Image of green turtle, coral reef, Australia, Great Barrier Reef. Image credit Nico Faramaz

Green turtle

Chelonia mydas


Adult green turtles have a smooth, high-domed carapace (shell), are olive green in colour, with occasional brown, reddish-brown or black highlights. Hatchlings have a black carapace with white margins around the carapace, flippers and on the plastron (bottom of shell).

Distribution and habitat

Green turtles are found in tropical, subtropical and temperate waters around the world and appear to be the most abundant of the six species of marine turtle found in the Great Barrier Reef World Heritage Area.

They are found in subtidal and intertidal coral and rocky reefs and seagrass meadows of the continental shelf. Green turtles are mostly herbivorous (plant eaters) as adults, eating algae, seagrass, mangrove fruit and jellyfish.

Two genetic stocks of green turtles breed within the Great Barrier Reef Marine Park, a southern and a northern stock. The southern stock has nesting concentrated in the Capricorn/Bunker group of islands, with an average annual nesting population estimated at 8000 females. The northern stock has nesting concentrated around Raine Island and Moulter Cay with an average annual nesting population of 30,000 females.

The proportion of a green turtle population that nests each year is highly variable and is influenced by variations in the El Niño Southern Oscillation (ENSO) Index. Green turtles are the only species of marine turtle for which this correlation has been shown and it may be based upon nutrition.

For green turtles nesting and foraging in the Great Barrier Reef Marine Park, research shows migration to Indonesia, Gulf of Carpentaria, Arnhem Land, Torres Strait, Papua New Guinea, the Solomon Islands, Vanuatu and New Caledonia (Figure 1).

Figure 1 Indicative migration path of Great Barrier Reef green turtles

Map of Queensland, indicating green turtle migration routes

Dashed lines (- - -) represent movements of southern Great Barrier Reef green turtles. Solid lines (-) represent movements of northern Great Barrier Reef green turtles.
All migration paths are indicative only.

Breeding areas

Low density nesting occurs on many islands and along Queensland's coastline (Figure 2). Although genetically distinct nesting aggregations are known, the stocks often occur in the same foraging habitat (feeding area).

To date, there have been no detectable declines in the number of nesting green turtles at Great Barrier Reef nest monitoring sites. However, the 20 to 25 years of data for the key sites (Raine Island, Heron Island) do not cover a single generation for green turtles, and trends are difficult to determine with the large fluctuations in nesting numbers that can occur because of the El Niño Southern Oscillation.

Additionally, research suggests that the northern and southern stocks may be exhibiting characteristics of populations under threat, including a decline in the size of nesting adults, increases in the non-breeding periods and a lack of expected increases of turtle numbers in dispersed feeding areas. In addition, climate change may be adversely affecting nesting success by, for example, increased flooding of nest chambers and altering chamber temperatures thus affecting the sex of hatchlings.

Figure 2: Great Barrier Reef green turtle nesting sites

Map of Queensland, indicating green turtle nesting sites

Green turtle facts

Breeding seasonLate October to February
Years between breeding Two to eight years
Age when female first breeds45 years
Age when move into feeding areaFour to seven years (30 - 40cm carapace length)
Nesting female length  107cm (range from 91 - 124cm)
Nesting female weight  130kg (range from 98 - 184kg)
Clutch size (number of eggs) 115 eggs (range from 62 - 153 eggs)
Hatchlings emerge  December to May
Hatchling success   84 per cent
Hatchling size   4.97cm (range 4.02 - 5.19cm)
Hatchling weight  24.83g (range from 19.8 - 28.4g)
Predators of hatchlings   Crabs, herons, dingoes and fish such as trevally and sharks