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
Just like birds and mammals carrying seeds through a rainforest, green sea turtles and dugong spread the seeds of seagrass plants as they feed. Our team at James Cook University’s TropWATER Centre has uncovered a unique relationship in the seagrass meadows of the Great Barrier Reef.
We followed feeding sea turtle and dugong, collecting samples of their floating faecal matter. Samantha then had the unenviable job of sifting through hundreds of smelly samples to find any seagrass seeds. These seeds range in size from a few centimetres to a few millimetres, and therefore can require the assistance of a microscope to be found. Once any seeds were found, they were stained with a chemical dye (Tetrazolium) to see if they were still viable (capable of growing).
Why is this important for turtles and dugong?
Green sea turtles and dugong are iconic animals on the reef, and seagrass is their food. Dugong can eat as much as 35 kilograms of wet seagrass a day, while sea turtles can eat up to 2.5% of their body weight per day. Without productive seagrass meadows, they would not survive.
This relationship was highlighted in 2010-11 when heavy flooding and the impact of tropical cyclone Yasi led to drastic seagrass declines in north Queensland. In the year following this seagrass decline there was a spike in the number of starving and stranded sea turtles and dugong along the entire Queensland coast.
The seagrass team at James Cook University has been mapping, monitoring and researching the health of the Great Barrier Reef seagrasses for more than 30 years. While coral reefs are more attractive for tourists, the Great Barrier Reef World Heritage Area actually contains a greater area of seagrass than coral, encompassing around 20% of the world’s seagrass species. Seagrass ecosystems also maintain vibrant marine life, with many fish, crustaceans, sea stars, sea cucumbers, urchins and many more marine animals calling these meadows their home.
These underwater flowering plants are a vital component of the reef ecosystem. Seagrasses stabilise the sediment, sequester large amounts of carbon from the atmosphere and filter the water before it reaches the coral reefs. Further, the seagrass meadows in the Great Barrier Reef support one of the largest populations of sea turtles and dugong in the world.
Seagrass meadows are more connected than we thought
Samantha’s research was worth the effort. There were seeds of at least three seagrass species in the poo of both sea turtles and dugong. And lots of them – as many as two seeds per gram of poo. About one in ten were viable, meaning they could grow into new plants.
Based on estimates of the number of animals in the coastal waters, the time it takes for food to pass through their gut, and movement data collected from animals fitted with satellite tags, there are potentially as many as 500,000 viable seeds on the move each day in the Great Barrier Reef. These seeds can be transported distances of up to 650km in total.
This means turtles and dugong are connecting distant seagrass meadows by transporting seeds. Those seeds improve the genetic diversity of the meadows and may help meadows recover when they are damaged or lost after cyclones. These animals help to protect and nurture their own food supply, and in doing so make the reef ecosystem around them more resilient.
Understanding recovery after climate events
This research shows that these ecosystems have pathways for recovery. Provided we take care with the environment, seagrasses may yet recover without direct human intervention.
This work emphasises how much we still have to learn about how the reef systems interconnect and work together – and how much we need to protect every part of our marvellous and amazing reef environment.
Samantha J Tol, PhD Candidate, James Cook University; Alana Grech, Assistant Director, ARC Centre of Excellence for Coral Reef Studies, James Cook University; Paul York, Senior Research Scientist in Marine Biology, James Cook University, and Rob Coles, Team leader, Seagrass Habitats, TropWATER, James Cook University
In the northern part of Australia’s Great Barrier Reef, the future for green sea turtles appears to be turning female.
A recent study has revealed that climate change is rapidly leading to the feminisation of green turtles in one of the world’s largest populations. Only about 1% of these juvenile turtles are hatching male.
Among sea turtles, incubation temperatures above 29ºC produce more female offspring. When incubation temperatures approach 33ºC, 100% of the offspring are female. Cooler temperatures yield more males, up to 100% near a lower thermal limit of 23ºC. And if eggs incubate at temperatures outside the range of 23-33ºC the risk of embryo malformation and mortality becomes very high.
