Sea Turtle Nesting in the Australian Territory of Ashmore and Cartier Islands, Eastern Indian Ocean

Scott Whiting, Michael Guinea, G. Desmond Pike

Abstract

The reproductive biology of sea turtles nesting on the three islands of Ashmore Reef was investigated between monsoons (September and October) from 1994 to 1998. Nesting was predominantly by green turtles (Chelonia mydas), with a small number of hawksbill turtles (Eretmochelys imbricata). Successful nesting by green turtles was hampered by dry, loose sand in sun-exposed areas. Green turtles emerged an average of 6.5 times for each successful nest. Hawksbill turtles usually nested successful on their first attempt. Preliminary evidence suggested that green turtle nesting activity hinders the establishment of the strand plant, Argusia argentea Green turtles also nest on Cartier Island, which is unvegetated Here, turtles succumb to the high temperatures during daylight hours and hunting by humans. The islands, reefs and surrounding waters at Ashmore Reef comprises areas set aside for conservation but contains culturally significant sites and traditional fishing areas. Being closer to Indonesia than to Australia, the Ashmore and Cartier island Territory requires careful management to maintain their high conservation value, while allowing traditional use by artisanal fishers from Indonesia.

Introduction

Sea turtles in Australia have been relatively un-exploited and a wealth of information now exists for selected rookeries in Queensland (Limpus et al. 1984, Limpus & Reed 1985) and Western Australia (Prince 1994). Little is known about the sea turtles of the islands on the north-western edge of the Australian continental shelf. Islands within the Ashmore and Cartier Island Territory, Indian Ocean, under Australian governance, are more than 600 km from Broome and 800 km from Darwin. Detailed descriptions of the habitats and lists of the fauna and flora of Ashmore Reef and Cartier Island are given in the management plan for the islands (ANPWS 1989), and by Berry (1993) and Pike & Leach (1997). Ashmore Reef was declared a National Nature Reserve by the Australian Commonwealth Government in 1983 (ANPWS, 1989), however, the great distance from Australia hampers protection of the fauna. The nearest islands of Indonesia are within 100 km. A Memorandum of Understanding (MOU) between Australian and Indonesian Governments established terms and conditions under which traditional Indonesian fishers were able to access Australian waters in order to carry out a range of fishing activities (ANPWS 1989, Vail & Russell 1989), but in the early years of the Nature Reserve, harvesting of nesting turtles by Indonesian fishers was virtually uncontrolled. This was in spite of the provisions of the MOU which specifically prohibited the killing of protected species.

In spite of numerous Australian patrols to the region since the early 1950's, there are few accounts from which the status of sea turtles can be assessed. Historical reports indicate the presence of green turtles and their nests on West Island, Ashmore Reef (Serventy 1952). A turtle monitoring program was commenced at Ashmore Reef in 1987, with tagging and limited satellite tracking (Spring & Pike 1998). Green (Chelonia mydas) and hawksbill (Eretmochelys imbricata) turtles nest on all of the islands in the reserve (Guinea 1995). A loggerhead (Caretta caretta) sea turtle was reported nesting on West Island (ANPWS 1989).

The aims of this study were to evaluate the number of sea turtles nesting on the islands, identify factors that hampered their nesting success, and identify the species of sea turtles and their relative abundance on the reef flat feeding ground.

Study Area

The Ashmore Reef National Nature Reserve includes three islands and numerous sand bars, extensive coral reefs and surrounding waters to a depth of 50m and encompasses an area of 583 square km (ANPWS 1989). The reef is approximately 28 km in length and approximately 16 km at its widest part. Three vegetated islands, West Island, Middle Island and East Island, sit along the east-west axis of the reef and are each about 8 km apart. Tides are semidiurnal with a maximum spring tide range of 4.58 m (ANPWS 1989). Sea turtle studies focused on West Island (12°15'S; 122^°58'E) with short trips to Middle and East Islands. The region was visited for 24 days in 1994 (19 September to 13 October; see Guinea 1995), 10 days in 1996 (2-11 September), 12 days in 1997 (22 September to 5 October) and 20 days in 1998 (20 September to 12 October). Cartier Island, 60 km south of Ashmore Reef, was visited for one day in 1992 (3 May; see Guinea 1993) and one day in 1998 (13 October).

