2. Biodiversity

The Region is home to 6 species of marine turtle, all of which are listed as either vulnerable or endangered under Commonwealth and Queensland legislation and recognised internationally as species of conservation concern. Marine turtles have significant cultural importance for Traditional Owners. For example, the green sea turtle is a totem species for the Mandubarra people ⁷⁶ and turtle nesting is one of the key indicators in the Woppaburra seasonal calendar.⁷⁸ Marine turtles also have economic importance for the tourism industry as an iconic Reef animal.

Globally important nesting and foraging areas exist in the Region for green, flatback, hawksbill and loggerhead turtles. Olive ridley and leatherback turtles forage in the Region but are generally not known to nest. However, in December 2023 an olive ridley turtle nested on a beach near Mackay.⁴³⁸ Marine turtles are ecologically important to reef ecosystems, both as herbivores and predators, and they contribute to the cycling of nutrients and sediment creation.⁴³⁹ Green turtle herbivory significantly influences the condition and structure of seagrass meadows,^142,440 and enhances seed dispersal and germination ⁴⁴¹ (Sections 2.3.4 and 2.4.2). Some Traditional Owner groups report that the numbers of turtle have declined on their Sea Country and are having less successful nesting seasons.^76,442 

A range of factors influence the health and condition of marine turtles. Although the risks vary between species and genetic populations, the key threats to marine turtles include climate change, marine debris, domestic and international take, terrestrial predation, entanglement, fisheries bycatch and light pollution.⁴⁴³ Significantly, climate change is leading to unsustainably high female to male ratios in sea turtle populations across the Region (and globally),^444,445,446 as hatchling sex is determined by sand temperature at nesting beaches.⁴⁴⁷ Rising sand temperatures lead to decreased egg survival.⁴⁴⁸ Nest temperatures remain cooler in areas that are shaded or experience rainfall during the nesting period,⁴⁴⁹ and manual application of fresh water or sea water to nests has been trialled as a cooling management measure in some locations.⁴⁵⁰ Other impacts of climate change, such as rising sea levels ⁴⁵¹ and increased storm events, are changing the availability (for example, sand depth) of turtle nesting habitat in some areas ⁴⁵² and affecting important food resources, such as seagrasses and corals ⁴⁴⁴ (Sections 2.3.4 and 2.3.5). 

Northern and southern green turtle populations on the Reef are genetically distinct. The southern population mainly nests at North West, Wreck, Hoskyn, Tryon, Heron, Lady Musgrave and Masthead islands. This population is considered to be moderately recovering from historical lows after the cessation of commercial harvest in 1950 and implementation of large-scale habitat protection.⁴⁴³ The population increase has continued since 2019, although the increase in numbers observed during the most recent nesting season was smaller than in previous seasons.⁴³⁸ In addition to key threats outlined for marine turtles generally, contemporary pressures on southern green turtles include exposure to reduced water quality.^{263,453,454,455} 

The northern green turtle population (for which 90 per cent of nesting occurs on Raine Island and Moulter Cay) ⁴⁵⁶ is considered to be in significant decline.⁴⁴³ Low nesting and hatchling success has been reported from Raine Island since the mid-1990s.^452,457 Since 2019, declines in hatchling production have continued despite some successes from management interventions implemented on the island across the past decade.^452,458,459 The decline is attributed to a number of potential factors, including reduced suitability of nesting habitat,⁴⁵² early embryo death,⁴⁶⁰ and adult mortality outside the breeding season from unquantified hunting pressure ^12,443. Very few male green turtles have been recruited to the northern green turtle population in recent years, reflecting effects of high sand temperatures on hatchling sex.⁴⁴⁶ 

Since 2019, green turtle strandings, which can occur as a result of both human and natural causes, have increased slightly (Figure 2.10).

