Green turtle (Chelonia mydas)

Green turtle (Chelonia mydas)

The Green turtle (Chelonia mydas) has a sub-circular to heart-shaped shell.

Also known as: Green Turtle

Local Names: Vonu dina, Ika du

Description

The Green turtle has a sub-circular to heart-shaped shell. Green turtles belong to the family Cheloniidae, the members of which are characterised by their hard shell as opposed to the soft shelled members of the family, Dermochelyidae (see Leather back turtle ).

It derives its common name from the green fat it stores which makes this species the most appealing in terms of taste. The average length of a Green turtle’s shell or carapace is 70cm and can grow up to 1.2m. Green turtles mature at around 85cm.

The carapace is light brown to greenish brown in colour, with the scutes not strongly overlapping as observed in the Hawksbill turtle (Eretmochelys imbricata). There are four costal shields on either side of the carapace, and four enlarged scales running through the centre or the bridge of the carapace. Green turtles can weigh up to 130 kg. There are two features that distinguish Green turtles from other members of its family that also occur in Fiji: the Green turtle has a single pair of prefrontals (the Hawksbill has two pairs); and the jaw of Green turtles is not beaklike (see the Hawksbill description).

For identification of the turtles, you may refer to the following website:

Turtle ID Sheet

Green turtle hatchlings have a shiny black dorsal surface and a while ventral surface.

Distribution

Green Turtles have been recorded in all parts of Fiji waters. Vomo Island was recorded as a major turtle nesting island by the Wilkes Expedition in 1840, but is no longer a nesting site. Current known nesting sites are: a sacred island in Rotuma (50 nests, representing 20-30 nesting turtles); Heemskereq and Ringgold Reef and Qelelevu (reported by Birdlife International during their expedition); the Mamanucas (very few); Namena lailai (with Hawksbills); Northern Lau (some resort islands). The map below only shows confirmed foraging sites as reported by Craig et al. (2004): Savusavu Bay, Northern Vanua Levu, North of Taveuni, Northern Lau and between Viti Levu and Ovalau.

 Green turtle (Chelonia mydas)

Green turtle (Chelonia mydas)

 Green turtle (Chelonia mydas) map

Green turtle (Chelonia mydas) map

Habitat Ecology and Behaviour

Recent research has shown that Green turtle nesting is currently confined to the isolated outer islands and cays that have high energy beaches and broad high platform. Green turtle hatchlings are largely carnivorous, the juveniles and sub-adults are omnivorous, while the adults are almost completely herbivorous. Adult Green turtles feed during the day in shallow waters on sea grasses, plankton, and fish eggs floating in the sea. During their hatchling, juvenile and sub-adult stages, the turtles are in what is referred to as their ‘Lost years’ whereby they swim into the open sea and get caught up in the ocean currents, feeding and growing along the way into adulthood.

Sea turtles are well known for their migratory behaviour, some travelling as far as 1400 km from where they hatched. A turtle which was tagged on Rose Atoll in American Samoa was found feeding the Fiji waters, having travelled more than 1400km. Research (Craig et al., 2004) showed that 58% the seven turtles that had been tagged in Samoa swam specifically to Fiji to forage. The turtles spend much of their adult life in Fiji waters, suggesting that Fiji is an important foraging ground for migrating turtles. Fiji’s sea grass and algal beds are probably more extensive than that east of Fiji, explaining the Green turtles’ tendency to migrate westwards.

There is little known about the behaviour of males, however, studies have shown that breeding males always return to the same courtship area every time they breed. Similarly, all sea turtles always return to the same foraging areas at the end of their breeding season, no matter how far away their breeding spot, and no matter how rich the foraging areas they encounter on the way back home. During the reproductive season, female turtles return to breed to the site where they hatched, and they mate with male turtles in the waters adjacent to their nesting sites.

In Fiji, mating begins in March, and the nesting season is from October to April. Female turtles do not start to reproduce, i.e. start to lay eggs until they reach the age of 30 – 40 years. Individual females have a nesting season every 2-3 years. One female Green turtle can lay up to seven clutches of eggs per nesting season, with 100-150 eggs per clutch. Sadly, not all eggs survive to become hatchlings, and not all hatchlings survive to become adults. Natural mortality of juveniles is likely to be over 90%. Green turtles take up to six hours to build their nest, lay their eggs and then return to the water. Other species can do all this in as little as two hours.

Threats

There is high natural mortality of hatchling and juvenile Green turtles which decreases as they get larger and older. Adult turtles have few natural predators other than man, though sharks are one of these. One of the main anthropogenic threats to the survival of Green turtles is the direct hunt of this particular species because of its green fat. Other contributing threats include the loss of their nesting sites, foraging grounds and predation of eggs and hatchlings. Female turtles lay their eggs in pits or egg chambers they have dug out, along sandy beaches, above the high tide line, where the eggs are not inundated by tides or the underlying water table. The substrate of the pits is usually moist and just fine enough to prevent the collapse of the egg chamber.

