But we weren’t really sure whether plastic eaten by turtles actually kills them, or if they just happen to have plastic inside them when they die. Another way to look at it would be to ask: how much is too much plastic for turtles?
This is a really important question. Just because there’s a lot of plastic in the ocean, we can’t necessarily presume that animals are dying from eating it. Even if a few animals do, that doesn’t mean that every animal that eats plastic is going to die. If we can estimate how much plastic it takes to kill a turtle, we can start to answer the question of exactly how turtle populations are affected by eating plastic debris.
In our research, published today in Nature Scientific Reports, we looked at nearly 1,000 turtles that had died and washed up on beaches around Australia or were found in nets. About 260 of them we examined ourselves; the others were reported to the Queensland Turtle Stranding Database. We carefully investigated why the turtles died, and for the ones we examined, we counted how many pieces of plastic they had eaten.
Some turtles died of causes that were nothing to do with plastic. They may have been killed by a boat strike, or become entangled in fishing lines or derelict nets. Turtles have even been known to die after accidentally eating a blue-ringed octopus. Others definitely died from eating plastic, with the plastic either puncturing or blocking their gut.
Some turtles that were killed by things like boat strikes or fishing nets nevertheless had large amounts of plastic in their guts, despite not having been killed by eating plastic. These turtles allow us to see how much plastic an animal can eat and still be alive and functioning.
The chart below sets out this idea. If an animal drowned in a fishing net, its chance of being killed by plastic is zero – and it falls in the lower left of the graph. If a turtle’s gut was blocked by a plastic bag, its chance of being killed by plastic is 100%, and it’s in the upper right.
The animals that were dead with plastic in their gut, but had other possible causes of death have a chance of death due to plastic somewhere between 0 and 100% – we just don’t know, and they can fall anywhere in the graph. Once we have all the animals in the plot, then we can ask whether we see an increase in the chance of death due to plastic as the amount of plastic in an animal goes up.
We tested this idea using our turtle samples. We looked at the relationship between the likelihood of death due to plastic as determined by a turtle autopsy, and the number of pieces of plastic found inside the animals.
Unsurprisingly, we found that the more plastic pieces a turtle had inside it, the more likely it was to have been killed by plastic. We calculated that for an average-sized turtle (about 45cm long), eating 14 plastic items equates to a 50% chance of being fatal.
That’s not to say that a turtle can eat 13 pieces of plastic without harm. Even a single piece can potentially kill a turtle. Two of the turtles we studied had eaten just one piece of plastic, which was enough to kill them. In one case, the gut was punctured, and in the other, the soft plastic had clogged the turtle’s gut. Our analyses suggest that a turtle has a 22% chance of dying if it eats just one piece of plastic.
A few other factors also affected the animals’ chance of being killed by plastic. Juveniles eat more debris than adults, and the rate also varies between different turtle species.
Now that we know how much is too much plastic, the next step is to apply this to global estimates of debris ingestion rates by turtles, and figure out just how much of a threat plastic is to endangered sea turtle populations.
Britta Denise Hardesty, Principal Research Scientist, Oceans and Atmosphere Flagship, CSIRO; Chris Wilcox, Senior Research Scientist, CSIRO; Kathy Ann Townsend, Lecturer in Animal Ecology, University of the Sunshine Coast, and Qamar Schuyler, Research Scientist, Oceans and Atmospheres, CSIRO
The Australian government plans to target invasive European carp with a herpes virus, leaving hundreds of thousands of tonnes of carp rotting in the river systems that supply our drinking water and irrigate the fruit and vegetables we eat.
The aim of “Carpageddon” is to return Australian aquatic ecosystems to their pre-carp state by eliminating or reducing the serious pest species.
Carp currently make up 83% of the fish biomass in the Murray-Darling Basin in New South Wales. They alter river and lake habitats in a way that reduces habitability for native species, including five threatened species. They also have a major impact on inland fisheries, with an estimated annual economic cost of A$22 million.
This all makes a substantial argument for releasing a carp killing herpes virus. However, dealing with the aftermath could cost A$30 million for NSW alone.
Cleanup costs could be reduced by introducing viruses to discrete populations. However, if the virus escapes into the Murray-Darling Catchment, we will lose control of the virus spread and carp death will be rapid and widespread.
Without a dedicated cleanup effort, the sudden influx of millions of dead fish could permanently pollute our waterways. A potential solution is to recruit nature’s cleaners to do our work for us – scavengers like turtles and crayfish. They could save us from carcass-choked rivers and wetlands, but only if we can protect them from endangerment and extinction.
Turtles and crayfish are our unlikely saviours
Carp carcasses are normally eaten by scavengers, a process that’s vital to the food web (the system of what eats what in a given environment). In fact, the majority of dead fish are consumed by scavengers.
