Crocodiles

Crocodiles do occur in the Great Barrier Reef World Heritage Area but they occur predominantly in coastal and estuarine areas adjacent to the Great Barrier Reef Marine Park.

The primary habitats for crocodiles are coastal and estuarine systems of the Queensland coast outside the boundaries of the Great Barrier Reef Marine Park. Nevertheless, satellite tracking data show that crocodiles do enter the Great Barrier Reef Marine Park when moving between these coastal habitats. As such, they are considered to be temporary migrants in the Great Barrier Reef. There is widespread perception that crocodile numbers have significantly increased since the crocodile gained protected species status in 1974. However, recent population surveys suggest that crocodile populations have only marginally increased since they were protected. Many human activities such as coastal development, mesh netting of tidal rivers and removal of riparian vegetation are believed to reduce the capacity for crocodile numbers to increase.

Marine Turtles

Condition

Marine turtles of the Great Barrier Reef

Individual female turtles return to nest at beaches in the same area in which they were born.

Of the world's seven species of marine turtles, six are found in the waters of the Great Barrier Reef: namely the green, hawksbill, loggerhead, flatback, olive ridley and leatherback turtles. The Great Barrier Reef is one of the remaining havens for marine turtles in the world. Four of these six species (green, loggerhead, hawksbill and flatback) have internationally significant populations in the Great Barrier Reef. The flatback turtle is endemic to Australia and only nests on Australian beaches.

All marine turtle species have the same general life cycle. Most are slow growing and take decades to reach sexual maturity: 20-25 years for flatback and loggerhead turtles, 30-50 years for green turtles. Females do not breed every year and have non-breeding periods of between 5-8 years for green turtles and 2-3 years for flatback turtles. This means that marine turtles require high annual survivorship of adults and near-adults in order for populations to be maintained.

Marine turtles breed at a limited number of nesting beaches. Many turtles travel thousands of kilometres between foraging and breeding grounds.

Marine turtles aggregate to breed at a limited number of nesting sites, with individual females returning to nest at beaches in the same area in which they were born. Such areas include:

• Raine Island and Heron Island which are both internationally significant green turtle nesting beaches;
• Milman Island which is an internationally significant hawksbill turtle nesting beach;
• Peak Island which is a major flatback turtle nesting site; and
• Wreck Island, a major loggerhead nesting site.

Marine turtles can migrate as far as 2600 km between nesting beaches and feeding areas, but repeatedly return to the same feeding and nesting sites throughout their lives.

The conservation status of marine turtles

Globally, marine turtle populations are under threat (see Table 1). The hawksbill is listed as 'critically endangered' on the 2002 IUCN Red List of Threatened Species. This means that the species is at 'extremely high risk of extinction in the immediate future'. Green, olive ridley, loggerhead and leatherback turtles are listed as 'endangered', meaning that they are at 'high risk of extinction in the near future'. In Australia, marine turtles are also considered threatened and are listed in Australia’s Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act 1999) and the Queensland Nature Conservation Act 1992. Collectively, these Acts and Regulations reduce the pressure on marine turtles through a variety of mechanisms (see Response).

Table 1: The conservation status of marine turtles found within the Great Barrier Reef Marine Park

¹.IUCN Red List categories: Extinct, Extinct in the Wild, Critically Endangered, Endangered, Vulnerable, Lower Risk, Data Deficient (Source: 2002 IUCN Red List of Threatened Animals).

².Commonwealth Environment Protection and Biodiversity Conservation Act 1999 categories: Extinct, Extinct in the Wild, Critically Endangered, Endangered, Vulnerable, Lower Risk, Data Deficient.

³.Queensland Nature Conservation (Wildlife) Regulation 1994 schedules: Presumed Extinct, Endangered, Vulnerable, Rare, Common.

The current condition of marine turtles in the Great Barrier Reef

In the Great Barrier Reef, most scientific studies of marine turtle populations have been focused on green, loggerhead, hawksbill and flatback turtles and long term survey data are available for some species. Nevertheless, the long lifespan of marine turtles means that even these long term studies do not cover a single generation, so that the impacts of current pressures may not become evident for decades.

Condition: loggerhead turtles

Since surveys began in the late 1970s the number of nesting females has steadily declined by 50-80% from about 1000 breeding females to a few hundred. The east Australian population of loggerhead turtles used to represent the bulk of the South Pacific stock (one of about eight loggerhead stocks globally). If this population disappears, it will mean the effective removal of the South Pacific stock. As female turtles return to nest in the area where they hatched, it is highly unlikely that a population that has ‘died out’ would be recolonised by turtles from another population somewhere else in the world.

Condition: green turtles

There are two genetically distinct breeding stocks of green turtles in the Great Barrier Reef. Nesting sites for the southern stock are concentrated in the Capricorn/Bunker group of islands, with an average annual nesting population of 8,000 females. The northern green turtle stock predominantly nests around Raine Island and Moulter Cay, with an average annual nesting population of 30,000 females. To date, there have been no detectable declines in the number of nesting green turtles at these sites. However, the 20-25 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, population models using accumulated demographic data suggest that the northern and southern stocks may be exhibiting characteristics of a population 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.

Numbers of nesting female hawksbill turtles are continuing to decline.

Condition: hawksbill turtles

Hawksbill turtles are found all over the world but tend to nest in low numbers. Around the Great Barrier Reef region, hawksbill turtle nesting areas are only found north of Princess Charlotte Bay and in the Torres Strait. The only nesting population for which there is sufficient information is at Milman Island, where data indicates a decline in the number of nesting females of about 3% per year. Tagging studies show that hawksbill turtles migrate to neighbouring countries where they may be harvested (see Pressure).

Condition: flatback, olive ridley and leatherback turtles

There are no indications that the east Australian flatback turtle stock is in decline with surveys showing relatively stable numbers of nesting flatback turtles. Olive ridley and leatherback turtles are uncommon in the Great Barrier Reef and have received little scientific attention. The long-term trends in the populations of these species are unknown. However, given the broad scale decline in the South Pacific leatherback turtle stock, the animals that occur in the Great Barrier Reef are likely to be part of that declining population, which raises concerns for this species in the Great Barrier Reef.