Australasian Science: Australia's authority on science since 1938

Garbage Guts

Photo: Denice Askebrink

A healthy adult hawksbill turtle checking out divers in the Maldives. Photo: Denice Askebrink

By Blake Chapman

Why are turtles attempting to eat shopping bags, balloons and other forms of human rubbish in preference to natural food sources?

The marine environment is an amazing place, filled with an assortment of species more diverse and wondrous than the most imaginative of human brains could ever concoct. This environment continually sustains, influences and inspires us, and many of the ocean’s inhabitants have won the hearts of the masses.

One of the more endearing families of animals in this environment is the marine turtles. Marine turtles are an important component of both traditional and modern cultures around the world. They are undeniably beautiful animals, with an inquisitive but laid-back behaviour that attracts snorkellers and divers to them the world over.

For the lucky few who have the opportunity to really get to know these animals, it becomes crystal-clear that the true depth of their beauty is often not fully recognised from fleeting glimpses, but is found in the slight but mesmerising natural variations in shell colour and pattern, the deep, dark eyes that beg for attention and understanding, and, above all else, the eccentricities in disposition among individuals.

The troubling reality, though, is that far out of our view is a silent but compounding threat to the health and vitality of these animals. It is completely man-made, affects every species of marine turtle and is getting worse every day. The threat is marine debris.

The term “marine debris” essentially encompasses any sort of unnatural debris that ends up in the marine environment. Common examples of this debris include plastics such as shopping bags and balloons, fishing line, nets and ropes. Unfortunately, most marine debris is very slow to decompose, resulting in a steady accumulation of waste in the marine environment.

Marine debris is often written off as an aesthetic concern or an isolated problem, and is easily overlooked because of its inconspicuousness and predominantly out-of-sight impacts. In reality, marine debris is a devastating problem that impacts the environment, the economy, human health and safety, and many coastal cultures.

The facts and figures emerging on marine debris are so overwhelming that they are hardly comprehensible. In terms of human health and dollar signs, the United Nations Environment Programme reported in 2004 that the annual global economic impact of death and disease from polluted coastal waters was US$12.8 billion per year, with the annual economic impact of hepatitis from contaminated seafood alone equating to US$7.2 billion.

Environmentally speaking, marine debris is an indiscriminate threat of staggering proportion. At least 267 species worldwide are documented to have either ingested marine debris or become entangled in it, including tiny crustaceans, the largest of marine mammals and everything in between – seabirds, turtles, seals, dolphins, fish… the list goes on and on.

The Australian Marine Conservation Society has reported that 5.5 million items of waste are discarded into the oceans every day. By weight, three times as much rubbish goes into the oceans compared with the amount of fish that comes out.

A 1997 report by David Laist, an ecologist at the Marine Mammal Commission in Washington DC, estimated that 86% of marine turtles, 36% of seabirds and 28% of marine mammals worldwide are affected by plastic debris alone. In one tragic but eye-opening display of the impacts of marine debris, an 8-metre Bryde’s whale was found in Cairns with 6 m2 of plastic in its stomach.

The effects of marine debris are wide-ranging. Animals can become entangled or entrapped, causing drowning, amputation, abrasion or a limited ability to capture or consume food. Debris can be ingested, causing gastrointestinal blockage, injury to the stomach lining, ulceration, toxicological implications, reduced reproductive ability or a reduction in appetite. Furthermore, many of these conditions leave survivors in a generally weakened state, making them more susceptible to predation, parasites and disease, and less fit for reproduction.

Plastics also pose a significant threat to regional marine biodiversity as they have the potential to transport encrusting organisms such as bacteria, algae, barnacles and hydroids to regions that are unable to cope with or control the spread of these alien species.

While reports have previously detailed the devastating impact of marine debris on the wildlife in other regions of the world, the situation was not thought to be as bad in Australia. However, when researchers at The University of Queensland began to take note of the frequency of debris ingestion in stranded marine turtles on the south-east coast of Queensland, it became alarmingly clear that our complacency was more of a soothing “ignorance is bliss” impression masking the reality.

Qamar Schuyler, a PhD student at The University of Queensland, is working to turn this situation around, dedicating her time and efforts to investigating the debilitating and often fatal interactions between turtles and marine debris. While her overriding aim is to determine the effects that marine debris is having on the marine turtle population of Queensland, her research is multifaceted and contributes to a much larger collaborative project between The University of Queensland, CSIRO, Earth Watch and Shell. “Turtles are my passion, but in this project they are just model species being used to show the impacts of marine debris on a large scale,” she says.

Over the course of her degree, Schuyler hopes to address large-scale questions such as how much debris is actually present in the marine environment and what percentage of turtles stranded have been affected by marine debris. Stranded turtles, in this case meaning those that are washed up on the beach or stuck in the surf and in too poor condition to return to their natural environment, are only one category of affected animals, but the ones that are most often in the public eye. These animals are generally either already deceased or at death’s door, and are usually “floaters” – meaning that for one reason or another they are unable to sink down to the safety and food resources found near the sea floor. Floating in turtles is a common symptom of a large range of ailments, including (but not limited to) debris ingestion, stress, respiratory infection, parasite overload or injury. When an animal becomes stranded, it is often exhausted, starving, in poor body condition, susceptible to sunburn and exposure, dehydrated and highly stressed.

The degree of damage that results from marine debris ingestion is based on a variety of factors, including the size, quantity and composition of the debris and the size, condition and anatomy of the animal that ingests it. The ingestion of marine debris could have negligible and fleeting impacts, lead to a slow and painful death, or have any range of intermediate effects. Therefore, through necropsies of the stranded turtles found along the Queensland coast, Schuyler hopes to identify if and how much debris was ingested and determine if debris was the cause, a compounding factor or even unrelated to the death of the animal.

