively complex littoral vegetation comprising reed beds of the genera Lepironia and Baumea. However, the less abundant turtles from the clear Lake MacKenzie are in far better condition, judged by the extent of internal fat bodies, than are those of the tea coloured lakes, so it is difficult to determine the habitat characteristics best suited to the turtles.

Diet

The Fraser Island short-neck is omnivorous. Plant food consists principally of the freshly sprouted shoots of sedges (Lepironia articulata and Baumea sp.) exposed when the turtles dig in the sand at the base of these plants (Georges, 1982b). Filamentous algae and the bladderwort, Utricularia sp., were found in substantial quantities in a few individuals inhabiting tea coloured lakes. In clear lakes, filamentous algae was much more abundant, and formed a major component of the diets of turtles there. Animal foods included caddisfly larvae (Leptoceridae), midge larvae and pupae (Chaoborinae, Chironomidae, Ceratopogonidae), dragonfly nymphs (Anisoptera), decapod crustaceans (Cherax robustus and Caridina indistincta). Mayfly nymphs (Leptophlebiidae), damselfly nymphs (Zygoptera), alderfly larvae (Sialidae) and beetle larvae were also present in the stomachs of a few individuals. Terrestrial foods, insects that fell upon the water, were important components in the diet, especially for smaller turtles. There is a distinct shift in diet with age, with small juveniles feeding almost exclusively on small insect larvae and crustaceans while larger turtles turn to vegetation and larger varieties of insect larvae and crustacean. The diets of mature males and females of the same size do not differ appreciably.

Reproduction

Males mature when the carapace attains a length of between 106 and 115 mm long and at an age of about seven to ten years (Georges, 1982a). Mature males exhibit a post-nuptial pattern of spermatogenesis typical of temperate-zone turtles elsewhere, with a peak in spermatogenic activity in autumn and a cessation of activity in the nesting season in spring and early summer (Georges, 1983). Sperm are abundant in the epididymal canals throughout the year. Mating occurs in autumn, late winter and spring.

Females mature at a carapace length of about 150 to 155 mm long, at an age of seven or eight years (Georges, 1982a). Yolk begins to accumulate in the ovaries of females in late summer, and the accumulation continues unabated through the mild winters (Georges, 1983). Ovulations occur from late winter to mid-summer and any developed follicles remaining in late summer are resorbed. Up to three clutches of hard-shelled ellipsoid eggs are laid per season. Eggs measure between 28.8 and 37.0 mm in length (mean 33.7 +/- 2.4 mm, n = 42) and between 17.5 and 20.9 mm in width (19.3 +/- 1.4 mm). They weigh between 5.2 and 8.7 g (mean 7.44 +/- 0.14 g) (Georges, 1983). Each clutch contains between four and 10 eggs; the number is strongly correlated with maternal body size. Reproductive potential ranges from 12 eggs per annum for a female that has recently matured (carapace length c. 150 mm) to 30 eggs per annum for a full-sized female (length c. 250 mm).

There are few data on the nesting. The distribution of nests disturbed by predators indicates that the turtles nest fairly close to water (typically less than 40 m) in open sandy patches. Nesting activity is greatest following rain.

Activity and Growth

The turtles are diurnally active, with peaks of activity in morning and afternoon, especially in the warmer months. Although active throughout the year , the turtles have a seasonal cycle of activity, most pronounced in small turtles, that bears a direct relationship to environmental temperature. Growth also correlates well with the annual cycles of daylength and ambient temperatures. Growth ceases in winter, even though the turtles remain active and feed in all months. Growth rates are poorly correlated with size, so it is impossible to calculate a satisfactory relationship between size and age. In general juveniles grow much faster than adults, and females grow faster than males.

Population Dynamics

A marked population of 668 turtles in Lake Coomboo (153 mature females, 142 mature males) has the potential to produce 3532 offspring in a single year. This potential is not realized because some mature females fail to produce the maximum number of eggs within their capability, and because of devastatingly high mortality on eggs and hatchlings. Desiccation and predation (goannas, dingoes, water rats) were likely causes of this mortality, and substantial recruitment may occur only in the occasional years when conditions are favourable. Once in the water, mortality of turtles of all sizes appears to be very low. Populations are probably sustained by a trickle-feed recruitment, possible only because of the longevity of adult turtles.

