Sea turtles and the perils of plastic

The problem with plastic
The explosion of plastic production in the 1950s could arguably be noted as one of the most pivotal moments in our recent history, revolutionising the way we eat, drink, shop, travel, and treat illness. It is a modern-day wonder material that saves lives, food, and money and has brought great benefits to our society. Yet, this incredible resource is both a blessing and a curse. The durability and affordability of plastic are key reasons for its popularity, but mismanagement and irresponsible use have resulted in what can only be described as an environmental time bomb. Cheap plastic items can be thrown away after only one or two uses that may last just a few minutes or even seconds. But where plastic is disposed of can vary greatly. Globally, only 9% of plastic is recycled and the rest (91%) may end up in landfills or, in many cases, our oceans, beaches, rivers, and lakes. It is estimated that 8 million metric tons of plastic enter the oceans every year. Compounding this, plastic does not biodegrade and persists within the natural environment for an indefinable amount of time. This means that the problem of pollution will only grow and accumulate if plastic input is not halted. Unfortunately, it’s not just about the plastic that we can see. At the sea surface and on beaches, wave action and ultra-violet radiation from the sun causes the plastic to fragment into smaller and smaller pieces, resulting in microplastics (less than 5 mm in size). These tiny and colourful pieces can be ingested by a plethora of marine organisms, from the microscopic, such as zooplankton at the base of the food chain, to the massive, such as filter-feeding baleen whales.

Why sea turtles?
The impact of plastic pollution on sea turtles is of particular concern. Turtles are highly mobile, travelling huge distances between foraging and breeding areas and they use both terrestrial and oceanic environments. Already under increasing pressure from a range of human stressors, including climate change, by-catch, and habitat destruction, these charismatic animals are undergoing a three-pronged attack from plastic in the form of ingestion, entanglement, and habitat degradation.  

Impact #1 – Ingestion

All seven sea turtle species and approximately 52% of all individuals are estimated to have ingested debris. Ingestion may occur for a number of reasons. Firstly, turtles are primarily visual feeders and may mistake anthropogenic items, for example, plastic bags, balloons, and sheet plastic, for food due to the visual similarity to prey, such as jellyfish. Secondly, plastic may become mixed up with the turtles natural dietary items. Grazers, like the herbivorous green turtle (Chelonia mydas), consume plastic pieces that have become entangled on the seagrass fronds they feed upon. Thirdly, carnivorous turtles, e.g. loggerheads (Caretta caretta), may ingest microplastics via their crustacean and mollusc prey, a process known as a trophic transfer. This is when microplastics are consumed by animals near the base of the food chain which is in turn, eaten by predators.

Macroplastics (more than 5mm in size) are particularly hazardous for sea turtles when ingested. Downward-facing spikes in their oesophagus, called papillae, help them swallow slippery prey while expelling seawater. This trait, however, did not evolve with plastic in mind and the papillae actually prevent plastic from being regurgitated. Instead, it enters the digestive tract where it can cause injuries, such as internal lacerations and punctures, or blockages. It can also make the turtle feel full and weaken the desire to feed, leading to malnutrition and eventually, starvation. Additionally, sea turtle buoyancy disorders, commonly referred to as “bubble butts,” are a growing issue. This condition is often caused by gas trapped inside the turtle’s digestive system. Disruption to digestive processes caused by marine debris is believed to be the reason for many of these cases. Sea turtles that cannot properly submerge will have trouble feeding and become more vulnerable to boat strikes, predation, and entanglements.

The consequences of microplastic ingestion for turtles are as yet poorly understood. When consumed by marine invertebrates, such as worms and zooplankton, they have been shown to reduce food intake, cause a decline in energy reserves, lower their ability to reproduce, and cause detrimental changes to the way their intestines function. For larger animals, such as turtles and marine mammals, any effects of microplastic ingestion are more likely to originate from the chemicals on or within them than the microplastics themselves. Chemical contaminants present within seawater, such as heavy metals and polychlorinated biphenyls (PCBs) which were used as flame retardants (and are now banned) among other things, are hydrophobic and so stick to plastic-like cooking oil sticks to the side of a washing-up bowl. These toxicants are known to cause cancer and disrupt hormonal systems. For their size, microplastics have a large surface area to volume ratio, meaning they can concentrate these chemicals. In addition, the plastic itself contains chemicals, called plasticisers, which are added during production. When plastic is ingested, this cocktail of chemicals is released and likely enters the tissue of the animal. We are still trying to work out what kind of effect this has on turtle health but it’s possible that it causes sub-lethal effects, such as lowered immune system function and reduced reproduction rates.