As such, simply removing the carp carcasses may reduce the overall amount of nutrients in the ecosystem. This would destabilise the food web, especially for scavengers such as turtles and crayfish who rely on them.
Instead, these scavenging species can provide crucial biocontrol. They would eat any decomposing flesh in our water systems, particularly in areas we can’t easily access with nets, boats and trucks. They would maintain the quality of our waterways in three ways:
Slow the spread of bacteria that break down dead fish, keeping water safe to drink and limiting deoxygenation that could devastate native fish species;
Digest carp directly into basic nutrients (fertiliser) that is more readily absorbed by plants and primary producers;
Semi-permanently store carp nutrients in their slow to decompose shells and exoskeletons, preventing or limiting toxic algal blooms caused by excess nutrients in water.
Our unlikely saviours are also dying
Threats to crayfish include agricultural and urban expansion, recreational fishing, pollution from surface runoff and insecticides, and introduced species such as trout and cane toads.
Turtles on the other hand, are in sharp decline throughout the Murray Catchment and elsewhere in Australia. A recent gathering of turtle experts in Canberra discussed major threats to turtles, and ways to protect them.
The meeting addressed major causes behind the 2% annual mortality rate of adult turtles that is leading the species to rapid extinction. Cars and foxes kill a significant number of adult turtles every year, and foxes destroy more than 95% of turtle nests in the Murray-Darling Basin.
Changes to the hydrology of the Murray Catchment may also impact turtles. Some species require permanent wetlands, while others prefer to move between temporarily flooded wetlands and more permanent waters.
All of these threats together may cause turtles to become functionally extinct in the near future, meaning they cannot play their significant role in the ecosystem anymore.
How can we help conserve the turtle population?
Such a sudden decimation of carp has potentially catastrophic consequences. But it may also be an excellent opportunity to recognise the importance of turtles and prioritise their conservation.
In a recent study, headstarting was named as the only management tool that could protect freshwater turtles from the multiple threats throughout their life cycle and eliminate all risks of extinction.
Headstarting involves rearing eggs or newborn animals in captivity, then releasing them into the wild. It has been controversial for decades, but releasing thousands of little turtles throughout the Murray River just might rescue us from the post-apocalyptic effects of Carpageddon.
Ricky Spencer, Associate Professor of Ecology, Western Sydney University; Claudia Santori, PhD candidate, University of Sydney; James Van Dyke, Postdoctoral fellow, Western Sydney University, and Michael B. Thompson, Professor in Zoology, University of Sydney
Recent calls for a ban on legal traditional hunting of dugongs and marine turtles imply that hunting is the main threat to these iconic species in Australia. The science indicates otherwise.
While more is being done to address traditional hunting than any of the other impacts, the main threats to their survival often pass unnoticed.
The real threat to sea turtles
The draft Recovery Plan for Marine Turtles in Australia evaluated 20 threats to the 22 populations of Australia’s six species of marine turtle. Climate change and marine debris, particularly “ghost nets” lost or abandoned by fishers, are the greatest risks for most stocks.
Indigenous use is considered to be a high risk for three populations: Gulf of Carpentaria green turtles, Arafura Sea flatback turtles and north-eastern Arnhemland hawksbill turtles.
However, in each of these cases it is the egg harvest, not hunting, that causes concern. International commercial fishing is also a high risk for the hawksbill turtle, whose future remains uncertain. Traditional hunting of marine turtles in Australia is limited to green turtles.
Is hunting a threat?
The Torres Strait supports the largest dugong population in the world and a globally significant population of green turtles. Archaeological research shows that Torres Strait Islanders have been harvesting these species for more than 4,000 years and the dugong harvest has been substantial for several centuries.
The situation for dugongs is very different in the waters of the Great Barrier Reef south of Cooktown. The Great Barrier Reef Outlook Report classifies the condition of the dugong population in this region as poor.
Modelling indicates that the southern Great Barrier Reef stock of the green turtle, which live and breed south of Cooktown, is increasing.
Nonetheless, both green turtles and dugongs died in record numbers in the year after the extreme floods and cyclones of the summer of 2010-11. Dugongs stopped breeding in the Great Barrier Reef region south of Cooktown.
Thankfully, our current aerial survey indicates that dugong calving has resumed as inshore seagrass habitats recover. There is no evidence that the 2011 losses significantly affected green turtle numbers.
Traditional owners are the first managers of our coastal waters, with cultural practices extending back thousands of years. They have the most to lose from any loss of turtles and dugongs. It is therefore in their best interests, and the government’s best interest, to work in partnership to protect and sustainably manage these species.
Longstanding tensions between traditional owners and tourist operators are behind much of the opposition to traditional hunting in the Cairns area. Some of these tensions have been relieved by the Gunggandji Traditional Use of Marine Resources Agreement signed in June 2016.