As current climate change models foresee increases in average global temperature of 2 to 3ºC by 2100, the future for these turtles is in danger. Worryingly, warmer temperatures will also lead to ocean expansion and sea-level rise, increasing the risk of flooding of nesting habitats.
How scientists are tackling the problem
Green sea turtles’ sensitivity to incubation temperatures is such that even a few degrees can dramatically change the sex ratio of hatchlings.
Sea turtles are particularly vulnerable because they have temperature-dependent sex determination, or TSD, meaning that the sex of the offspring depends on the incubation temperature of the eggs. This is the same mechanism that determines the sex of several other reptile species, such as the crocodilians, many lizards and freshwater turtles.
Scientists and conservationists are well aware of how future temperatures may threaten these species. For the past two decades they have been investigating the incubation conditions and resulting sex ratios at several sea turtle nesting beaches worldwide.
This is mostly done using temperature recording devices (roughly the size of an egg). These are placed inside nest chambers among the clutch of eggs, or buried in the sand at the same depth as the eggs. When a clutch hatches (after 50 to 60 days) the device is recovered and the temperatures recorded are analysed.
Research has revealed that most nesting beaches studied to date have sand temperatures that favour female hatchling production. But this female bias is not immediately a bad thing, because male sea turtles can mate with several females (polygyny). So having more females actually enhances the reproductive potential of a population (i.e. more females equals more eggs).
But given that climate change will likely soon increase this female bias, important questions arise. How much of a female bias is OK? Will there be enough males? What is the minimum proportion of males to keep a sustainable population?
These questions are being investigated. But, in the meantime, alarming reports of populations with more than 99% of hatchlings being female stress the urgency of science-based management strategies. These strategies must be designed to promote (or maintain) cooler incubation temperatures at key nesting beaches to prevent population decline or even extinction.
The challenge of reversing feminisation
There are two general approaches to the problem:
- mitigate impacts at the most endangered nesting beaches
- identify and protect sites that naturally produce higher proportions of males.
Several studies emphasise that the natural shading native vegetation provides is essential to maintain cooler incubation temperatures. Thus, a key conservation action is to protect beach vegetation, or reforest nesting beaches.
Coastal vegetation also protects the nesting beach against wave erosion during storms, which will worsen under climate change. This strategy further requires coastal development to allow for buffer zones. Construction setback regulations should be enforced or implemented.
When natural shading is not an option, clutches of eggs can be moved either to more suitable beaches, or to hatcheries with artificial shading. Researchers have tested the use of synthetic shade cloth and found it is effective in reducing sand and nest temperatures.
Other potential strategies involve adding light-coloured sand on top of nests. This can help by absorbing less solar radiation (heat) compared to darker sand. Beach sprinklers have also been tested to simulate the cooling effect of rainfall.
The effectiveness of these actions has yet to be fully tested, but there is concern about some potential negative side effects. For example, excess water from sprinklers may cause fungal infections on eggs.
Finally, as much as mitigation measures are important, these are always short-term solutions. In the long run, prevention is always the best strategy, i.e. protecting the nesting beaches that currently produce more males from deforestation, development and habitat degradation.
Our recent research on the largest green turtle population in Africa reports unusually high male hatchling production. We found almost balanced hatchling sex ratios (1 female to 1.2 males). We attributed this mostly to the cooling effect of the native forest.
This, and similar nesting beaches, should be designated as priority conservation sites, as they will be key to ensuring the future of sea turtles under projected global warming scenarios.
Sea turtles face an uncertain future
Sea turtles are resilient creatures. They have been around for over 200 million years, surviving the mass extinction that included the dinosaurs, and enduring dramatic climatic changes in the past.
There is potential for these creatures to adapt, as they did before. This could be through, for example, shifting the timing of nesting to cooler periods, changing their distribution to more suitable habitats, or evolution of critical incubation temperatures that produce males.
But the climate today is changing at an unprecedented rate. Along with the feminisation of these turtles in the northern Great Barrier Reef, sea turtles globally face many threats from humans. These include problems associated with by-catch, poaching, habitat degradation and coastal development, plus a history of intense human exploitation.
In 2018, the prevalence of these species depends now more than ever on the effectiveness of conservation measures.
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.