Pressure

Many human activities are known, or thought likely to have, negative impacts on marine turtle populations. The Recovery Plan for Marine Turtles in Australia outlines the major impacts associated with each turtle species in Australia. This information is based on impacts from various activities upon each genetic stock  known for the species within Australia. While marine turtles are relatively well protected in Australian waters, they regularly make migrations of up to thousands of kilometres and subsequently are exposed to significant pressures in overseas waters (e.g. harvesting of green turtles in Indonesia).

Entanglement in fishing gear is one of many human pressures on turtles. This turtle is entangled in a crab pot. © QPWS

Within Australia, the Environment Protection and Biodiversity Conservation Act 1999  lists the following factors as Key Threatening Processes:

• Predation by the European Red Fox which affects green, loggerhead and leatherback turtle nests. A threat abatement plan has been developed for this key threatening process.
• Predation, Habitat Degradation, Competition and Disease Transmission by Feral Pigs which affects flatback turtle nests. A threat abatement plan is considered desirable, although one has yet to be developed.
• Incidental catch (bycatch) of Sea Turtle during coastal otter-trawling operations within Australian waters north of 28 degrees South. A threat abatement plan was not warranted at this stage given current actions by industry to implement bycatch mitigation devices (e.g. Turtle Excluder Devices - TEDs). TEDs are now mandatory in the Great Barrier Reef Marine Park (see Response).
• Injury or fatality to marine vertebrates caused by marine debris. A coordinated threat abatement plan could be an effective and efficient means of abating the threatening process by incorporating action to address both land-based and shipping and boating sources of marine debris. 

Within the Great Barrier Reef Marine Park, direct human pressures on turtle populations include bycatch in commercial fishing gear (e.g. trawl nets), traditional hunting, boat strike, habitat degradation, incidental catch in shark control gear, and ingestion of, or entanglement in, floating rubbish such as plastic bags and fishing line which can block guts after being eaten.

Pressure: boat strike

Boat strike is a major human pressure on turtles. © QPWS

In 1996, the Marine Wildlife Stranding and Mortality Database was established by the Queensland Parks and Wildlife Service (QPWS) to record information on marine wildlife strandings. For the years 1999 and 2000, the data show that human activities accounted for between 78% and 81% of mortality where the cause of death could be identified. Boat strike was the primary cause of mortality accounting for up to 60% of the known and reported human caused deaths. The increasing number of high speed vessels in the Great Barrier Reef will increase the risk of boat strike in areas frequented by turtles. Green turtles are especially at risk because of their habit of basking at the surface of the water.

Pressure: marine debris

Ingestion of fishing line is another human pressure on turtles. © QPWS

Marine debris consists of a variety of objects including litter and rubbish from boats and ships, items washed into the Great Barrier Reef from coastal waterways and lost fishing gear. As turtle hatchlings associate with converging zones of ocean currents, they come into close proximity to floating marine debris and consequently risk becoming entangled. Turtles may also mistake marine debris for food and ingest foreign objects, which may result in stomach and intestinal blockages. Entanglement in marine debris such as discarded fishing line may cause turtles to drown or become so encumbered that they cannot swim and feed properly. Debris washed up on nesting beaches may interfere with nesting turtles and prevent hatchlings from reaching the sea. 

Pressure: incidental catch

Prior to the introduction of TEDs, a large number of turtles were captured as bycatch in trawl nets.

Prior to the introduction of Turtle Excluder Devices (TEDs), a large number of marine turtles were drowned in trawl nets. Research suggests that some 5295 (± 1231) turtles were caught every year by trawlers operating in the waters off the East Coast of Queensland and the Great Barrier Reef, with between 1.1% and 7.8% of the turtles caught drowning. The full adoption of TEDs  in the trawl fleet has progressed slowly, due in part to ambiguities in guidelines and definitions relating to the use and design of TEDs, although this has been addressed (see Response). Turtles are also taken as bycatch  in the Queensland Shark Control Program (QSCP) which sets nets and baited ‘drum lines’ along popular swimming beaches. Currently, six nets and 127 drum lines are deployed within the Great Barrier Reef Marine Park. Research has showed that nets result in higher bycatch of marine turtles and the replacement of nets with drum lines has significantly reduced turtle bycatch. Relatively few turtles are now killed by QSCP gear (see Response). Nevertheless, these changes have not completely eliminated bycatch of turtles, with loggerhead turtles still being caught on drum lines in southern Queensland.

