• There are numerous knowledge gaps that need to be bridged in order to better understand the impacts of the present-day marine environment
• Seven species of sea turtles are reported to exist in our blue planet and Sri Lankan marine waters are home to five out of these seven species
• Research shows that the amount of plastics that can be recovered from the gut would be greater in animals that have died due to plastic ingestion

Never have we been so unfortunate to observe and experience a chain of marine turtle deaths in our seas. At the time of this writing, more than 80 dead marine turtles are reported to have been washed ashore. On June 16, when the world was celebrating the marine turtles in our blue planet, we were losing these wonderful creatures from our marine environment. This article looks at the global picture of marine turtles in the face of human impacts on the marine environment and how we should systematically investigate into the cause of death of an individual turtle or a population of marine turtles. 

Why are marine turtles unique?

Marine turtles are a fascinating group of marine reptiles. Unlike most other turtles and tortoises, marine turtles cannot retract their head and limbs into their shells, in threatened situations. They can be found in all oceans except those in the Polar Regions. During their nesting season, female turtles swim ashore to lay their eggs on sandy beaches. After an incubation period of 50-60 days, baby turtles hatch out, dig through the sand, and crawl to the sea. This is a magical sight which needs to be witnessed by anyone who loves the sea and its wonderful creatures! Being a country, which is globally recognised as one of the few places in the world where marine turtles can be observed in their natural habitat, numerous foreign tourists visit Sri Lanka each year to witness this fascinating behaviour of this special marine creature.

 At present, seven species of sea turtles are reported to exist in our blue planet and Sri Lankan marine waters are home to five out of these seven species, the green turtle, ‘olive ridley’ turtle, ‘hawksbill’ turtle, ‘loggerhead’ turtle and ‘leatherback’ turtle. Claimed to be the largest out of these five species, the leatherback turtle is critically endangered and is reported to be rare in Sri Lankan waters. Overall, six out of seven species of sea turtles in our world, are listed as vulnerable, endangered, or critically endangered on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. It is important and timely to look at how the existence of these amazing creatures  being threatened and what we can do to prevent them getting extinct from this planet.

Threats to marine turtle life 

To date, several threats have been identified for the existence of marine turtles. Different coastal communities of the world hunt sea-turtles as a source of food, adding more pressure on the numbers of this threatened creature. The use of turtle shell in the production of ornaments and the use of turtle-oil in the production of cosmetics and medicine have added to this threat. Many countries have banned the collection of eggs of this animal, but illegal harvest is not quite rare. According to research carried out by the National Aquatic Resources Research and Development Agency (NARA), Sri Lanka, collection of marine turtle eggs from the rookeries for human consumption and marketing eggs to marine turtle hatcheries have been identified as major threats to the existence of marine turtles in Sri Lanka. Globally, longline and trawl fishing have long been accused for accidental capture of marine turtles. The number that is affected has been estimated to be a staggering 150,000 turtles of all species killed in shrimp trawls, more than 200,000 loggerheads and 50,000 leatherbacks affected by longline fisheries, and great numbers drowned in gill nets. Among all threats that are faced by these wonderful marine creatures, marine debris and chemical pollution in the marine environment have caught the greatest attention. Accumulation of plastics in the marine environment is a growing environmental concern. Annually, over 1 million marine animals are estimated to be killed because of plastic debris in our oceans. Marine turtles are particularly vulnerable because of the risk of ingesting plastic debris which can become lethal to these creatures. It is a well-known fact that leatherback turtles cannot distinguish between a jellyfish (which is one of their prey) that is swimming in the sea, from a floating plastic bag. Moreover, entanglement by marine debris, including discarded or lost fishing gear and netting materials, and plastic onion sacks have resulted in the loss of many marine turtle lives.