Schuyler and her colleagues recently reported that 34% of stranded marine turtles from the south-east coast of Queensland had marine debris in their gastrointestinal tract. While this number may seem low, it only represents animals that have ingested debris and become stranded.

It is a common misconception that debris instantly kills animals. This is not the case. Schuyler reiterates the fact that the issue of marine debris ingestion is more complicated than this, especially if there are toxicological implications. “Marine debris directly kills animals,” she says, “but even if the animal isn’t killed it doesn’t mean that the debris hasn’t affected their reproductivity or led to other complications”.

In an effort to further understand the reasoning behind debris ingestion, Schuyler is looking for variations in debris ingestion between marine turtle species, life stages and the areas they are found. These are all very important concepts because they represent such a great amount of diversity.

Marine turtles are well-known for their significant but mysterious age-related ecological shifts. For example, green and hawksbill turtles spend their developmental years in the open ocean, where it is thought that they drift with downwelling currents. While this behaviour probably provides the growing turtles with the added benefits of a greater food source (generally plankton) and protection from predators, it also exposes them to a greater volume of debris, which is carried and aggregated by the same currents.

These turtles are then recruited back to coastal waters as late-stage juveniles, where they remain for the rest of their lives. At this time, their diet also shifts to predominantly benthic resources, such as seagrasses, crustaceans, sponges, algae and the occasional jellyfish.

While this component of Schuyler’s study is still ongoing, preliminary data has shown that there is no significant difference between the debris ingestion rates of green and hawksbill turtles. However, younger turtles of both species occupying pelagic habitats had a significantly increased debris ingestion rate (54.5%) than older turtles inhabiting coastal regions (29.0%).

Most people think that carelessly discarded rubbish doesn’t go anywhere near the oceans; or, if it does, it will enter the waterways but wash ashore along local rivers or catchments; or, at most, the rubbish would make its way to the ocean only to wash back onto the beach. While debris does impact creatures in these environments, the most dangerous debris floats in the oceanic currents well out of sight and mind to most, but becoming a floating time bomb for juvenile turtles and other marine animals.

Green and hawksbill turtles are both selective in their natural food sources, preferring certain items over others even when these items are not as readily available as other suitable food sources. Hence it should not be surprising that Schuyler found that pelagic and coastal turtles select different types of debris. Pelagic feeders were less selective in their feeding but tended towards positively buoyant rubber items, while coastal benthic feeders were more likely to ingest soft, clear plastics most likely due to their visual resemblance to jellyfish, a natural prey item of every species of marine turtle.

This leads Schuyler to her final research question: why are these selective feeders choosing marine debris as a food item?

While turtles do use their sense of smell to help them find food, they are predominantly visual foragers. However, while we know how we visualise the underwater environment, we need to take a step back to fully understand how turtles perceive their environment.

Each species of marine turtle has a visual system that is specially tuned to its environment, ecology and physiology. As a result, they don’t see the marine environment in the same way that we do. Therefore, to fully realise how these animals are sensing their environment, the inner workings of their visual system needs to be understood. Then, using that information, a model or blueprint of their visual environment, natural food sources and unnatural objects can be formed. This modelling essentially provides us with the knowledge to see through the eyes of the turtle.

“While this is a complex undertaking, the outcomes will be very rewarding and could lead to a much better understanding of why turtles selectively ingest marine debris and, more importantly, new developments that would help to deter turtles from this potentially lethal behaviour,” Schuyler says.

Despite the finite timeframe of her PhD project, Schuyler has some long-term goals for her work. She hopes to come away from the project with messages about marine debris that the general public can hold onto and use in their everyday lives, and she is already working to ensure that there is a broader dissemination of information to the general public.

While 20% of marine debris is thought to come from ocean-based sources such as abandoned nets and fishing gear or waste from ships and boats, a staggering 80% comes from land-based sources, including beach rubbish, materials washed down into storm drains or sewers, and rubbish that enters streams or rivers and is carried into the oceans. These statistics demonstrate how important the role of education is in the battle against marine debris. “The great thing with marine debris is that it is a problem that people can really do something about,” Schuyler says.

While ongoing research and clean-up efforts are certainly important, the benefits of prevention far outweigh those of combative measures. Therefore, following simple rules such as not littering, carrying reusable cups or cutlery with you, using biodegradable bags, and encouraging education and awareness on the topic are all great environmental tips and will help to restore natural beauty to the marine environment and help to make it a safe, productive environment for the animals that inhabit it.

“I don’t believe people change by being confronted, so I try to work by example and I create opportunities to talk to people. People have come back to me and said that they do things differently because of me, and I heard that after one of my talks at a primary school, parents were coming back months later saying that they couldn’t even drive along the road anymore without their kids seeing a plastic bag and screaming: ‘STOP, turtles are going to die!’.”

For every person that makes a concerted effort to make better choices themselves and inspire those around them through their actions, fewer animals will be confronted with the threat of marine debris. And, as marine debris is such a compounding issue, the more proactive we are with the controlled disposal and management of our waste, the less reactive we will need to be to make the same amount of difference through exhaustive, expensive and technically difficult clean-up efforts.

Ultimately, the decision lies with all of us. Is our natural environment to be trashed or treasured?

Dr Blake Chapman completed her PhD in marine biology and neuroscience at The University of Queensland, and is a freelance science communicator.