Hawksbill turtles nest on islands in the far north of the Region, including a significant rookery at Milman Island.⁴⁶² Key foraging areas in the Region include reefs north of Princess Charlotte Bay and the Capricorn-Bunker reefs. Individuals foraging in the Region migrate to nesting sites within the Region, other parts of northern Queensland, and the wider Coral Sea region.^463,464,465 Historically, this species was commercially harvested for tortoiseshell. Ongoing legal and illegal harvest in the wider Coral Sea region may still affect the species,⁴⁴³ and survival to breeding may be low for turtles outside the Region.⁴⁶⁶ The numbers of clutches at the Milman rookery has steadily declined since monitoring began in 1990.⁴⁶² The population is currently considered to be in significant decline and modelling suggests a risk of complete loss of nesting locally (and potentially for the north Queensland stock) within the next 10 to 15 years.^443,462

Loggerhead turtles in the south-west Pacific population nest exclusively along the east coast of Australia and New Caledonia. Important nesting beaches are found on the mainland coast between the Woongarra coast (Mon Repos beach) and Wreck Rock, as well as on Tryon, Heron and Erskine islands. Key foraging areas and migration routes occur throughout the wider Region.⁴⁶⁷ Hatchlings disperse as far as South America in ocean currents before returning to Australian foraging and nesting grounds as young adults approximately 16 years). Significant declines in the number of turtles returning to Australian nesting beaches have been attributed to bycatch mortality and ingestion of marine plastic, either in Australian waters or throughout the oceanic journey.⁴⁶⁸ Moderate population recovery was observed for mainland nesters after the introduction of mandatory turtle exclusion devices in prawn trawl fisheries in 2001.⁴⁶⁹ More recently, an overall decline in the number of nesting loggerhead turtles was recorded on the Woongarra coast over the 5 years since 2016 ⁴⁷⁰ and on the Wreck Rock coast over the past 6 summers.⁴⁷¹ The annual recruitment of young female loggerheads into the Woongarra coast breeding population has declined over the past 2 decades.⁴⁷⁰ Island nesters have not demonstrated strong recovery post 2001. Mainland and island nesters have recently been shown to use different foraging grounds.⁴⁶⁷ Coastal development and predation continue to affect mainland nesting sites, while in-water life stages are affected by overseas fisheries, boat strikes, marine debris and changes to foraging habitat. Initial beach cooling studies suggest that measures such as artificial rain and beach shades can improve hatchling emergence success.⁴⁷⁰ Overall, the species is endangered, and the population is considered to be severely depleted.⁴⁴³

Flatback turtles are endemic to the Australian continental shelf. Several genetically distinct populations have been identified. The eastern Queensland stock are known to forage throughout the Region, nesting on southern mainland and island beaches (including Peak, Curtis, Wild Duck and Avoid islands).⁴⁷² The nesting population at Peak Island declined by 50 per cent from 1980 until 2001 but is now increasing.^443,473 Nesting populations at Curtis Island have also increased since about 2010,⁴⁷³ while Wild Duck Island continues to be the largest rookery on the east coast.⁴⁷⁴ Nevertheless, the species remains vulnerable ⁴⁴³ due to the relatively low population size overall and reductions in the number of young females recruiting into the nesting population. 

Leatherback turtles from the north-west Pacific population forage and migrate through the Region. Historically, a small population used to nest in eastern Queensland. It is unknown whether these animals were part of the wider north-west Pacific population or a genetically separate population. Although a mating pair was sighted off Bundaberg in 2021,⁴⁷⁵ no nesting has been reported from the east coast of Australia since 1996.⁴⁴³ The species is endangered following significant decline regionally.^443,476

Olive ridley turtles are sighted rarely on the Great Barrier Reef. Individuals foraging in the Region likely originate from the significantly depleted western Cape York Peninsula nesting population, where they are affected by sandmining.⁴⁴³

Declining population trends are reported for most species and distinct genetic populations of marine turtles in the Region. Since 2019, the southern green turtle population and eastern Queensland flatback turtles have shown moderate recovery. The nesting population of hawksbill turtles is in significant decline and could be at risk of loss within the next 10 to 15 years. Key threats include climate change, marine debris, national and international take, terrestrial predation, fisheries bycatch and light pollution.