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Marine Turtle Newsletter 67:5-6, © 1994
Robert I. T. Prince
Wildlife Research Centre, Department of Conservation and Land Management,
P. O. Box 51, Wanneroo, Western Australia 6065, Australia
Work undertaken as part of the Western Australian Marine Turtle Project (WAMTP) prior to the 1993/94 summer nesting season for sea turtles had indicated the importance of the Turtle Bay area of northern Dirk Hartog Island, Shark Bay (ca. 25°30'S, 113°00'E), as one of only two major breeding locations for the loggerhead turtle (Caretta caretta) in the western Australian region, the other location being the Muiron Islands off Exmouth Gulf. It was also thought that some green turtle (Chelonia mydas) nesting might occur here, but logistic and resource limits had prevented any detailed on-site work before this time.
Funding made available during 1993 to assist in preparation of management plans for the Shark Bay World Heritage Area allowed an on-site visit to be made from 10-21 January 1994. During these 11 nights, two Department of Conservation and Land Management (CALM) personnel (Dr. R. I. T. (Bob) Prince, WAMTP Leader, and Wildlife Officer Don Noble) and four volunteers (Carol Davies, Karen Girardin, Mike Bascombe, Max Coyne) investigated nesting turtle activity on the Turtle Bay - Cape Levillain beach complex on Dirk Hartog Island.
Rough seas over two nights reduced turtle nesting activity substantially, but on other favourable nights up to 100 or more female loggerheads beached to attempt nesting. No other species were seen onshore. The few recently dead animals found on the beaches (seemingly dead from post-nesting exhaustion) and dried skeletal material found in the dunes backing the Cape Levillain beaches were identified to be loggerhead. During the mid- 1993/94 nesting season, and probably at all times, this beach complex appears to be predominantly, if not exclusively, a loggerhead breeding site.
The comparative abundance of nesting females found here was not expected, but a pleasant surprise. Four hundred and thirty turtles of the over 500 seen were tagged with WAMTP 'WA' series titanium tags. Shortage of tags available on-site prevented a better result. One hundred and thirty of these turtles were double-tagged, the remainder single-tagged. Blood samples for population genetics work were taken from 20 adult turtles. The population of breeding adult female loggerheads attending the Dirk Hartog Island rookery for the 1993/94 nesting season can reasonably be assumed to be greater than the 500+ individuals seen during this island visit. The expected total could well be 800-1000 turtles.
Aerial assessment of overnight beach use was made on four separate occasions outside the period of the visit. These flights were undertaken in the early morning around peak spring tide to take advantage of the fresh swept exposed inter-tidal zone allowing clear distinction of turtle movements off the beach attributable to animals on-shore the previous night and leaving after the nocturnal high tide. Turtle tracks do not generally remain visible here after the next (daytime) high water, and those on the western-most fine sandy beaches of the complex can be obliterated in a few hours by strong winds (often to 35 kph, or more). Flight observations revealed very low activity in early November 1993, which increased substantially in late November, and appeared to peak in late December at levels consistent with on-site observations in January 1994. The final flight observations of 3 April 1994 showed moderate continuing activity of nesting adults (around expected mid-November level).
Observations during the latter flight also detected evidence of many nest emergences. We saw no hatchlings mid-January, and eggs washed (by the very rough seas mentioned above) from beach-front nests that we examined were all in an early phase of embryo development. A late summer peak of nesting activity at this rookery with substantial hatchling emergence through autumn as suggested is consistent with past observations we have made of beach-washed post-hatchling loggerheads found on the lower west coast and south coast beaches of Western Australia, generally from late June through September-October. These strandings result we believe from interaction between transport of the young turtles via the Leeuwin Current and shoreward forcing of surface waters by the westerly storms which bring rain to this part of the world. We do not yet have any other evidence of where the assumed majority of post-hatchling loggerheads produced from the western Australian rookeries are distributed.
In summary, the Dirk Hartog Island, Shark Bay nesting site may accommodate up to 75% of the breeding loggerhead turtles of the western Australian populations. It is the major breeding site for loggerheads on the eastern rim of the southern Indian Ocean. Its importance to conservation of the regional loggerhead turtle populations, and current status of these populations has yet to be properly assessed. The site is, however, clearly a major feature of the Shark Bay World Heritage Area.