How much plastic is too much plastic for sea turtles?

In a video that went viral in 2015, researchers spent nearly 10 minutes pulling out a 10-centimeter (4-inch) plastic straw from the nostril of a male olive ridley turtle (Lepidochelys olivacea) off the coast of Costa Rica. For the entire duration of the extraction, the turtle writhed in pain.

Plastic straws are just one among the trillions of pieces of plastic trash that have ended up in the ocean, many sinking right down to the deepest, darkest depths. Past research has found that about half of the world’s sea turtles may have ingested some form of plastic. Floating pieces of plastic and balloons can resemble jellyfish or squid — foods that turtles eat out in the sea. Not all turtles that eat plastic die because of the plastic, though. Some die after getting entangled in fishing nets, while others are struck by ship propellers.

Do all age groups of turtles consume plastic? And how much plastic is lethal for a sea turtle? A new study has some answers.

Researchers in Australia examined the digestive tracts of 246 dead sea turtles collected from along the coast of the state of Queensland, and found that 58 individuals had plastic in their guts, with the number of pieces ranging from one right up to 329.

Younger turtles, both post-hatchlings (or a baby turtle that has started feeding in the ocean) and juveniles, were found to have consumed considerably higher amounts of plastic than adult turtles, the researchers report in the study published in Scientific Reports.

This could be because young turtles are less selective about what they eat, and eat whatever they encounter, said Britta Denise Hardesty, principal research scientist at the oceans and atmosphere unit of the Commonwealth Scientific and Industrial Research Organisation, Australia. Young turtles also tend to feed at the ocean’s surface, where floating bits of plastic are more likely to occur. Moreover, young turtles drift with ocean currents, just like plastic debris tends to do, and both might end up aggregating in the same places.

“We know that not only do these smaller, younger turtles eat more plastic, they also tend to eat more different types of plastic, including balloons and other positively buoyant types of plastic,” Hardesty told Mongabay in an email. “It may be that they simply are less selective and they encounter higher concentrations of floating debris which they then ingest.”

The likelihood that plastic could be a key threat at this early life stage of turtles is worrying, Brendan Godley, a professor of conservation science at the University of Exeter, U.K., who was not involved in the study, told Mongabay.

“This is of particular concern as pieces of plastics and baby turtles are both likely to be aggregated together in similar areas,” he said. “It is thought that most marine turtles undertake at least the first few years drifting on the open ocean.”

In addition to data from the 246 autopsied turtles, the team also looked at 706 autopsy records of sea turtles in a government-maintained stranding database to determine the likelihood of death due to plastic ingestion.

The team used turtles that had died from a known cause, such as those that had drowned in fishing gear or died after a boat strike, as their control group; these animals’ chances of having being killed by plastic was zero. The researchers compared this control group to turtles that had died of unclear causes (their guts had plastic at the time of autopsy, but their deaths could have been from other possible causes like infections or propeller strike), and those that died due to plastic perforating or blocking their gut. Then they plotted all the turtle data to see if there was a relationship between the likelihood of death due to plastic ingestion and the concentration of plastic in the turtles’ guts.

As it turns out, there is a relationship. The higher the number of plastic pieces a turtle has inside its gut, the higher the chance of it being killed by the plastic. For an average-sized turtle, ingesting more than 14 pieces of plastic translates into a 50 percent likelihood of death.

“I think it’s pretty confronting to learn that you have a 50% likelihood of death if you’re a turtle who has eaten 14 pieces of plastic, even if they’re very small pieces,” Hardesty said. “Even one piece of plastic means you have more than a 20% chance of dying from eating it.”

Sharp, hard plastic items are more likely to result in a gut perforation, Hardesty said, while thin, film-like plastics are more likely to block the gut, not allowing other food to pass through.

Hardesty’s team next plans to see how the size or type of plastic affects the turtles. “This is an obvious next step and something of great interest,” she said.

Godley said the study was “a great first step towards measuring what we know to be a broad and pernicious threat to marine vertebrates such as turtles and seabirds.”

“I think the Australian researchers have put together a generalized framework that can be tested throughout the world,” he said. “I would, however, be very concerned if people started using their preliminary findings as global risk factors. There are many other factors that still need to be considered such as the susceptibility of different species or life stages and the relative distribution of plastic and the type, size and shape of pieces. These latter three factors are likely to have profound impacts on the likelihood of ingestion and possible associated damage.”

Citation:

Wilcox, C., Puckridge, M., Schuyler, Q. A., Townsend, K. and Hardesty, B. D. (2018) A quantitative analysis linking sea turtle mortality and plastic debris ingestion. Scientific Reports, 8 (12536). DOI: 10.1038/s41598-018-30038-z