Mass mortalities of marine turtles in our beaches

Unlike the individual stranding of marine turtles reported in various beaches of the world, mass mortality outbreaks of marine turtles are more alarming and catch the attention of various parties. The recent occurrence of mass stranding of marine turtles in our beaches which created so much interest, triggered this article. According to the reports of the Department of Wildlife Conservation (DWC), Sri Lanka, an annual average of five turtle deaths are usually reported in our shores, excluding the death of baby turtles. In the year 2020, three deaths of turtles were reported before the fire of the vessel MT New Diamond which caught fire in our seas. 15 dead turtles were reported on our shores, following this fire. To date, more than 80 dead turtles have been reported on our shores following the devastation of the X-Press Pearl vessel. The mystery of dead marine turtles washed ashore, still remains. Is it an aftermath of the fire in X-Press Pearl? If so, what actually has happened to the turtles? What can be the cause of this mass mortality outbreak? How are we going to investigate this matter? It is important to find solid, scientific answers to these questions. Afterall, it is our marine turtle population that is dying.

Harmful algal blooms (HABs)

Toxic or harmful algal blooms (HAB) have been reported to cause mass mortalities in marine turtles in various parts of the world including Florida, USA, and in the Gulf of Mexico. HABs are thought to have a great impact on marine turtles. A mass stranding of 318 marine turtles were reported in one such outbreak that occurred in the west coast of Florida, USA, during the period from 2005-2006. More than 90% of both live and dead stranded animals tested positive for the toxin produced by the single-celled algae Karenia brevis.  High concentrations of the algal toxin were recorded in the stomach contents of affected turtles. It was also thought that the turtles inhaled this toxin when they came to the surface for breathing, which was present in the air by the action of wind and waves. Presence of suspected toxic algal blooms around the fire-stricken X-Press Pearl, were suggested by scientists. However, as to now this has not been confirmed and has not been proved to be associated with the mass turtle stranding in our shores.

Impact of chemical pollution in the marine environment

Besides the possibility of a toxic algal bloom, the X-Press Pearl which was reported to carry loads of hazardous toxic substances and plastic hurdles may have created direct impacts on these already threatened creatures. A variety of fish species, dolphins and whales were also reported dead on our shores following this tragic incident of X-Press Pearl. Can there be an adverse impact of these hazardous chemicals on our marine turtles? If so, how are we going to investigate this impact? While the impacts of chemical pollution on marine life such as that of whales, seals and fish, are well-studied, the threats of chemical contamination of the marine environment on marine turtles are mostly understudied. With the increasing interest and attention in the potential association between seawater contamination and mortality in marine turtle populations, it is particularly important to follow correct procedures and collect standard samples from stranded turtles, for toxicological analysis. Apart from potential, short-term toxicities, bioaccumulation of hormone-mimicking chemicals such as atrazine, lead, zinc and copper have been proposed to alter reproductive hormones (oestrogens and testosterone) in turtles, potentially reducing the reproductive capacity in nesting turtles. Studies on heavy metals such as copper, cadmium, lead, and mercury in marine turtle populations have shown varying levels of these heavy metals depending on the species, region, stage of life cycle and tissue type sampled. Considering the concentration of toxic substances such as heavy metals in the top level predators (carnivorous fish such as tuna) in the marine food web, the different levels in the marine food web occupied by marine turtles and the long life span of marine turtles, there is a high risk of chemical contaminants present in the marine environment to get accumulated in the body of turtles. This can lead to long-term impacts on marine turtles, such as reduced reproductive performances. This will further aggravate the existing threats experienced by these animals. Some research has also shown passing down of chemical contaminants to younger generations of turtles, through eggs.

Legal protection to marine turtle life
With all this background, what measures have been taken to protect this threatened animal population? Marine turtles are protected by law in most countries, including Sri Lanka.

Relevant permits should be obtained before carrying out any investigation including post-mortem examinations, on these animals. In Sri Lanka, marine turtles are protected under the Fauna and Flora Protection Ordinance administered by the Department of Wildlife Conservation since March 1, 1938 (amended in July 20, 1972) and the Fisheries and Aquatic Resources Act of 1996. Independent organisations such as the Turtle Conservation Project (TCP) are actively working on marine turtle conservation strategies through education, research and community participation. Considering the threats faced by marine turtles, it is a challenging task for veterinarians, marine scientists and other responsible parties to investigate the cause or causes of the recent mass mortality outbreaks in marine turtles, within a legal framework. 