Threats to Survival

The Fraser Island Short-necked turtle is not directly threatened by activities such as harvesting or trade, but should be considered vulnerable because of the fragility of its habitat. Dune lakes, as closed oligotrophic systems, are considered to be particularly sensitive to disturbance in their catchments. Natural events such as rainfall and fire influence the flow of nutrients and other materials into the lakes where it accumulates and presumably influences production (see Kennett and Georges, 1990). The same may be true of some forestry activities involving prescribed burns and logging in lake catchments, of sand-mining and of the incidental effects of tourism. Between the end of the 1960's and the mid 1970s, the annual number of visitors increased from 5,000 to 20,000 and by 1985 almost 200,000 people visited the region each year (Fitzgerald, 1991). In 1988-90, Fraser Island attracted 213,000 visitors. A 2-3% annual increase in Australian tourists and a 20-25% annual increase in overseas tourists has been forcast (Fitzgerald, 1991). The lakes form a focus for tourist activity for those who venture inland. Gradual eutrophication of the dune lakes is a real possibility, though the likely impact on the turtles is not known. Already Lake Wabby and Ocean Lake are showing signs of eutrophication (Angela Arthington in her submission to Fitzgerald, 1991).

Conservation Measures Taken

The Fraser Island Short-neck benefits from State and Federal legislation prohibiting the collection and exploitation of native wildlife. The turtles are protected under Queensland wildlife legislation. Export is prohibited under the Wildlife Protection (Regulation of Exports and Imports) Act 1982 administered by the Australian National Parks and Wildlife Service.

Until 1971, all but a few freehold leases and a strip of coastal dunes 1 km wide and extending the length of the island, came under the jurisdiction of the Queensland Forestry Department. They set aside 23 "Beauty Spots" for special protection, 16 of which included dune lakes. Logging was not conducted in the vicinity of these localities and activities of campers was regulated so as to minimize the potential impact on the lakes through nutrient enrichment by detergents and sewage. In 1971, the Queensland Government set aside a National Park at the northern end of the Island, but it included none of the beauty spots previously designated by Forestry and none of the lakes mentioned above as home to the Fraser Island Short-neck. Under continued public pressure, the boundaries of the park were reluctantly extended in each of 1973, 1977, 1979 and 1990. It now includes Lake Bowarrady and the Boomerang Lakes, each inhabited by a large population of turtles. These populations are afforded the protection of management by the Queensland National Parks and Wildlife Service.

Following the report of the Fitzgerald Inquiry (Fitzgerald, 1991), the Queensland Government has stopped logging of the island's forests and is supporting moves to have the island recognized under World Heritage Legislation.

Conservation Measures Proposed

A current proposal to further extend the boundaries of the park to encompass Lakes Freshwater, Deepwater, Allom, Coomboo and Hidden Lake deserves the strongest support. Consideration should be given to creating reserves to include populations in Lakes Birrabeen, Benaroon, Boemingen, Wabby and McKenzie. These lakes are set in spectacular country and are subject to the highest levels of tourist activity on the island. Adequate management leading to their long-term conservation requires the active participation of a management authority such as the National Parks and Wildlife Service.

Consideration should also be given to declaring some of the island as scientific reserves to be available primarily for scientific research, as pristine reference stations for environmental monitoring, and for non-mechanized, non-disruptive forms of ecotourism. These could be declared as Category I Protected areas in line with the IUCN General Assembly Resolutions 16/34 and 17/36 (Eidsvik, 1990). The region including Lake Coomboo, Lake Freshwater, Deepwater Lake and perhaps Hidden Lake would be an eminantly suitable candidate for such a declaration. Deepwater lake is a water table window whereas the other lakes are perched. Lake Freshwater is currently inaccessible by road.