Under this agreement, the traditional owners decided to cease hunting turtles and dugongs in the waters surrounding Green Island, Michaelmas Cay and Fitzroy Island.
The Gunggandji agreement is the seventh to be signed between the Great Barrier Reef Marine Park Authority and traditional owners. In addition, there are two Indigenous land use agreements that address hunting issues in the Great Barrier Reef.
In the Torres Strait, dugong and turtle hunting is managed through 14 (soon to be 15) management plans. There are similar agreements with traditional owners and management agencies in other regions in northern Australia.
Indigenous rangers are crucial to implementing all these agreements in collaboration with management agencies and research institutions. Rangers deliver the practical, on-the-ground arrangements to conserve these species in their Sea Country.
The Great Barrier Reef Marine Park Authority has implemented an Indigenous Compliance Program that authorises trained Indigenous rangers to respond to suspicious and illegal activities that they encounter as part of their work.
Indigenous rangers also remove marine debris from remote beaches. The community-based organisation GhostNets Australia has worked with 31 coastal Indigenous communities to protect over 3,000km of northern Australia’s saltwater country from ghost nets. These community projects have been instrumental in rescuing turtles, clearing ghost nets off beaches and identifying key areas to aid management agencies to better understand the impact.
Traditional owners from the Torres Strait and the northern Great Barrier Reef also play a valuable role in intervention works at Raine Island, one of the world’s most significant green turtle rookeries. This includes rescuing stranded turtles, using fences to stop turtles from falling over cliffs, and altering beach profiles.
What about welfare?
Traditional hunting raises animal welfare issues. The turtle and dugong management plans developed by the Torres Strait communities explicitly address animal welfare. The Torres Strait Regional Authority has been working with a marine mammal veterinarian and traditional owners to develop additional methods of killing turtles humanely.
Indigenous hunters who breach state and territory animal welfare laws can be prosecuted. But more widespread animal welfare problems, not associated with hunting, are largely hidden and ignored. The Queensland Strand Net Program reported that 879 turtles died of their wounds from vessel strike between 2000 and 2011.
Other serious animal welfare issues are associated with animals drowning in nets and being caught in and ingesting marine debris. In addition, the potential impact of emerging threats like underwater noise pollution and water quality remain as substantial knowledge gaps. These matters tend not to make the headlines.
Australian waters are home to some of the world’s largest populations of marine turtles and dugongs. A comprehensive and balanced approach to their conservation and management is required to enable our grandchildren and their children to enjoy these amazing animals.
James Whitmore, The Conversation
Immersion in seawater kills sea turtle eggs, suggesting that sea turtles are increasingly at risk from rising seas, according to research published today in Royal Society Open Science.
In a laboratory experiment, researchers immersed green turtle eggs in seawater for varying lengths of time. The researchers tested eggs of various ages, and then counted the number of eggs that hatched. They found that immersion for six hours reduced survival by a third.
The study partly explains reduced numbers turtle of hatchlings recorded at Raine Island, home to the largest population of green sea turtles in the world.
David Pike, lecturer in tropical biology at James Cook University and lead author of the study, said turtle nests low down on beaches could be underwater for six hours during abnormally high “king” tides or storm surges.
Michele Thums, ecologist at the Australian Institute of Marine Science, said that given climate projections for increased severe weather events, this could mean fewer hatchlings survive in the future.
But every beach will see different impacts from rising seas, said Tim Dempster, senior lecturer in marine biology at University of Melbourne.
“You can’t just take [a…] scenario of a certain degree of warming, say that will lead to a certain amount of sea level rise, project how much land will be inundated and then project what proportion of nesting habitat will be affected,” he said.
Turtle embryos need oxygen to develop into baby turtles, and immersion in water prevents oxygen from the soil entering the eggs. The embryos effectively suffocate, a process known as “hypoxia”.
Thums said that while most turtles nest above the high tide line and are rarely immersed for six hours, “there are always inexperienced turtles that will lay further down the beach and also there is competition at high density nesting sites like Raine Island”.
Compared to the rest of the world, green sea turtles on Raine Island have a much lower level of breeding success, which could lead to a large decline in the number of breeding adults in the future.
Pike said the low level of success could be partly explained by inundation, but there were likely other factors at work.
“One possibility is that the sand is full of bacteria from all of the rotting eggs that are beneath the sand, and that any fresh eggs laid there may be exposed to bacteria that overgrow the egg and kill the embryo,” he said.
“Another possibility is that contaminants (heavy metals, pesticides) are being passed from the mother turtle to the eggs, and that may cause the embryos to die.”
The Queensland Department of Environmental Heritage and Protection is currently trying to raise low lying spots on Raine Island by moving sand. The island could lose between 7 and 27% of its area thanks to rising seas.
With Janelle Braithwaite, editor at The Conversation.