Pressure: traditional hunting

The direct take of turtles in the Great Barrier Reef is legally restricted to traditional hunting by Aboriginal peoples and Torres Strait Islanders living adjacent to the Marine Park. Turtle meat and eggs are an important traditional element of the diet of Australia's Indigenous peoples, particularly for celebrations and family gatherings.

The Great Barrier Reef Marine Park Authority (GBRMPA) is aware that illegal take occurs by non-Traditional Owners without appropriate permission and without following cultural protocols. The level of this take is unknown but may be high in certain areas of the Great Barrier Reef.

A recent survey of Indigenous fishing in northern Australia estimates that over a one year period in 2000-01, 3851 marine turtles (species unspecified) were taken by Indigenous hunters in north Queensland waters excluding the Torres Strait. Whilst the GBRMPA has reservations about the survey methodology, the current level of marine turtle hunting (constituting traditional and illegal catch) on the Great Barrier Reef appears to be unsustainable. Turtles are also hunted in neighbouring countries that share turtle stocks with the Great Barrier Reef. This combined direct mortality is thought to exceed the capacity to replace losses particularly for the northern Great Barrier Reef green turtle stock and hawksbill turtles.

Pressure: habitat degradation and disturbance

A variety of human activities can destroy or damage beach habitats that are nesting sites for marine turtles. Coastal developments may totally remove beach sand and restrict access to nesting areas. Lights from buildings and in particular, street lights, can confuse turtle hatchlings, causing them to move inland instead of moving out to sea. Additionally, unrestricted camping and vehicle traffic on beaches can cause damage to turtle nesting areas by compacting the sand, increasing dune erosion and by creating wheel ruts that can trap turtle hatchlings. Uncontrolled camping may disrupt nesting turtles and disturb or damage nests. Disturbance through noise, direct harassment of turtles and increasing vessel traffic may displace turtles from their normal nesting or foraging grounds.

Habitats such as seagrass meadows and coral reefs are also under increasing pressure from human impacts such as pollution. Seagrass and coral reef habitats are important foraging grounds for species such as green turtles that feed on seagrasses, or loggerhead turtles that feed on coral reef animals. A national review by an independent panel of scientists concluded that that the runoff of sediments from the land into the Great Barrier Reef has greatly increased since European settlement and that declining water quality poses a serious threat to the Great Barrier Reef. The increasing runoff of nutrients, pesticides and increasingly fine sediments can smother turtle habitats such as coral reefs and seagrass meadows, cause algal blooms and decrease the amount of light reaching seagrasses and corals. Declining water quality may also directly affect marine turtles by increasing the incidence of disease. For more information about the impacts of declining water quality on turtle habitats, see Environmental status – water quality, Environmental status - seagrass and Environmental status - corals.

Pressure: climate change

While there is little information about the specific effects of climate change on marine turtles, these impacts can be predicted by examining sea turtle biology and ecology. The changing temperatures and weather patterns associated with climate change are likely to have both direct physiological impacts on marine turtles, as well as indirect effects through impacts on of critical turtle habitats.

Climate change may alter the sex ratio of sea turtle populations, and changes to sea level and an increase in the frequency and intensity of storms may destroy critical nesting sites.

The sex of marine turtle hatchlings is determined by the incubation temperature of the eggs, with warmer incubation temperatures giving rise to higher numbers of female hatchlings. Climate change may alter beach sand temperatures and thus, cause changes in the male/female sex ratio in marine turtle populations. Once hatched, changing sea temperatures may affect the growth rate of hatchlings and juvenile turtles.

Climate change may also affect critical turtle habitats. Rising sea levels and an increase in the frequency and intensity of severe storms may erode critical nesting beaches and reduce the availability of suitable nesting sites. Climate change may also alter ocean circulation patterns and disrupt marine food webs, both of which would have significant impacts on turtles during pelagic phases of their lifecycle. For example, the El Niño Southern Oscillation is thought to have a significant impact on the numbers of green turtles nesting in the Great Barrier Reef. Higher than normal sea temperatures have resulted in major coral bleaching episodes in 1998 and 2002, as well as seagrass burning. Continued increases in sea temperatures may lead to an increasing frequency and intensity of coral bleaching and seagrass burning events (see Environmental status – corals and Environmental status – seagrass), reducing the amount and/or quality of both shelter and food for marine turtles. The long life span and generation times of marine turtles reduces the ability of these animals to cope with rapidly changing climatic conditions.

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