The Majestic Turtles of the Great Barrier Reef

• Common name: Hawksbill turtle
• Scientific name: Eretmochelys imbricataI
• UCN (World Conservation Union): Critically Endangered
• Australian Environment Protection and Biodiversity Conservation Act 1999: Vulnerable
• Queensland Nature Conservation (Wildlife) Regulation 1994: Vulnerable

Adult hawksbill turtles have extensively variegated patters of brown and black on their carapace (shell). The plastron (bottom of shell) is cream with occasional black spots. A distinguishing feature of the hawksbill turtle is their beak-like mouth and narrow head. The carapace has overlapping scutes made of material much like fingernails. Hatchlings are dark brown.Distribution and habitatHawksbill turtles occur in all oceans usually in tidal and sub-tidal coral and rocky reefs in tropical and subtropical areas. In Australia, hawksbills feed in rocky areas and on coral reefs. Their main feeding area is on the east coast, including the Great Barrier Reef.Sponges make up a major part of the hawksbill’s diet, although they also feed on seagrasses, algae, sea cucumbers, soft corals and shellfish. The pointed jaws are well adapted to prising food from crevices in and around coral.Research on hawksbill turtles that nest or forage in the Great Barrier Reef Marine Park shows migration to Indonesia, Papua New Guinea, the Solomon Islands, and Vanuatu (Figure 1).Breeding areasHawksbill turtles are found all over the world. The entire far northern section of the Great Barrier Reef Marine Park and the Torres Strait region is internationally significant for hawksbill turtle nesting as numbers have declined in many parts of the world.There are three main breeding areas in Australia – northern Great Barrier Reef (several thousand nesting females), north-eastern Arnhem Land (about 2000 nesting females), and Western Australia (several thousand nesting females).Hawksbill turtles tend to nest in low numbers and in the Great Barrier Reef Region. Nesting areas mainly occur north of Princess Charlotte Bay and in the Torres Strait (Figure 2).The only Great Barrier Reef nesting population for which there is sufficient information is at Milman Island, where data indicates since 1990 numbers of nesting females have been declining at about three per cent each year.Figure 1 Indicative migration path of Great Barrier Reef flatback and hawksbill turtlesDashed lines (- – -) represent movements of hawksbill turtles.
Solid lines (-) represent movements of flatback turtles.
All migration paths are indicative only.

Hawksbill turtles facts
Breeding season – Year round, concentrated from November to February
Years between breeding – Two to five years
Ages when female first breeds – 30 – 35 years
Age when move into feeding area – Four to seven years (30cm carapace length)
Nesting female length – 81.6cm (range from 63.5 – 91cm)
Nesting female weight – 50.4kgs (range from 32 – 72kg)
Clutch size (number of eggs) – 122 eggs (range from 18 – 210 eggs)
Hatchlings emerge – Year-round concentrated from February to April
Hatchling success – 79 per cent
Hatchling size – 3.98cm (range 3.2 – 4.36cm)
Hatchling weight – 13.8g (range from 8 – 17.5g)
Predators of hatchlings – Gulls, reef egrets, ghost crabs, crocodiles, fish, sharks, crabs and dingoes

Common name: Flatback turtle

Scientific name: Natator depressus

IUCN (World Conservation Union): Data deficient

Australian Environment Protection and Biodiversity Conservation Act 1999: Vulnerable

Queensland Nature Conservation (Wildlife) Regulation 1994: Vulnerable

The flatback turtle has a distinct low-domed flat carapace (shell) with upturned edges and covered by a thin skin and their plastron (bottom of shell) is white. Adults have olive grey flippers and head. Hatchlings are grey and have a white plastron.

Distribution and habitat

Flatback turtles are only found on the continental shelf of Australia. Although they feed around Papua New Guinea and Indonesia as well as within the Great Barrier Reef Marine Park, they nest only in Australia.

