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Four species of sea turtles nest on the islands and coastline of the North West Shelf of Western Australia; hawksbill turtles (Eretmochelys imbricata), green turtles (Chelonia mydas), loggerhead turtles (Caretta caretta) and flatback turtles (Natator depressus) (Prince 1994). The nesting season commences around September-October for hawksbill turtles and October-November for the other three species. Nesting typically ends by March, with hatchlings emerging until May.

In March 1998, the authors were members of a survey team mapping the subtidal, intertidal and supratidal habitats of the near shore island chains, consisting of the Mangrove Islands, Mary Anne Group and the Passage Islands. The habitat information was required for the environmental impact assessment and management of a proposed oil and gas-drilling program. A specific objective of the March survey was to identify and quantify any sea turtle presence. This report constitutes a summary of some of the observations arising from the survey.

In the area extending from the Passage Islands to the Mangrove Islands, hard substrates were dominated by macroalgae and shallow sediments often supported seagrass meadows. The aptly named Mangrove Islands, supporting dense stands of flowering Rhizophora stylosa and fruiting Avicennia marina. Small submerged coral reefs (known locally as bomboras) were also scattered throughout the area.

The subtidal and intertidal zones were dominated by Sargassum, Padina and Dictyota macroalgae while the most common seagrasses were Halophila spp. (H. spinosa, H. ovalis, H. ovata and H. minor), Halodule and Cymodocea. Common coral genera included Porites, Turbinaria and Goniopora. Coral bleaching was widespread at the time of the survey with approximately 80% of the corals observed in the Mangrove and Mary Anne Island groups exhibiting severe bleaching (M. Ford, pers. comm.).

Sea turtles were commonly observed at sea throughout the region, however the greatest concentrations were noted around the islands at the southern end of the island chains and over seagrass beds in near shore (mainland coast) waters. An area of particular importance was a broad shallow limestone platform between Middle and North Mangrove Island. The platform substrate supported a dense meadow of Halodule mixed with Halophila and Cymodocea seagrass growing on the thin veneer of soft sediments. While no animals were observed, the seagrass beds were characterized by Dugong dugon scars. The area was also favored as an area for green turtles. Whilst standing in water 40 cm deep it was possible to count in excess of 10 individuals within a 50 m radius. Most animals appeared to be resting or feeding in the warm shallow water and could be approached easily. Of note was the fact that most of the animals observed were mature males (determined by obvious elongated tails).

On the North West Shelf male turtles are typically only seen in waters off nesting beaches during the mating season. Occasionally they are found on their backs on the nesting beaches where they have rolled off a female while mating in the surf zone on a falling tide. Observation of such a large congregation in shallow water is unusual in the region.

The team also found another area of high turtle density amongst the mangroves on the northern coast of North Mangrove Island. Numerous sharks, rays, fish and sea turtles were observed following the rising tide across the intertidal flats. Most of the sea turtles were observed sheltering or feeding in the water under the mangrove canopy which was 50 cm deep. A single large female green turtle was observed on the seaward edge of the mangrove fringe, raising her head clear of the water to break off and eat the mangrove (Avicennia marina) leaves. Only the physical impact of the slowly drifting inflatable boat bumping against her carapace that caused her to break from her feeding and move a short distance away. A closer inspection of the branch showed that the animal had cleanly removed parts of a number of leaves. The leaves appeared free of epiphytic algae or invertebrates. Although the authors were unable to find reference to mangroves as a sea turtle food source in the scientific literature, feeding on leaves and fruits of mangrove by sea turtles in King Sound is known to the people of One Arm Point Aboriginal community in the West Kimberley region of north-western Australia (Prince, pers. comm.).

PRINCE, R.I.T. (1994) Status of the Western Australian Marine Turtle Populations: The Western Australian Marine Turtle Project 1986-1990. In R. James (Compiler). Proceedings of the Australian Marine Turtle Conservation Workshop, November 1990. pp 1-14.