Methods and Materials

Standard methods followed those described by Limpus et al. (1983a, b), including recording dates from mid-day for the following 24 hours. Beaches were patrolled on foot from the time at which the rising tide flooded the reef flat at night. Ambient light was used to locate turtles. Painted numbers on the carapace served as a temporary mark before laying and a more permanent, individually- numbered titanium tag was applied to the axial scale of each front flipper of the turtles as they either laid or returned to the water. Curved carapace length (CCL) was taken with a flexible fiberglass tape measure ( ± 0.5 cm) from the leading edge of the nuchal scale to the posterior edge of the notch between the supracaudal scales. Curved carapace width (CCW) was measured at the widest part of the carapace with the same tape.

Results

A total of 44 green turtles were tagged on West Island. A total of 29 nests resulted from 193 attempts (established by tracks on the beach), for a gross ratio of 6.58 sets of tracks for each successful nest. The numbers of tracks, nests and individual turtles, and the average tracks and nests per night varied considerably between years (Table I). In 1994, 15 nests resulted from 106 tracks made by 19 turtles; in 1996, nine nests resulted from 53 tracks made by 23 turtles; in 1997, a single nest resulted from 18 tracks by at least four turtles; and in 1998, four nests resulted form 16 tracks by at least two turtles. It was common for turtles that attempted to nest in open sand to excavate three egg chambers, generally unsuccessfully. Up to five body pits and four egg chambers were attempted on a single night by one turtle. Another returned on three successive nights, and three separate times in one night. Another turtle attempted to nest on six nights over an eight day period, all unsuccessful.

As green turtle eggs were found on the reef flat (Guinea 1995), some turtles may have released eggs into the sea after repeated unsuccessful nesting attempts. Prolonged nesting activity during the night resulted in several turtles being stranded on the reef flat at low tide the following morning, as they made their way from the island back to sea.

Table I: Number of tracks, successful nests and green turtles, C. mydas, on West Island, Ashmore Reef from 1994 to 1998.

Carapace lengths (CCL) of 44 green turtles ranged from 92.0 to 113.0 cm (mean = 101.1 cm, SD = 4.48). Carapace widths (CCW) ranged from 81.5 to 101.5 cm (mean = 91.5 cm, SD = 4.36). Three individuals were recaptures of turtles which had been tagged prior to 1994. Two other turtles retained rectangular scars and tears between scales that indicated their tags had been lost. The frequency distribution of curved carapace lengths was biased towards smaller individuals (Fig. 1).

Nesting hawksbill turtles were less common than green turtles during the visits. Hawksbill tracks were identified on West Island at the start of the 1994 visit, but none nested during that survey (Guinea 1995) or in 1996 and 1997. Two hawksbill turtles nested on West Island during the 1998 visit. Both nested under Argusia bushes.

On Cartier Island, there was evidence of green turtle nesting in 1992 and 1998. In 1992, a single green nested and several nests hatched. In 1998, one turtle nested the night before our visit, but there was no sign of hatched nests. On both occasions, a number of dead adults were found in the central depression of the island. As all were decomposed and dehydrated, it was not possible to estimate how recently the animals had died.

In 1996, the carcass of an adult female turtle was found buried on West Island. The animal had been butchered and the flippers, body organs and meat had been removed. On Cartier island in 1998, two adults, one of which was buried, had the flippers, body organs and meat removed.

Discussion

Of the three vegetated islands at Ashmore Reef, only West Island experienced any significant sea turtle nesting during the surveys. Green turtles were the most abundant species nesting on West Island. Their nesting activity was greater during the period of spring tides, but their nesting success remained low. Turtles on West Island spent considerable energy attempting to dig a suitable egg chamber and made on average 6.58 visits ashore before either laying or presumably releasing the eggs at sea. The nesting success rate was unlike that at rookeries on the southern Great Barrier Reef, where green turtles usually nest successfully on their first visit ashore after digging an average 2.4 egg chambers per clutch (Bustard & Tognetti 1969).