There is an urgent need to decide the status of the Fraser Island Short-neck as a species or distinctive subspecies. Lack of a formal description and recognition that goes with it impedes decisions on the conservation priority that should be given the form.

Populations of the turtles in different lakes show considerable variability in colouration and to a lesser extent in morphology of the shell, perhaps consequences of founder effect or lenghty isolation of small populations. In the absence of immediate demographic threats, perhaps research should be focussed on genetic variability among lake populations to form the foundation of a strategy for minimizing the impact on the Fraser Island Short-neck of the inevitable intrusion of tourist-related development and other human activities should it lead to habitat degredation in the future.

References

Bayly, I.A.E. (1964). Chemical and biological studies on some acidic lakes of east Australian sandy coastal lowlands. Aust. J. Marine Freshwat. Res. 15:56-72.

Bayly, I.A.E. (1975). Interactions between terrestrial and aquatic environments: Queensland's coastal lakes. Proc. Ecol. Soc. Aust. 9:325-328.

Bayly, I.A.E., Ebsworth, E.P. and Wan, H.F. (1975). Studies on the lakes of Fraser Island, Queensland. Aust. J. Mar. Freshwat. Res. 26:1-13.

Cogger, H.G. (1988). Reptiles and Amphibians of Australia. A.H. and A.W. Reed: Sydney.

Eidsvik, H.K. (1990). A framework for the classification of terrestrial and marine protected areas. Report to the Meeting of the IUCN Commission on National Parks and Protected Areas, Perth, Australia, November 1990.

Fitzgerald, G.E. (1991). Report of the Commission of Inquiry into the conservation, management and use of Fraser Island and the Great Sandy Region. Report to the Queensland Government, Brisbane, 13-May-91. ISBN 0 7242 4383 6 and 0 7242 4390 9.

Georges, A. (1982a). Ecological studies of Krefft's River Tortoise, Emydura krefftii (Gray), from Fraser Island, Queensland. Ph.D. Thesis: University of Queensland, Brisbane.

Georges, A. (1982b). Diet of the Australian freshwater turtle Emydura krefftii (Chelonia: Chelidae). Copeia 1982:331-336.

Georges, A.(1983). Reproduction of the Australian freshwater turtle Emydura krefftii (Chelonia: Chelidae). J. Zool., London. 201:331-350.

Georges, A. (1985). Reproduction and reduced body size of reptiles in unproductive environments. Pp. 311-318 in Biology of Australasian Frogs and Reptiles ed. by G. Grigg, R. Shine, and H. Ehmann, Royal Society of New South Wales, Sydney.

Janzen, D.R. (1974). Tropical blackwater rivers, animals, and mast fruiting by the Dipterocarpaceae. Biotropica 6:69-103.

Kennett, R.M. and Georges, A. (1990). Habitat utilization and its relationship to growth and reproduction of the eastern long-necked turtle, Chelodina longicollis (Testudinata: Chelidae), from Australia. Herpetologica 46:22-33.

Miller, G.J. (1975). The potential threat of sand mining to the nutrient status of Fraser Island Lakes. Fraser Island Environmental Enquiry, Exhibit 453 (Australian Archives: CRS A3911, item 453).

Wetzel, R.G. (1975). Limnology. W.B. Saunders: London.

Whitehouse, F.W. (1968). Fraser Island -- geology and geomorphology. Queensl. Nat. 19:4-9.

Sources

This article was prepared by Arthur Georges [Institute for Applied Ecology, University of Canberra, P.O. Box 1, Belconnen ACT 2616, Australia] and John Legler [Biology Department, University of Utah, 201 South Biology, Salt Lake City, Utah 84112, U.S.A.] for the volume The Conservation Biology of Freshwater Turtles currently being prepared by the IUCN/SSC, Gland, Switzerland. Information on this volume can be obtained from Dr Anders Rhodin, Chelonian Research Foundation, 168 Goodrich Street, Lunenburg, Massacheusetts 01462, USA [E-mail: RhodinCRF@aol.com].