Post-mortem examinations

One of the main approaches in solving the mystery of the mass stranding of marine turtles in our beaches would be to carry out post-mortem examinations on these dead turtles and to carry out appropriate laboratory examinations for further diagnosis. Unfortunately, the diagnosis of the cause of death through post-mortem examinations is hampered by numerous factors. By the time the dead turtles are found ashore, most have undergone severe post-mortem changes, making it difficult to identify and assess the damages that are related to the true cause of death. This makes it difficult to assess the carcase both by gross examination and microscopic examination of body tissues of the dead turtles. Nevertheless, the degree of decomposition can be grossly assessed from corneal opacity in the eyes of turtles, colour changes/darkening and sloughing of the skin, pooling of blood in lower parts (dependent parts) of the body of the turtle, degree of swelling, and odour. While the eyes of a marine turtle can be sunken due to dehydration in a fresh carcase, bulged out eyes were noted in almost all turtle carcases that washed ashore in the past few days, indicating post-mortem changes.  It should be noted that veterinarians face practical limitations and difficulties in examining these large marine creatures in the coastal environment. However, it’s the need of the hour to find ways and means to carry out all possible investigations on these poor creatures.

Ingestion of marine plastics

In the past, what observations have been made by veterinarians and marine biologists during post-mortem examinations of dead marine turtles? Accidental capture of marine turtles in capture fisheries results in damages to the gut following fishing line ingestion which is visibly evident even without opening up the gut. However, deaths caused by ingestion of marine plastic debris require thorough post-mortem examination before confirming the cause of death. Sometimes, plastic bags protruding from the cloaca/back of the turtle can be observed during preliminary examination of the carcases. Globally, it has been estimated that about 52% of all marine turtles have ingested plastic debris in their gut. However, research shows that the amount of plastics that can be recovered from the gut would be greater in animals that have died due to plastic ingestion. On the other hand, turtles can die due to the impacts of plastic ingestion, even when they have ingested a single item of plastic. This results when the animal’s gut gets perforated or obstructed due to the plastic item that it had ingested. Both the structure of the marine turtle gut and the composition of debris that have been ingested by the turtle play a part in mortality caused by plastic ingestion. Unlike most other animals that we know, marine turtles cannot vomit the food/debris that they ingest. As a result, the gut of marine turtle is prone to accumulation of debris. On the other hand, rather than passing out the ingested plastic item as it is, pieces of soft plastic that were ingested at different times, can get assorted and pass as a single compacted item, leading to obstruction and damage of the gut. While small plastic particles can remain in the gut of a marine turtle up to four months, a 10 × 10 cm plastic sheet can even remain up to six months. Thus, the results of plastic ingestion may be seen long after the plastic item has been ingested. When attempting to study the gut contents to find out the cause of death of a marine turtle, the contents of the gut can be sieved to extract any plastic present. Marine turtles of different age groups can be exposed to different types of debris. Adult turtles living in the coast might come across larger plastic debris that has recently washed into the ocean. On the other, young turtles living in the open sea (pelagic region) might get exposed to smaller fragments that have degraded. 

Way forward

With the ever-increasing amount of pollution and frequent accidents happening in our marine environments, there is an increasing role for veterinarians to play in diagnosing the contribution of various environmental factors to marine aquatic life including marine turtles. Yet there are numerous knowledge gaps that need to be bridged in order to better understand the impacts of the present-day marine environment, on the health of marine turtles. Overall, a holistic approach promoting collaborative research studies among marine biologists, environmental scientists, oceanographers and chemical scientists coupled with the role of veterinary diagnosticians performing post-mortem examinations in marine turtles, will be required to improve our knowledge and to combat the multiple threats faced by this amazing marine creature to ensure its sustainability in this blue planet.