The loss of turtle nesting beaches to development is one of the main threats to the nesting sites of these turtles. There are very few if any Green turtle nesting sites remaining in Fiji and these are going to need total protection if this species is to survive as a Fijian breeding species. While natural predators of turtle eggs and hatchlings do exist (crabs, birds, sharks, fish), the demise of the turtle population in Fiji is attributable almost entirely to predation of eggs and adults by humans. Fiji’s turtles have been harvested without control since man first arrived in the islands. However, the uncontrolled harvesting of adult turtles in the peak of the turtle industry (for meat) from the 1800s until the early 1990s in Fiji and the Pacific has made the Pacific Green turtle population one of the most endangered in the world.

Conservation Status

The plight of Fiji’s turtles was first officially recognised in the Fiji’s State of the Environment Report in 1992, and in 1995, the Fiji Sea Turtle Working Group was established to address the obvious decline in the Fiji marine turtle population. By 1997, the Fiji Sea Turtle Working Group was able to initiate a 3-year harvest ban of turtles and this has continued until today. Currently turtles may only be harvested for traditional purposes with a permit issued by the Fisheries Department. Penalties for illegal capture of turtles are quite severe (see Leatherback Turtle for details). The Fisheries Department research station on Makogai Island has been successfully rearing turtles in captivity and releasing them in the wild for the past 10 years but is very poorly resourced. The Institute of Marine Resources (USP) is currently implementing a major regional turtle protection project and WWF have been active in turtle conservation for several years. They have been conducting awareness workshops in existing, known turtle nesting sites on their vulnerability, and measures on how to protect our marine turtles.

Recent research by the IMR have identified some Green turtle nesting grounds and that there are 50 – 100 nesting Green turtles in Fiji. These nesting areas are: a sacred island in Rotuma (50 nests, representing 20-30 nesting turtles); Heemskereq and Ringgold Reef and Qelelevu (reported by Birdlife International during their expedition); the Mamanucas (very few); Namena lailai (with Hawksbills); Northern Lau (some resort islands).

Remarks and Cultural Significance

The plight of Fiji’s turtles was first officially recognised in the Fiji’s State of the Environment Report in 1992, and in 1995, the Fiji Sea Turtle Working Group was established to address the obvious decline in the Fiji marine turtle population. By 1997, the Fiji Sea Turtle Working Group was able to initiate a 3-year harvest ban of turtles and this has continued until today. Currently turtles may only be harvested for traditional purposes with a permit issued by the Fisheries Department. Penalties for illegal capture of turtles are quite severe (see Leatherback Turtle for details). The Fisheries Department research station on Makogai Island has been successfully rearing turtles in captivity and releasing them in the wild for the past 10 years but is very poorly resourced. The Institute of Marine Resources (USP) is currently implementing a major regional turtle protection project and WWF have been active in turtle conservation for several years. They have been conducting awareness workshops in existing, known turtle nesting sites on their vulnerability, and measures on how to protect our marine turtles.

Recent research by the IMR have identified some Green turtle nesting grounds and that there are 50 – 100 nesting Green turtles in Fiji. These nesting areas are: a sacred island in Rotuma (50 nests, representing 20-30 nesting turtles); Heemskereq and Ringgold Reef and Qelelevu (reported by Birdlife International during their expedition); the Mamanucas (very few); Namena lailai (with Hawksbills); Northern Lau (some resort islands).

References

Batibasaga et al. (2003;),
Bjorndal and Zug (1995);
Boyle (1998);
Bustard (1972);
Carr (1965);
Carr et al.(1980/1981);
Craig et al. (2004);
Daley (1990, 1991);
Diamond (1976);
Ehrhart (1982);
Ernst and Barbour (1989);
Ernst et al (1994);
Guinea (1993);
Laveti and Fong (2007);
Laveti and MacKay (2007);
Leslie (1996 a, b, c);
Limpus (1971, 1972),
Limpus and Reed (1985);
Limpus et al. (1992) ;
Limpus et al. (1983) ;
Lohmann et al. (1996) ;
Lutcavage et al. (1996) ;
MacKay, K (personal communication) ;
Meylan et al. (1990) ;
Mortimor (1982) ;
NRC (1990) ;
Sovaki (1997) ;
Stanyck (1982) ;
Sue (1996) ;
Weaver (1996) ;
Wilkes (1985).

Front Page Photo: Jurgen Freund