受威脅物種

綠海龜

© Adam Minu

Marine turtles have already existed on the earth for over 100 million years, and some early fossil records date back 230 million years. There are seven living species of marine turtle namely, Dermochelys coriacea, the Leatherback turtle, Caretta caretta, the Loggerhead turtle, Chelonia mydas, the Green turtle, Eretmochelys imbricata, the Hawksbill turtle, Lepidochelys kempii, the Kemp's Ridley turtle, Lepidochelys olivacea, the Olive Ridley turtle and Natator depressus, the Flatback turtle.

Among them, five species are found in the tropical and sub-tropical waters of the oceans and the other two have restricted ranges: the Flatback turtle occurs around northern Australia and southern New Guinea and Kemp's ridley mainly in the Gulf of Mexico. Most species spend most of their lives in continental shelf waters, while leatherback and Olive ridley turtle tend to live in the high seas.

Male turtles do not leave the sea and the females only come ashore to lay their eggs on sandy beaches during the spawning seasons. During the nesting season, mature males and females migrate from feeding grounds and mate near the nesting beaches. Females mate with several males and store the sperm for repeated fertilisation of eggs without the need for further mating.

Female turtles typically crawl out of the water and up the beaches at night to lay many eggs, often more than 100, in a single nest and return to the water before dawn. This process may be repeated several times in a single nesting season. An individual nesting female may lay all her clutches within the same few square metres, showing high nest-site fidelity, and more remarkably, some females have a strong tendency to return to the nest on the same beaches on which they were hatched.

Fertilised eggs incubate for 50 to 75 days, depending on the temperature. Like many reptiles, the sex of the hatching turtles depends on the temperature in the nest, with cooler temperature favouring the development of males, and warmth females. Although successful hatching rates may reach 80%, there may be high predation, for example by ghost crabs, rats, dogs and other animals. On average, only one in 1,000 can survive to adulthood.

Lucky survivors disperse into pelagic waters and spend several years at sea. Some species do not return to coastal water until fully mature, while others may do so as large juveniles.

To adapt to the marine environment, different species of marine turtle have evolved special mechanisms. For example, the Green turtle will hibernate in seabed mud to avoid cold shock and the Hawksbill is immune to the toxins contained in their sponge diet.

Four marine turtles occur in Hong Kong waters, the Hawksbill, the Olive ridley, Leatherback and Green turtles. Although both Olive ridley and Leatherback turtles were recorded in Hong Kong in the 1990s, they are extremely rare. However, nesting of the Green turtle has been reported at Sham Wan, southern Lamma Island and other sites.

The Green turtles are so called because of the colour of the cartilage and fat deposit around their internal organs. The carapace of the Green turtle is dark black-brown or greenish yellow in colour. The largest female Green turtles are found in the Atlantic and western Pacific Ocean and weight ranges between 90 and 200 kg. They feed on sea grasses and flowering plants which grow in shallow coastal waters.

Although symbolising longevity, fertility, strength and protection from harm in many cultures and civilisations, marine turtles all over the globe have been threatened by direct consumption, pollution, by-catch, disease, habitat destruction, etc.

Direct consumption: Turtles generally make good eating and their eggs are easy to collect and highly nutritious. In many parts of the world turtle eggs are prized by humans as a rich source of protein, and are often reputed to have aphrodisiac properties.

By-catch: It is estimated that about 200,000 to 300,000 marine turtles are accidentally killed by shrimp trawling nets and offshore longlines. Shrimp trawl is probably one of the major causes of decline in the Kemp's ridley turtle while Leatherback and Loggerheads turtles attracted to the bait of longlines for tuna could become snared and tangled in the line and drown.

Habitat destruction and alternation: The construction of seawalls, hotels, marinas and the associated infrastructures for coastal tourism have destroyed large areas of turtle nesting beaches around the world. For example, the only remaining nesting site in Hong Kong is Sham Wan, southern Lamma Island.

Pollution: Heavy metals, including aluminium, cadmium, mercury, etc., have been recorded in the tissue, especially the kidneys and livers, of stranded turtles. Solid debris, like tar balls and styrofoam pellets, could be mistaken as sargassum floats and eaten by young turtles.

Disease: Since the 1970s, debilitating and disfiguring tumours have been identified in many populations of turtles, especially Green turtles. Tumours can blind the turtles or inhibit their ability to feed.

Over-exploitation and other anthropogenic impacts have driven marine turtles to the edge of extinction and all marine turtle species, except the Flatback turtle, have been categorised as 'Critically Endangered' or 'Endangered' by the International Union for the Conservancy of Nature (IUCN)/SCC Marine Turtle Specialist Group. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) also lists all marine turtles species in Appendix I, thus prohibiting trade of them by CITES member nations (Table 1).

Locally, the beach at Sham Wan, southern Lamma Island, the only known nesting site, has been declared a Site of Special Scientific Interest and Restricted Area since 1999. Access to the beach is prohibited during the nesting season (June to October). In addition, all marine turtle species are protected under the Wild Animal Protection Ordinance (Cap. 170), the Animals and Plants (Protection of Endangered Species) Ordinance (Cap. 187) and other Ordinance.

To conserve marine turtle, one could:

• Stop/avoid the consumption of marine turtle meat and eggs.
• Stop the purchase of products made of marine turtle shells.
• Support marine turtle conservation research and programmes.
• Report sightings of marine turtles and nesting.
• Report illegal trading activities of marine turtle products.