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Around the world, sea turtles swallow pieces of plastic floating in the ocean, taking them for delicious jellyfish, or simply unable to avoid the debris that surrounds them.
A new Australian study is now trying to catalog the damage.
While some sea turtles have swallowed hundreds of pieces of plastic, only 14 pieces significantly increase their risk of death, according to the study published Thursday in scientific reports.
The study reveals that young sea turtles are the most vulnerable because they drift with currents where floating debris also accumulates and they are less demanding than adults about what they will eat.
Worldwide, more than half of all seven species of sea turtles have consumed plastic debris, says Britta Denise Hardesty, lead author of the newspaper and principal researcher at the Tasmanian Commonwealth Scientific and Industrial Research Organization. "No matter where you are, you'll find plastic," she said.
Six of the seven species of sea turtles are considered threatened, although many populations are recovering.
The study examined data from two sets of Australian marine turtles: an autopsy of 246 animals and 706 records from a national database on strandings. Both showed that animals that died for reasons unrelated to plastic consumption had less plastic in their intestines than those who died from unknown causes or direct ingestion.
But deaths are hard to pin down. "Just because a turtle has a plastic in it, you can not say that she died, except in very extenuating circumstances," said Dr. Hardesty. Even a single piece of plastic can occasionally cause death. In one case, a turtle was found with its digestive tract blocked by a piece of soft plastic; in another, his intestine was perforated by a piece of sharp plastic.
In others, a variety of plastic material was found inside their digestive tract – no less than 329 pieces in a sea turtle. Because of their anatomy, sea turtles can not vomit anything once they have swallowed it, said Dr. Hardesty, which means that they cross their intestines or get stuck.
For a juvenile of typical size, half of the animals should die if they ingest 17 plastic items, the study concluded. According to Dr. Hardesty, sea turtles can live up to 80 years or more, with juveniles too young to breed until they are 20 to 30 years old.
The innovation of the study was to try to determine this point of inflection, where the plastic load becomes deadly, said T. Todd Jones, a biology researcher at the National Oceanic and Atmospheric Administration in Hawaii.
"There has always been this question of when plastic is too much?" Said Dr. Jones.
An animal that swallows a lot of plastic may look healthy, said Dr. Jones, but could be weakened by the plastic in his gut, which limits the absorption of food.
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Mark Hamann, a turtle expert and associate professor at James Cook University in Townsville, Australia, said he hoped that studies like this one would give some idea of the scale of the problem. In some areas with heavy plastic pollution, such as the Mediterranean and the South Atlantic, turtles are not able to avoid debris, while in others the problem is less.
"We know that every turtle is dying, but we still do not know if enough turtles are dying to cause the population decline, and that's where we are heading now," said Dr. Hamann.
Jennifer Lynch, research biologist at the National Institute of Standards and Technology in Hawaii, questioned how the study measured vulnerability to plastic.
In her own research, she saw animals that were not injured after swallowing 300 pieces of plastic. She does not believe that 14 coins pose such a high risk of death. "They ate a lot of plastic but it did not hurt them," said Dr. Lynch about the animals she examined. "They swallow it and they make it poo."
The difference between the two studies, Dr. Lynch said, was animal health. "In their study of dead and very sick animals, there is a very strong bias," she said. "We watched only healthy live animals that died because they drowned in a hook."
Dr. Lynch said the new study should have focused on the weight of the plastic rather than the number of pieces. One piece could go from a microplastic grain to a whole snack bag, she noted.
"I think that magic number of 14 pieces is too low," said Dr. Lynch. "I think we have a lot to do before we know what concentration of plastic causes physiological and anatomical impacts."
Dr. Lynch agrees that sea turtles eat too much plastic. "We need to control this pollutant if we do not want to kill half of our sea turtles."
The vast majority of plastics off Hawaii, she said, comes from the international fishing industry, which is prohibited from throwing out its old fishing lines and crates, but the often does not suffer any consequences. "Teeth are what is needed," said Dr. Lynch.
Dr. Hardesty believes that it is possible to reduce the exposure of turtles to plastic through a variety of approaches, from incentives to bans for high-impact and often congested items.
"Things that end up in the ocean were in the hands of someone at some point," she said.
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