Fig 1: Frequency distribution of curved carapace length (CCL) of green turtles, C. mydas, nesting on West Island Ashmore Reef from 1994 to 1998.

It is suggested that the dry sand was responsible for the low nesting success (multiple nesting attempts) during the dry season surveys (Guinea 1995). Mortimer (1990) found that egg chambers in dry sand repeatedly collapsed and eventually turtles attempted another body pit and egg chamber elsewhere. Bustard & Greenham (1968) found that egg chambers held their shape where the sand was moist with either sea water (in the intertidal region) or from moisture associated with the Argusia bushes, or alternatively when the rootlets of grass supported the walls of the chamber. The origin of the moisture beneath Argusia bushes not known, but could have been generated by either internal dew formation (Mortimer 1990), decomposition of leaf litter, or capillary action of ground water. At Cartier and Ashmore Island Territory, the dry sand caused successive nesting attempt failure, which placed the eggs in danger by being either deposited in the intertidal zone, or released at sea.

Multiple nesting attempts and the prolonged time on the beach increased the probability of adult females being stranded on the reef flat by the ebbing tide and exposed to (possibly lethal) daytime temperatures. A study of the thermal and hydric stability of the dunes throughout the year and the impact of sea turtle nesting on the dune vegetation would provide useful information on the environmental factors that contribute to egg survivorship. If human activities have reduced the vegetation on West Island, then measures should be taken to prohibit the removal of foliage from the island and initiate an active program of revegetation on the dunes. It is believed that rain in December would enhance successful nest construction, and that during the wet season, the number of tracks on the island should be similar to the number of nests. This track to nest ratio must also be considered when conducting aerial surveys to establish the seasonality of, or changes in, nesting activity by green turtles on the dry islands of the eastern Indian Ocean.

Nesting female green turtles were similar in size (CCL) to nesting green turtles in Western Australia (R. Prince pers. comm.), but smaller than those reported on the Great Barrier Reef, (Limpus et al. 1984), and within the global range reported for the species (Hirth 1980). Data are too scarce to elaborate on the apparent bias to smaller individuals in the nesting population, when larger females were missing during the surveys on Ashmore Reef. Further surveys during the peak nesting season are required to determine the size and age structure of the nesting population.

Mitochondrial DNA studies indicated the nesting population on Ashmore Reef comprise a discrete genetic stock of green turtles (C. Limpus pers. comm.). If the absence of large turtles can be attributed to harvesting, it is unlikely that recruitment from neighbouring areas will take place, as Scott Reef, the Australian mainland and Indonesia each comprise their own discrete stocks. The resident (non-nesting) green turtles on Ashmore Reef are likely to belong to other gene pools in the South East Asian and Australian region.

In the past, an Environment Australia contract vessel on station at Ashmore Reef has deterred turtle harvests for many months of the year but in recent years the vessel has been recalled to Australia during the December to March cyclone season, when most green turtle nesting is thought to occur. During the brief periods of this survey, poaching of sea turtles was evident at West Island and Cartier Island and on Scott Reef (Guinea 1995) in spite of sustained aerial and maritime surveillance by Australian authorities.

Conclusions

Conservation of sea turtles on Ashmore Reef and Cattier Island Teritory also requires management of the people that harvest the sea turtles. Neighbouring governments must work towards joint management of shared turtle resources through enhanced cooperation, possibly by strengthening the existing MOU between Australia and Indonesia. Efforts should be directed at building the capacity of coastal communities in Indonesia and Australia to manage their remaining sea turtle populations, many of which may either nest or feed in the Ashmore Reef and Cartier Island Territory. Scientific endeavours should be directed towards quantifying the critical life history parameters of recruitment to feeding grounds, growth rates, age to maturity, recruitment to, and remigration within the nesting population, egg production and hatchling survivorship. These parameters are crucial for the construction of viable population models which are necessary for the management of sea turtles.

