Flatback turtle  (Natator depressus)

One of just seven species of sea turtle, the flatback turtle is distinguished by its restricted distribution and its very flat carapace, or upper shell (2). The fleshy carapace (3), which is composed of thin, bony scales (2), has an upwards turned rim, particularly towards the rear (2). The carapace may be grey, pale grey-green or olive in colour (3). The head and soft flippers are also olive-grey (2), while the underside is pale yellow (6). Flatback turtle hatchlings are olive-green with scales edged in black (3).

Having one of the most restricted ranges of any marine turtle (1), the flatback turtle is found only in the tropical waters of northern Australia, Papua New Guinea and Irian Jaya, and nests only in Australia (3).

The flatback turtle inhabits coastal waters over soft-bottomed sea beds (3). Like other marine turtles, its lays its eggs on sandy beaches, either on the Australian mainland or on offshore islands (2).

The breeding season of the flatback turtle varies depending on the location. For example, in southern Queensland, the flatback turtle begins mating in October and nesting occurs between October and January with a peak in December, while in northern Australia the flatback turtle nests between June and August. Each nesting season, the female lays between two and three clutches, 15 days apart (3).

The female hauls herself out of the ocean, usually at night (2), and finds a suitable spot on the beach; most nests are constructed on top of dunes or as high as possible on seaward slopes (7). A hole is then excavated using alternate sharp jerks of the hind-flippers to throw sand away. Into the hole, measuring around 22 centimetres across and 30 centimetres deep, the female lays an average of 50 to 60 eggs within ten minutes. The hole is then filled with sand, and further sand is piled on top to form a mound. The female then leaves for the sea (2).

Around six weeks later, the incubating eggs hatch and the hatchlings emerge from the nest chamber (2). The temperature at which the eggs have been incubating is critical; below 29°C and the clutch will be male, above 29°C and females are produced (3). The hatchlings instinctively head for the sea, fanning out as they run to the water rather than following each other (2). Unlike other sea turtles, flatback turtles do not spend any part of their life in the deep ocean, remaining instead in the surface waters of the continental shelf (3). Flatback turtles spend much of their day floating on the sea surface, basking in the sun, and it is not unusual to see birds resting on the turtle's back (2).

While little is known about the diet of the flatback turtle (3), it is thought to be a predominantly carnivorous species (7), feeding on organisms that are found on the sea floor. This includes cuttlefish, hydroids, soft corals, crinoids, molluscs and jellyfish (3).

There is a large range of threats which may be affecting populations of the flatback turtle. Flatback turtle eggs and hatchlings are threatened by tourism and recreation disturbing nesting beaches, the effects of light pollution, and harvesting by indigenous people. They are also vulnerable to predation by feral pigs, particularly on the Cape York Peninsula (8). Adult flatback turtles are harvested for their meat, and face additional threats such as entanglement in lost or discarded fishing nets, ingestion of marine debris, being struck by boats, and being caught as by-catch. Flatback turtles comprise the majority of the turtle by-catch (59 per cent) in trawls in the Northern Prawn Fishery (8). However, as one of the most poorly understood marine turtle species, there is insufficient information to determine to what extent the flatback turtle may be affected by these threats, and thus it has been classified as Data Deficient on the IUCN Red List (1).

The flatback turtle is classified as Vulnerable in Australia under the Environment Protection and Biodiversity Conservation Act 1999 (8), and there are a number of measures in place to protect the marine turtles of Australia. Much of the flatback turtle's habitat is protected. For example, 75 per cent of its nesting habitat is protected in Queensland and it occurs on the Great Barrier Reef, one of the world's most extensive protected areas. In addition, in an effort to reduce by-catch, Turtle Excluder Devices were made compulsory in the Northern Prawn Fishery in 2000, resulting in turtle mortalities being reduced to just five per cent of what they were in 1989-90 (8). A Recovery Plan for Marine Turtles in Australia was developed in 2003 and outlined a number of further measures necessary to reduce detrimental impacts on flatback turtles and other marine turtles of Australia. These included restricting boat speeds in areas of important marine turtle habitat, developing management plans for nesting beaches and a code of conduct for tour operators on beaches, as well as creating plans to ensure traditional indigenous harvests are undertaken in a sustainable manner (8).

For further information on turtle conservation in Australia see:

• Cape York Turtle Rescue:
  www.capeyorkturtlerescue.com
• WWF Australia:
  www.wwf.org.au/ourwork/oceans/turtles

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk