Six species of marine turtles are found in the Indian Ocean - South-East Asia region, as described below.
Green turtle (Chelonia mydas)
The green turtle (Chelonia mydas) is the largest of all the hard-shelled sea turtles, growing up to one meter long and weighing 130-160 kg. In the western Indian Ocean adult females are larger than males, and have a more "bullseye" pattern in the scutes of the carapace.
Adult green turtles are unique among sea turtles in that as large immatures and adults they are primarily herbivorous, feeding on seagrasses and algae. Green turtles take between 20 and 50 years just to reach sexual maturity. Females return to their natal beaches (i.e., the same beaches where they were born) every 2 to 4 years to nest, laying several clutches of about 125 eggs at roughly 14-day intervals several times in a season. However, very few hatchlings survive to reach maturity – perhaps fewer than one in 1,000.
The green turtle is globally distributed and generally found in tropical and subtropical waters along continental coasts and islands roughly between 30°N and 30°S. Green turtles primarily use three types of habitat: oceanic beaches (for nesting), convergence zones in the open ocean, and benthic feeding grounds in coastal areas.
Adults migrate from foraging areas to mainland or island nesting beaches and may travel hundreds or thousands of kilometers each way. After emerging from the nest, hatchlings enter the sea and swim offshore where they enter longshore currents that take them to various oceanic areas where they are believed to get caught up in major oceanic current systems and live for several years, feeding close to the surface on a variety of pelagic plants and animals. Once the immatures reach a certain age/size range, they leave the pelagic habitat and travel to nearshore foraging grounds. Once they move to these nearshore benthic habitats, large immature and adult green turtles are almost exclusively herbivores, feeding on sea grasses and algae.
The Indian Ocean hosts some of the largest nesting populations of green turtles anywhere, particularly on oceanic islands in the southwest and on islands in SE Asia. Many of these populations are now recovering after intense exploitation in the last century dramatically reduced the populations. However, some populations are still declining. The green turtle is one of the most widely distributed and commonest of the marine turtle species in the Indian Ocean. Interactions with fishing operations are especially important threats in coastal fisheries where nets are employed, but trawl fisheries may also have important impacts.
During the 19th and 20th centuries intense exploitation of green turtles provided onboard red meat for sustained cruises of sailing vessels before the time of refrigeration, as well as meat and calipee for an international market. Several nesting populations in the Indian Ocean were devastated as a result.
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Hawksbill turtle (Eretmochelys imbricata)
The hawksbill (Eretmochelys imbricata) turtle is small to medium-sized compared to other sea turtle species. In the Indian Ocean region, adults weigh 45 to 70 kg, but can grow to as large as 90 kg. Female hawksbills return to their natal beaches every 2-3 years to nest. A female hawksbill may lay 3-5, or even more, nests per season, each of which contains an average of 130 eggs.
Hawksbill turtles, like other marine turtles, use different habitats at different stages of their life cycle, but this species is most commonly associated with coral reefs. Post-hatchlings (oceanic stage juveniles) are believed to occupy the pelagic environment. After a few years in the pelagic zone, small juveniles recruit to coastal foraging grounds. This shift in habitat also involves a shift in feeding strategies, from feeding primarily at the surface to feeding below the surface, primarily on animals associated with coral reef environments. Their narrow, pointed beaks allow them to prey selectively on soft-bodied animals like sponges and soft corals.
Hawksbill turtles are circumtropical, typically occurring from 30°N to 30°S latitude. In modern times hawksbills are solitary nesters (although some scientists postulate that before their populations were devastated they may have nested on some beaches in concentrations) and, thus, determining population trends or estimates on nesting beaches is difficult.
Although generally not found in large concentrations, hawksbills are widely distributed in the Indian Ocean. There, the largest nesting populations – which are among the largest in the world – occur in the Seychelles, Indonesia and Australia. Adult hawksbill turtles are capable of migrating long distances between nesting beaches and foraging areas, although the migration distances are generally not as long as those for green turtles.
Exploitation of hawksbills, primarily for their "tortoise shell" (the thick, overlapping, horny scutes that cover the shell), dates back millennia in the Indian Ocean. The shell was sought after for manufacture of diverse articles in both the Orient and Europe, and it constituted one of the most important trade commodities in a well developed trade network in the Indian Ocean. Many populations were decimated. However decades-long protection programmes in some places, particularly at several beaches in the Indian Ocean, have resulted in population recovery. Today, interactions with fisheries are especially important in coastal fisheries where nets are used.
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Leatherback turtle (Dermochelys coriacea)
The leatherback (Dermochelys coriacea) is the largest turtle and the largest living reptile in the world. Mature males and females can grow to 2 m in total length and weigh almost 900 kg, although individuals of this large size seem to be rare today. Females lay clutches of approximately 100 eggs, including many abnormally small, "yolkless" eggs. Like all other marine turtles they use sandy, beaches, but unlike other marine turtles, leatherbacks can nest on both tropical and subtropical beaches. They nest several times during a nesting season; there are records of some individuals nesting as many as 7, 8, or 9 times in a single season.
The leatherback is the only sea turtle that lacks a hard shell: there are no large external keratinous scutes and the underlying bony shell is composed of a mosaic of hundreds of tiny bones. Adults are capable of tolerating water temperatures well below tropical and subtropical conditions, and special physiological adaptations allow them to maintain body temperature above cool water temperatures. In subtemperate areas they can maintain internal body temperature up to 17 degrees above ambient water temperature.
The leatherback's beak is unique in having a conspicuous cusp on either side of the upper jaw. They specialise on soft bodied invertebrates found in the water column, particularly jelly fish and other sorts of “jellies.”
The leatherback is the most wide ranging marine turtle species. Animals regularly migrate enormous distances, crossing ocean basins, or even moving between ocean basins, such as between the Indian and south Atlantic Oceans. They are commonly found in pelagic areas, but they also forage in coastal waters in certain areas. The distribution and developmental habitats of juvenile leatherbacks are poorly understood. While the leatherback is not as common in the Indian Ocean as other species, important nesting populations are found in and around the Indian Ocean, including in Indonesia, South Africa, Sri Lanka and India’s Andaman and Nicobar Islands. The leatherback's markedly oceanic habits make it especially subject to interactions with modern high seas fisheries, particularly longlines.
For more information on the conservation status of leatherback turtles in and around the Indian Ocean, please consult the IOSEA Leatherback Species Assessment.
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Loggerhead turtle (Caretta caretta)
The loggerhead turtle (Caretta caretta) may grow to over one meter long and weigh around 110 kg or more. It reaches sexual maturity at around 35 years of age. This turtle is characterised by the large head and powerful jaws, which are used by immatures and adults in benthic habitats to crush the shells of molluscs and crustacean prey.
Loggerheads are circumglobal, occurring throughout the temperate and tropical regions of the Atlantic, Pacific, and Indian Oceans. They occupy three different ecosystems during their lives: the terrestrial zone, the oceanic zone, and the "neritic" zone The greatest cause of decline and the continuing primary threat to loggerhead turtle populations worldwide is incidental capture in fishing gear, primarily in longlines and gillnets, but also in trawls, traps and pots, and dredges.
Loggerheads nest in relatively few countries in the Indian Ocean, and the number of nesting females is generally small, except on Masirah Island (Sultanate of Oman) which supports one of only two loggerhead nesting beaches in the world that have greater than 10,000 females nesting per year. Studies in the Atlantic and Pacific Oceans show that loggerheads can pass decades crossing from one side of the ocean basin to another, before taking up residence on benthic coastal waters. This makes them especially subject to interactions with modern high seas fisheries, particularly longlines; coastal trawl and net fisheries may also be important.
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Olive ridley (Lepidochelys olivacea)
The olive ridley (Lepidochelys olivacea) turtle is considered the most abundant sea turtle in the world, with an estimated 800,000 nesting females annually. Adults are relatively small, weighing on average around 45 kg. As with other species, their size and morphology varies from region to region. The olive ridley is omnivorous, feeding on algae, lobsters, crabs, tunicates, mollusks, shrimp, and fish.
The olive ridley is globally distributed in the tropical regions of the South Atlantic, Pacific, and Indian Oceans. It is mainly a pelagic species, but it has been known to inhabit coastal areas, including bays and estuaries; where they can dive to depths of about 150 m to forage on benthic invertebrates. Olive ridleys often migrate great distances between feeding and breeding grounds. They mostly breed annually, involving an annual migration from pelagic foraging, to coastal breeding and nesting grounds, back to pelagic foraging. Olive ridleys reach sexual maturity in around 15 years, a young age compared to some other sea turtle species. Many females nest every year, once or twice a season, laying clutches of approximately 100 eggs.
In certain places, the olive ridley has one of the most extraordinary nesting habits in the natural world. Large groups of turtles gather off shore of nesting beaches. Then, all at once, vast numbers of turtles come ashore and nest in what is known as an "arribada". During these arribadas, hundreds to thousands of females come ashore to lay their eggs. In the northern Indian Ocean, arribadas occur on three different beaches along the coast of Orissa, India. Gahirmatha used to be one of the largest arribada nesting sites in the world. However, arribada nesting events have been less frequent there in recent years and the average size of nesting females has been smaller, indicative of a declining population. Declines in solitary nesting of olive ridleys have been recorded in Bangladesh, Myanmar, Malaysia, and Pakistan. In particular, the number of nests in Terengganu, Malaysia has declined from thousands of nests to just a few dozen per year. Solitary nesting also occurs extensively throughout this species' range.
Despite the enormous numbers of olive ridleys that nest in Orissa, India, this species is not generally common throughout much of the Indian Ocean. Its pelagic habits make it especially subject to interactions with modern fisheries, such as longlines. Coastal trawl and net fisheries are also important sources of incidental catch and mortality.
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Flatback turtle (Natator depressus)
The flatback turtle (Natator depressus) nests exclusively along the northern coast of Australia. It gets its name from its relatively flat, smooth shell, unlike other marine turtles which have a high domed shell. The Flatback is a medium-sized marine turtle, growing to up to one meter long and weighing up to 90 kg. It is carnivorous, feeding mostly on soft-bodied prey such as sea cucumbers, soft corals, jellyfish, molluscs and prawns.
Flatback turtles are found in northern coastal areas, from Western Australia's Kimberley region to the Torres Strait extending as far south as the Tropic of Capricorn. Feeding grounds also extend to the Indonesian Archipelago and the Papua New Guinea Coast. Although flatback turtles do occur in open seas, they are common in inshore waters and bays where they feed on the soft-bottomed seabed.
Flatbacks have the smallest migratory range of any marine turtle species, though they do make long reproductive migrations of up to 1300 km. This restricted range means that the flatback is vulnerable to habitat loss, especially breeding sites. Flatbacks are caught as 'bycatch' in commercial fishing operations, getting tangled in discarded ('ghost') fishing nets and ingestion of marine debris.
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The preceding biological information on marine turtle species found around the Indian Ocean is derived partly from the NOAA Fisheries, Office of Protected Resources, website:(http://www.nmfs.noaa.gov/pr/species/turtles/), supplemented by other sources (such as a website of the Australian Government, Department of Environment, Water, Heritage and the Arts -- for information on the Flatback turtle), and additional information supplied by Dr. Jack Frazier (IOSEA Advisory Committee Chair).