Our burgeoning plastic waste—arising from the packaging of everything from food and drinks to toiletries and medicines—isn’t just littering the oceans, it is choking marine life. Almost twice as many endangered green sea turtles are ingesting plastics compared with 25 years ago, according to a study last year, led by Qamar Schuyler, a postdoctoral fellow from the University of Queensland.
But why are foraging sea turtles going after plastic debris? Can they not distinguish between real prey and plastic? Now, Schuyler’s team has found the answers, confirming what scientists have long suspected. Their recent study revealed that sea turtles eat plastic debris mistakenly because it resembles their natural prey; they prefer soft and flexible plastics, such as clear plastic bags that look like jellyfish.
Plastic debris, in all its forms—bags, bottles, foam, pellets, and many more—can be swept away by winds and tides reaching far into the oceans where few humans have ever been. Over time, large plastic debris undergoes fragmentation whereby it degrades into smaller pieces, sometimes only a few millimeters in size.
When marine organisms such as small fish, zooplankton, and sea turtles encounter these unfamiliar items, they may eat them, and the fragments end up inside their guts.
Sea turtles possess color-vision and primarily search for prey visually rather than relying on chemical cues.
“Adult green sea turtles are herbivorous,” says Schuyler, “but they do eat jellyfish when they encounter them.” Older sea turtles feed at the bottom of the sea floor closer to the shore, whereas juveniles feed mainly at the ocean surface and are omnivorous, consuming a variety of prey including jellyfish, sponges, and crustaceans.
The researchers investigated whether sea turtles are choosing to eat specific types of debris, such as those of a particular color, texture, or brightness, over others.
To find out, Schuyler’s team opened up the guts of 115 dead, stranded sea turtles in southeast Queensland and probed them for plastic debris. In total, 19 turtles—most of which were green sea turtles—had large quantities of plastic debris, and were chosen for the study.
As a representation of the types of plastics that turtles may be exposed to in the ocean, they also surveyed beaches for debris. They analyzed 20 debris items from the beach as well as from the turtles’ guts in terms of color, texture, flexibility, translucency, brightness, and contrast.
To visualize how the debris would look like from the perspective of the turtles, the team created a color model of the green sea turtle’s visual system, showing the colors of the spectrum their eyes are sensitive to.
Sea turtles are, in fact, picky eaters, the results revealed; they chose to eat debris items that were more flexible and translucent than other items available. Their strong preference for soft and transparent items—akin to jellyfish, their natural prey—supports the theory that turtles gravitate toward prey-like items. Clear plastic bags mimic tasty jellyfish passively floating in the ocean, which makes an easy catch for foraging turtles. Schuyler was not surprised with these findings because her team previously reported that older turtles prefer soft and clear plastics.
More dark debris was discovered inside the turtles, probably because they appear prominent against the bright ocean environment. But the researchers caution that the brightness may have been dampened by the digestive juices inside the gut.
Compared to the debris on the beach, fewer blue-colored items were found inside the turtles. It is unclear why turtles may avoid blue items. One possibility, suggested by the researchers, is that they could blend in with the ocean background, making them less visible.
Schuyler even noticed an unusual item inside one of the turtles. “One interesting finding was that we found two apple stickers in one small turtle!” she said.
It is unknown why the turtles died but ingestion of plastic fragments can be lethal for turtles. Not only can the fragments pierce their intestines, but also they can release toxins into the body. Schuyler explains that debris inside turtles’ guts can leave them malnourished by “essentially filling the stomach up so that they just don’t take in as much food, but also by blocking the intestine and then gasses build up and the turtle floats, unable to feed normally.”
While all plastics are harmful to marine life, the study demonstrates that clear and flexible plastics, such as plastic bags, are most likely to be ingested by sea turtles, and conservation efforts should focus on ensuring their proper disposal.
“A lot of the plastics that turtles are consuming are end-user items, such as packaging and food storage materials. We all need to be more responsible and aware of our choices and reduce the amount of these consumer items that end up in the waste stream,” said Schuyler.
The team is now studying the risks of ingesting plastic debris among different sea turtle species.
Despite increased awareness of the perils of plastics, supermarkets around the world still widely distribute plastic bags, some of which end up invading our oceans. So far only a few countries have successfully implemented a charge on plastic bags to curb overuse, notably Hong Kong, which introduced a levy ($0.06, £0.04) on each supermarket bag in 2009. Within a year, disposal of plastic bags declined by 83%. Similar reductions were seen in Wales after they imposed a charge (£0.05) on each plastic bag in 2011.
In an attempt to emulate the success of Wales, the UK is set to introduce a similar charge next year.
So next time you’re at the beach and spot plastic bags, or any other plastic trash, floating in the sea or at the shore, pick it up; the turtles will thank you for saving them.
Schuyler et al. Mistaken identity? Visual similarities of marine debris to natural prey items of sea turtles. BMC Ecology 2014 14:14.