As it stands, we do not fully understand the scale of turtle mortality caused by plastic ingestion or the resulting potential population-level effects. It’s likely that the occurrence and consequences of ingestion varies with species, age class, and population due to their differing feeding ecologies and diet, as well as habitat use. For example, loggerhead turtles are at higher risk of consuming plastic due to their generalist feeding strategy, but a more robust digestive tract (in adults and sub-adults) enables foreign objects to be excreted. Hatchling and juvenile turtles are likely to be more vulnerable to encountering and becoming injured as a result of plastic ingestion due to three reasons. Firstly, six of the seven species of turtles undergo a period of time in the open ocean, called the lost years. After hatchlings enter the water for the first time, ocean currents often transport them away to highly productive areas, called oceanic convergence zones, where food, along with debris, is concentrated. This spatial overlap potentially creates an ecological trap for young turtles because it increases the likelihood of exposure to plastic. Secondly, their naivety may mean they are more likely to consume debris. As they learn to identify and select the best things to eat, young turtles will invariably encounter and potentially ingest plastic. Thirdly, young turtles, that are small in size, maybe at higher risk of mortality from plastic ingestion due to their smaller, less robust, digestive tracts. Of all life stages, the lost years are the least studied and the least understood in terms of the impacts of plastic ingestion. It is possible, however, that turtles in this age class are the most vulnerable.

Impact #2 – Entanglement

In the ocean, sea turtles are susceptible to becoming entangled in floating plastics, since they tend to use oceanic fronts, currents, and drift lines where floating rubbish and debris are concentrated. Entanglement may lead to injury, amputation, or choking which may ultimately cause drowning or death by starvation. Lost or discarded fishing gear, known as “ghost gear”, has become a huge issue since the 1950s when the fishing industry replaced natural fibers, such as cotton, jute, and hemp, with synthetic plastic materials such as nylon, polyethylene, and polypropylene, which do not biodegrade in water. Turtles are also known to become entangled in a variety of other items, including plastic twine, six-pack rings, plastic packaging, plastic chairs, balloons and their string, sheet plastic, and boat mooring line. Hatchlings and juvenile sea turtles are particularly susceptible to becoming entangled as they may ‘set up home’ near floating debris, as it provides shelter, and can remain there for years. Based on beach stranding records, it is estimated that more than 1,000 turtles die a year globally after becoming entangled. However, large knowledge gaps exist about the severity of entanglement of sea turtles, since not all turtles that die from entanglement wash onshore as they decay at sea. Therefore, rates of entanglement are likely a gross underestimate. Nevertheless, a survey of sea turtle experts indicated that plastic and other pollution pose a long-term impact on the survival of some turtle populations and that they perceive entanglement as a greater threat to turtles than oil spills.

Impact #3 – Habitat degradation

Large quantities of plastic are also found in coastal areas, which are crucial for sea turtle reproduction; nesting females emerge onto land to lay their eggs, and after about two months, hatchlings emerge and make their way to the ocean. Plastic at nesting grounds can cause obstruction and entanglement to both nesting females and emerging hatchlings. Large pieces of plastic and debris may make it difficult for ascending turtles to lay their eggs, affecting nesting success and the overall reproductive output at a nesting beach. For emerging hatchlings, entanglement in or obstruction by debris can increase the likelihood of predation and dehydration, potentially leading to injury and mortality. Additionally, these tiny turtles will need all their energy when they enter the ocean and start the ‘swim frenzy’. This is a highly important stage where hatchlings swim continuously for a day or two until they reach offshore currents, which transport them away from land and out into the relatively safe open ocean. Any energy that gets wasted as a result of trying to get around, through or over obstacles and detangling could reduce their ability to run the gauntlet of predators waiting in the shallow water off the beach. Naturally, only approximately 1 in 1000 hatchling turtles will survive to adulthood but interactions with human debris at this early life stage could reduce this survival rate even further.

