MTN 52:4-5 Nesting Seasonality of the Flatback Turtle at Bare Sand Island, Northern Territory Australia

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Nesting Seasonality of the Flatback Turtle at Bare Sand Island, Northern Territory Australia

Michael L. Guinea, Paul G. Ryan, Lynda Umback and Leslee Hills
Faculty of Science, Northern Territory University, P. O. Box 40146 Casuarina 0811, Northern Territory Australia

The flatback sea turtle (Natator depressus Garman) nests during the summer (October to January) at the southern limits of its eastern Australian nesting distribution at Mon Repos (24°48'S, 152°27'E) (Limpus 1971), yet nests throughout the year on Crab Island (10°59'S, 142°06'E) in the north-eastern Gulf of Carpentaria (Limpus et al. 1983) and on Deliverance Island (09°32'S, 141°31'E) in Torres Strait (Limpus et al. 1989). Year-round nesting is normal for populations in the Northern Territory (Fry 1913; Worrell 1963; Cogger and Lindner 1969; Pritchard 1979). Nearly a century ago, Mr. Hugh Christie nominated Bare Sand and Quail islands (12°32'S, 130°25'E) as year-round nesting localities for flatback sea turtles (Fry 1913).

Since 1986, the flatback rookery on the mainland coast at Fog Bay (12°41'S, 130°21'E) has been monitored on a near monthly basis and found to be strongly seasonal from May to October, with a peak in nesting in July. After one of us (L.H.) observed five and six flatbacks nesting on Bare Sand Island on 15 and 16 July 1989, respectively, Bare Sand and Quail islands (24 km north of Fog Bay) were surveyed on 16-17 December 1989 and again on 26 May 1990. Quail Island is a Royal Australian Air Force Bombing Range and as such is a prohibited entry area; thus, the dunes were inspected for evidence of recent turtle nesting by boating parallel to the shore. There were no tracks or fresh or hatched nests to indicate that any nesting had occurred on these islands during November-December 1989 or April-May 1990. On Bare Sand Island, shells of the characteristically large eggs of the flatback were exposed in the sand dunes by wind and wave erosion. The discovery of the dehydrated remains of a hatchling in the interior of the island further supports our identification of flatback egg shells.

We conclude that nesting on Bare Sand Island is seasonal and in synchrony with that at Fog Bay. The apparent contradiction with earlier reports may be explained by any of the following:

Mr. Christie was incorrect in his comments on the seasonality of nesting (Fry 1913). This is unlikely in view of the years he spent as lighthouse keeper at Charles Point, some 30 km to the east of Bare Sand Island.
The population nesting on Bare Sand and Quail islands was disrupted when the area was used as an allied air force bombing range from 1942 to the present day.
A change in environmental conditions since the early years of this century now constrains flatback nesting to the winter months.

We would welcome hearing from other researchers regarding the feasibility of any of our possible explanations.

Cogger, H. G. and D. A. Lindner 1969. Marine turtles in Northern Australia. Aust. Zool. 15(2):150-9.

Fry, D. B. 1913. On the status of Chelonia depressa Garman. Rec. Aust. Mus. 10(7):159-85.

Limpus, C. J. 1971. The flatback turtle Chelonia depressa Garman in south east Queensland, Australia. Herpetologica 27(4):431-446.

Limpus, C. J., C. J. Parmenter, V. Baker, and A. Fleay. 1983. The Crab Island sea turtle rookery in the north-eastern Gulf of Carpentaria. Aust. Wildl. Res. 10:173-84.

Limpus, C. J., D. Zeller, D. Kwan, and W. MacFarlane. 1989. Sea turtle rookeries in the north-western Torres Strait. Aust. Wildl. Res. 16:517-25.

Pritchard, P. C. H. 1979. Encyclopedia of Turtles. T.F.H. Publications, New Jersey.

Worrell, E. 1963. Reptiles of Australia. Angus and Robertson, Sydney.