Hatchling flatback turtles are unique in that they do not have an oceanic pelagic phase, rather they are believed to inhabit inshore areas of clear reefal waters.

Flatback turtles inhabit subtidal soft-bottomed habitats of the continental shelf and feed on things such as soft corals, sea pens and jellyfish. Numbers in the east coast population in Queensland appear to be stable.

Tag returns up to 1300km between nesting and foraging areas have established movement between Australia and Indonesia (southern Irian Jaya) (Figure 1).

Breeding areas

All known breeding sites of the flatback turtle are in Australia. Breeding is centred in the southern Great Barrier Reef around Peak, Wild Duck, Curtis and Facing Islands (Figure 2).However, low density nesting by flatbacks occurs on many mainland beaches and offshore islands north of Gladstone.The largest amount of nesting occurs on Crab Island in western Torres Strait.

Whilst trends in population numbers of breeding flatback turtles are generally uncertain, their numbers are thought to be relatively stable on the Australian east coast including the Great Barrier Reef.

Figure 1 Indicative migration path of Great Barrier Reef flatback and hawksbill turtles

Dashed lines (- – -) represent movements of hawksbill turtles.
Solid lines (-) represent movements of flatback turtles.
All migration paths are indicative only.

Flatback turtle facts

• Breeding season – November to February
• Years between breeding – One to three years
• Ages when female first breeds – 20 years
• Nesting female length – 94cm (range from 88 – 96cm)
• Nesting female weight – 82kgs (range from 70 – 90kg)
• Clutch size (number of eggs) – 54 eggs
• Hatchlings emerge – January to April
• Hatchling success – 80 per cent
• Hatchling size – 6.1cm (range 5.66 – 6.55cm)
• Hatchling weight – 44g (range from 33.3 – 49.1g)
• Predators of hatchlings – Pelicans, ghost crabs, crocodiles and sharks

The Flatback Sea Turtles

The odd one out. 

If you lined up all the sea turtles that visit Australian waters, they would all look different, but one major similarity would be they all have a rounded shell. The flatback turtles therefore can be easily identified as they are the only species that don’t. Because of this their shells are not as strong and easily crack under pressure so they rarely go deeper than 60 metres. 

What is the flatback turtles favourite country?
Australia! In fact these sea turtles love this country so much that this is the only place they will breed and nest their eggs. In comparison to other species, the flatback only lay around 50 eggs per nest, that’s half the amount of other turtles. They also don’t migrate to other countries, but rather stay in the shallower waters to feed. The flatback turtle eggs will begin hatching in December and continue until March. 

Tasty, said the crocodile.

As these turtles favour the warm waters of Australia, it means their main predators are saltwater crocodiles. However their population faces other threats such as direct hunting for meats and eggs, nesting beaches are disrupted for coastal development, and pollution. 

Wild Facts.

The gender of turtles is determined by the temperature of the sand in which the eggs have been laid. If the sand is below 29 degrees the turtles will be males and if the temperature is above 29 degrees the turtles will be female. 

Size: 75-95 cms 

Weight: 70-90 kgs

Location: Australia, Papua New Guinea 

Lifespan: 100 years 

• Common name: Green turtleScientific name: Chelonia mydasIUCN (World Conservation Union): EndangeredAustralian Environment Protection and Biodiversity Conservation Act 1999: VulnerableQueensland Nature Conservation (Wildlife) Regulation 1994: VulnerableAdult 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 Indicative migration path of Great Barrier Reef green turtles

  

  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

  

   

  Green turtle facts

  • Breeding season – Late October to February
  • Years between breeding – Two to eight years
  • Ages when female first breeds – 45 years
  • Age when move into feeding area – Four to seven years (30 – 40cm carapace length)
  • Nesting female length – 107cm (range from 91 – 124cm)
  • Nesting female weight – 130kgs (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 from 19.8 – 28.4g)
  • Predators of hatchlings – Crabs, herons, dingoes and fish such as trevally and sharks