Six species of sea turtles occur in the waters of the Great Barrier Reef (GBR). Five of these belong to the family Cheloniidae: green turtle (Chelonia mydas) (Fig. 29.1A), flatback (Natator depressus), olive ridley (Lepidochelys olivacea), hawksbill (Eretmochelys imbricata) (Fig. 29.1B) and loggerhead (Caretta caretta). The remaining species, the leatherback or luth (Dermochelys coriacea), is a member of the family Dermochelidae.
Each of these species has its own feeding and nesting grounds but there is broad overlap among species. In the non-breeding season the turtles disperse widely to feeding grounds within and well beyond the GBR region. Those nesting on or near the GBR may go as far as Indonesia, New Guinea, the Solomon Is, Vanuatu, New Caledonia and Fiji. Only the flatback is endemic to Australia.
Green turtles nest on the Capricorn-Bunker islands and on the cays of the outer barrier from the Swain Reefs north to Princess Charlotte Bay. Loggerheads (Fig. 29.1C) nest on the cays of the Swain Reefs and Capricorn-Bunker islands as well as on beaches of the adjacent Queensland coast. Hawksbills lay their eggs mostly on the cays of the inner shelf north to Princess Charlotte Bay. The flatback has a restricted nesting area on the inshore continental islands between Gladstone and Mackay, and leatherbacks lay their eggs on the mainland coast just south of Bundaberg. The olive ridley does not appear to nest in the GBR region. Except for the flatback, all species also nest well beyond the GBR, and indeed are distributed worldwide in subtropical and tropical waters. The leatherback also ranges into cooler waters.
Female turtles return repeatedly to lay their eggs on beaches in the area from which they hatched. In the GBR region, sea turtles aggregate and begin to court near nesting beaches in spring (September to November) and females come ashore and dig pits in the sand in which they lay their eggs between October and March. Eggs incubate in the sand and the temperature of the nest determines the sex of the hatch-lings: cooler nests produce males whereas warmer nests produce females. Hatchlings emerge in summer and autumn (January to May) and go to sea where they drift with currents and feed on macroscopic plankton. After 5-10 years they move into their traditional feeding grounds and adopt the adult diet.
Adult green turtles feed mainly on algae and seagrass on coral reefs and inshore seagrass flats. The loggerhead eats mainly molluscs and crabs in sandy lagoons on the reef or in inshore bays and estuaries. The olive ridley consumes small crabs in inter-reefal areas. Hawksbills eat algae, seagrasses, sponges, as-cidians, bryozoans and molluscs, mainly on coral reefs. The flatback feeds on soft corals and other soft-bodied animals on soft bottoms in inter-reefal areas.
Figure 29.1 A, green turtle (Chelonia mydas) (photo: GBRMPA); B, hawksbill turtle (Eretmochelys imbricata) (photo: GBRMPA); C, loggerhead sea turtle (Caretta caretta) digging a nest pit on Heron I., Great Barrier Reef (photo: H. Heawole); D, head of olive sea snake (Aipysurus laevis) showing the two semicircular nostril valves (closed) (photo: V. Lukoschek); E, various sea snake species in an aquarium illustrating the differences in size, shape, and colour pattern within the group. On the left with saddle-blotches is Astrotia stokesii and the banded one is a species of Hydrophis (note that the head is small and protruding above the water and the paddle-shaped tail is large and submerged). The three on the right are Aipysurus laevis (note their marked differences in colour). Colour patterns vary greatly within many species, especially from one locality to another. (Photo: H. Heatwole.); F, Two turtleheaded sea snakes, Emydocephalus annulatus, courting. Male is the smaller (dark) snake pursing the larger female. There is enormous colour pattern variation in both sexes of this species ranging from entirely black (melanistic) to very pale salmon. (Photo: M. Berger.); G, Juvenile olive sea snake, Aipysurus laevis, showing strongly banded pattern of this species in its first year of life. (Photo: V. Lukoschek.); H, Aipysurus duboisii swimming over a sea grass bed in sandy habitat adjacent to a coral reef. (Photo: V. Lukoschek.); I, Two turtleheaded sea snakes, Emydocephalus annulatus, courting. The male is the smaller snake on top of the larger female. (Photo: M. Kospartov.); J, Adult olive sea snake, Aipysurus laevis, showing typical colour pattern (olive head and uniform grey to brown body) found on the Great Barrier Reef. (Photo: V. Lukoschek.); K, Astrotia stokesii, the bulkiest of all sea snake species, may reach 2 m in length and has a large head and an exceptionally wide gape. Its fangs are long and it is one of the few species capable of penetrating a wet suit. (Photo: AIMS.)
The leatherback eats jellyfish and salps, mainly in deeper water away from reefs.
A number of threatening processes affect marine turtles at all stages of their life cycles and in different parts of their geographic ranges. Because of this, marine turtle conservation requires a multifaceted approach and international co-operation. Throughout the world key threatening processes include the loss of nesting habitat on key nesting beaches, artificial lights at nesting beaches that attract hatchlings towards the light and away from the ocean, direct harvest of eggs for human consumption, increased predation on nests by feral animals (such as wild pigs), direct harvest of adults for consumption of meat and for 'tortoiseshell', tourism, boat strike, ingestion and/or entanglement in marine debris, and incidental capture in fisheries gear. In the GBR region (as with many other places), numerous conservation initiatives are in place, which aim to reduce or eliminate these impacts. These initiatives include the protection of key nesting beaches and important internesting and foraging habitats, assisting indigenous hunters (traditional owners) to manage their harvests within sustainable limits, increasing awareness of the need for boats to 'go slow' in key turtle habitats, increasing awareness of the impacts of pollution and litter in the marine environment, and a legal requirement that turtle exclusion devices (TEDs) be used by trawl fishers at all times. Ecotourism ventures in Queensland allow visitors the opportunity to observe females dig their nests and lay eggs (Fig. 29.1 C) and to participate in the release of hatchlings; these activities aim to educate people about the fascinating lives of marine turtles.
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