Acknowledgements

This study would not have been possible without the assistance of Environment Australia. We thank the officers and crew of the Royal Australian Navy patrol boats, Australian Customs Service and staff of Natural History New Zealand and the crew of the 'Evohe'. We thank the staff of the Environment Australia contract vessels 'Ocean Reaper' and 'Aurelia IV', Steve and Kate Tester and student volunteers, Keith Glenn and Andy Prindaville, for their assistance. This study was undertaken, in part, during Professional Development Leave (by MLG) from the Northern Territory University. Research permits were kindly provided by Environment Australia and the Animal Experimentation Ethics Committee of the Northern Territory University.

References

ANPWS, 1989. Ashmore Reef National Nature Reserve Plan of Management Canberra, Australian National Parks and Wildlife Service: 65pp.

Berry, P.F., 1993. Marine faunal surveys of Ashmore Reef and Cartier Island Northwestern Australia. Records of the Western Australian Museum, Supplement No. 44: 91pp.

Bustard, HR. & P. Greenham, 1968. Nesting behaviour of the green sea turtle on a Great Barrier Reef island. Herpetologica, 2: 93-102.

Bustard, H.R. & K.P. Tognetti, 1969. Green sea turtles: a discrete simulation of density dependent population regulation. Science 163: 939-941.

Guinea, ML., 1993. Reptilia, Aves and Mammalia. In: Survey of Marine Biological and Heritage Resources of Cartier and Hibernia Reefs, Timor Sea. (Russell, B.C. & J.R. Hanley, eds). Darwin, Northern Territory Museum of Arts and Sciences: 74-83.

Guinea, M.L., 1995. Sea turtles and sea snakes of Ashmore Reef National Nature Reserve. Report of Australian Nature Conservation Agency. Darwin, Northern Territory University: 116 p.

Hirth, H.F., 1980. Some aspects of the nesting behaviour and reproductive biology of sea turtles. American Zoologist 20: 507-523. 

Limpus, C.J. & P.C. Reed, 1985. The green turtle, Chelonia mydas, in Queensland: A preliminary description of the population structure in a coral reef feeding ground. In: Biology of Australasian Frogs and Reptiles (Grigg, G., R. Shine, & H. Ehmann, eds). Royal Zoological Society of New South Wales, Sydney: 47-52.

Limpus, C.J., C.J. Parmenter, V. Baker & A. Fleay, 1983a. The Crab Island sea turtle rookery in the North-Eastern Gulf of Carpentaria. Australian Wildlfe Research 10: 173-84.

Limpus, C.J., A. Fleay & M.L. Guinea, 1984. Sea turtles of the Capricornia Section of the Great Barrier Reef Marine Park. In: The Capricornia Section of the Great Barrier Reef past, present and future (Ward, W.T. & P. Saenger, eds). The Royal Society of Queensland. Brisbane: 61-78.

Limpus, C.J., J.D. Miller, V. Baker & E. McLachlan, 1983b. The hawksbill turtle, Eretmochelys imbricata, in north eastern Australia: the Campbell Island rookery. Australian Wildlife Research 10:185-97.

Mortimer, J.A., 1990. The influence of beach sand characteristics on the nesting behaviour and clutch survival of green turtles (Chelonia mydas). Copeia 1990(3): 802-817.

Pike, G.D. & G.H. Leach, 1997. Handbook of the Vascular Plants of Ashmore Reef and Cartier Island. Parks Australia, Canberra: 154 p.

Prince, R.T., 1994. Status of the Western Australian marine turtle populations: the Western Australian Marine Turtle Project 1986-1990. In: Proceedings of the Australian Marine Turtle Conservation Workshop. 14-17 November 1990. Sea World Nara Resort, Cold Coast (James, R. ed). Qld. Department of Environment and Heritage and Australian Nature Conservation Agency, Canberra: 1-14.

Serventy, D.L., 1952. The bird islands of the Sahul Shelf. The Emu 52: 33-59.

Spring, C.S. & G.D. Pike, 1998. Tag recovery supports satellite tracking of a green turtle. Marine Turtle Newsletter 82: 8.

Vail, L. & B. Russell, 1989. Indonesian fishermen of Australia's North-West. Australian Natural History 23(3): 210-219.