In the Mediterranean, the ingestion rate of plastic by loggerhead turtles varies from 15 to 80% depending on the region. As opportunistic feeders, turtles do not differentiate plastic from its natural prey. This weakness has given it “indicator species” rank for the European Community.
In the near future, scientific monitoring of loggerheads will reveal the Mediterranean basin’s state of health. Françoise Claro, coordinator of Marine Turtles Group France, was aboard Tara – the opportunity to learn more about these reptiles.
What species of sea turtles do we find in the Mediterranean basin?
The loggerhead turtle and green turtle are the two species that breed in the Mediterranean. We can also observe leatherback turtles passing through. The loggerhead is the most common species on our shores and breeds in the eastern Mediterranean, from Greece to Lebanon. It’s exceptional to observe spawning in the western part of the basin, and yet this year we have identified 4 instances in Sardinia, and 2 attempts in Corsica.
What are the threats to these species?
In the Mediterranean, turtles are confronted with many human activities, which can be dangerous and also obstacles to migration. If we mention only the cases in French waters, catching turtles is accidental because all species are protected by the Ministerial Decree of October 14, 2005. It is absolutely forbidden to take or detain a turtle. Fishermen who capture them do so unintentionally since they can be caught in their nets or longline hooks. We also see a lot of collisions with motor propellers that result in shell fractures and deep lesions. In fact, turtles breathe on the surface and if a boat arrives at high speed in the same spot, there is a collision. Regarding breeding sites (spawning grounds), there are other threats. Light pollution is one: nesting turtles may be disturbed by lights when spawning. And conversely, small newly hatched turtles can be attracted to artificial lights, take a wrong turn and get run over by cars. Normally, they find their way to the sea thanks to the bright horizon. The destruction of habitat is another threat: some beaches are too busy or too degraded. And stray dogs can dig up the eggs and eat the hatchlings as they leave the nest. And of course, the turtles also suffer from ingestion of our litter.
Why are turtles so affected by plastic pollution?
Studies on diet and stomach contents show that they are very opportunistic feeders. In general they eat gelatinous organisms like jellyfish or sea squirts — organisms which can resemble drifting clear plastic.
A colleague of the French network Mediterranean Marine Turtles working in Antibes, just brought me more recent observations: a 20cm loggerhead turtle was found floating on a bed of trash and another was found dead with a plastic bag in its mouth and trash in its digestive tract. And all of this took place on our Riviera!
Is this impact quantifiable?
Yes, to some extent. For example we can measure the amount of plastic found in the digestive tract of dead turtles that have washed ashore. But the plastic is not necessarily the cause of death. An Italian colleague told me about a turtle found with its gut perforated by a cotton swab stick. The animal died. Sometimes a turtle ingests so much waste, it causes an obstruction and if necrosis of the digestive mucosa occurs, the turtle can not be saved. In the latter two cases, it is easy to make the connection between the waste and its impacts.
However, other effects are more difficult to assess, but we do have hypotheses. For example, what we call the dilution effect: when a turtle ingests a lot of waste, there is no more room for its natural foods, it doesn’t absorb enough nutrients, and in the long-term it may grow more slowly and be more vulnerable to predators and disease. Another example is when a turtle ingests too much waste, the digestive transit can be disrupted causing gas formation, so the turtle floats. In this case the animal can not dive for food.
The loggerhead turtle has been designated an indicator species for the Marine Strategy Framework Directive* . Why have you have selected this species?
Besides the procedures to estimate the amount of waste in our marine environment, the loggerhead turtle will let us assess whether the measures taken to reduce the amount of litter at sea are effective or not. For the Atlantic and Channel region, the northern fulmar (Fulmarus glacialis) has been designated as indicator. These species therefore will monitor the health of our European seas.
Interview by Noëlie Pansiot
Marine Environment Strategy Directive*: European directive which defines common objectives for the protection and conservation of the environment.
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