Plastic fragments, in particular microplastics, can also alter the specific conditions needed for beaches to provide a suitable nesting environment. As hatchling success and sex ratio is influenced by temperature, alterations to the incubating environment could negatively affect the development of hatchlings, the proportion of males to females produced, and mortality rates. Plastics, particularly those containing a dark pigment, warm up when exposed to heat, and their presence within the sand may increase the nest temperature, potentially leading to a higher proportion of female hatchlings being produced. This phenomenon has already been observed as a result of rising temperatures related to climate change, but plastic debris could exacerbate the issue. Nevertheless, research is still underway to determine what type and quantities of plastic would be required to alter the incubating environment and negatively impact sea turtle reproduction. Further, chemical additives found in plastic can also be transferred to the incubating environment potentially causing detrimental effects to the health of sea turtles.

Ocean Optimism

With so many negative impacts, it is easy for the problem of plastic pollution to seem so huge and widespread that the damage is irreparable. Yet, many of us working on the issue feel it is not too late. Indeed we are seeing a rise in the cases of sea turtles ingesting and getting entangled in plastic, but there has also been a rapidly growing grass-roots movement to tackle the issue from the bottom up. Pressure is building on businesses and governments to mitigate the waste created by unnecessary plastic packaging on food and other goods, and there has been a huge uptake in the number of individuals saying ‘No!’ to single-use disposable items, such as drinks bottles, cutlery, shopping bags, and drinking straws. The age-old mantra of ‘Reduce, Reuse, Recycle’ is one which many of us will be familiar with but perhaps forget to strive for the first, and most effective, the step of reducing our plastic consumption. There are now many alternatives to plastic products and making simple changes in our everyday lives, for example by using refillable water bottles, reusable cloth bags, and declining unnecessary plastic packaging can lead to a significant reduction in waste.

Ghost gear collection initiatives, such as Net-WorksTM, integrate business with conservation by enabling local people to earn an income collecting derelict fishing nets from the ocean and recycling them into products like carpets. This ‘circular economy’ approach is growing in popularity and gives cause for hope that plastic may one day be seen as a valuable resource, rather than something to be discarded. Cleaning up what has already entered the environment is another approach to tackle plastic pollution. Citizen-led beach cleans not only remove vast quantities of litter from our coastlines, but they have also been proven to improve ocean literacy, particularly in children, as well as lead to positive changes in behaviours and attitudes. As with any great environmental issue, it is important that people feel empowered to create change and contribute to the solution, not just the problem. The tangible nature of plastic pollution means that it is visible in our everyday lives and the public is easily connected to it. This has inspired many to become aware of the other threats faced by our oceans in such a way that plastic pollution has inadvertently become a catalyst for marine conservation.

With the tipping point looming, it is of great importance that we remain proactive in capitalising on this newfound awareness and continue gathering evidence, developing policies, pressuring governments, and designing innovative solutions. Above all, we must remain optimistic that the plastic tide can be turned.

Further Reading

Beckwith, V. K., & Fuentes, M. M. P. B. (2018). Microplastic at nesting grounds used by the northern Gulf of Mexico loggerhead recovery unit. Marine Pollution Bulletin, 131(January), 32–37. doi:10.1016/j.marpolbul.2018.04.001

Duncan, E. M., Botterell, Z. L. R., Broderick, A. C., Galloway, T. S., Lindeque, P. K., Nuno, A., & Godley, B. J. (2017). A global review of marine turtle entanglement in anthropogenic debris: A baseline for further action. Endangered Species Research, 34, 431–448. doi:10.3354/esr00865

Jambeck, J. R., Geyer, R., Wilcox, C., Siegler, T. R., Perryman, M., Andrady, A., … Law, K. L. (2015). Plastic waste inputs from land into the ocean. Science, 347(6223), 768–771. doi:10.1126/science.1260352
Nelms, S. E., Duncan, E. M., Broderick, A. C., Galloway, T. S., Godfrey, M. H., Hamann, M., … Godley, B. J. (2016). Plastic and marine turtles: a review and call for research. ICES Journal of Marine Science, 73(2), 165–181. doi:10.1093/icesjms/fsv165

Schuyler, Q. A., Wilcox, C., Townsend, K. A., Wedemeyer-Strombel, K. R., Balazs, G., van Sebille, E., & Hardesty, B. D. (2016). Risk analysis reveals global hotspots for marine debris ingestion by sea turtles. Global Change Biology, 22(2), 567–576. doi:10.1111/gcb.13078

This article is from issue


2018 Jun