For sea turtles, a diet worse than junk food

A Pacific leatherback turtle in Monterey Bay breaks the surface about every two hours, taking a deep breath of air before going back under to hunt for jellyfish. Leatherbacks use their powerful flippers to propel themselves forward and grab a gelatinous mouthful.

“Mabibi – LEATHERBACK TURTLE” by Jeroen Looyé is licensed under CC BY 2.0.

Only it might not be a jellyfish.

It might be a plastic bag, perhaps one of the 13 billion disposable grocery bags that Californians use each year. Scientists are finding single-use bags, cosmetic microbeads and other types of plastic litter throughout the ocean, even in the deepest submarine canyons. Globally, an estimated 8 million metric tons reach the ocean every year.

Plastic doesn’t biodegrade. Instead, it breaks down into smaller and smaller pieces, persisting in the environment. That makes plastic pollution a major threat to marine ecosystems—and sea turtles are among the most vulnerable ocean animals.

“The enormity of the problem, the scale of the pollution and the vast impact have only really been appreciated in the last decade,” says Kyle Van Houtan, Monterey Bay Aquarium’s director of science. “Sea turtles are good indicators of the challenges the ocean faces right now.”

Plastic pollution starts early

To assess the risks that threatened sea turtles face, scientists need to know where they spend their vulnerable early years. In a paper published in the journal Ecology and Evolution in March, Kyle focused on the world’s smallest sea turtle population, the Hawaiian hawksbill.

The Endangered Species Act has protected Hawaii’s green sea turtles and hawksbill turtles since 1978. Green turtles have rebounded, but hawksbill numbers are still dangerously low; only about 100 breeding adults remain. Why did conservation efforts succeed for one species, but not the other?

Kyle had a hunch the answer lies in what he calls hawksbill turtles’ “lost years.”

Researchers think most sea turtle species—green and loggerhead, for example—spend the bulk of their early years in the open ocean before returning to the coast as large juveniles. But Kyle discovered that juvenile hawksbills spend their early years in shallow reefs and coastal estuaries, which are hotspots for plastic pollution.

Kyle Van Houtan’s research found more plastic than food, by mass, in the guts of baby hawksbill turtles.

That may explain why every juvenile hawksbill Kyle and his colleagues examined was either entangled in, or had eaten, plastic.

“Those early years are very important to turtles’ survival,” Kyle says. “If we’re compromising that with plastic pollution, we’re adding insult to injury.”

In clean water, a healthy young hawksbill turtle would eat mostly gelatinous zooplankton, like tiny jellies, tunicates and salps. But Kyle’s research shows young Hawaiian hawksbills are eating more plastic, by mass, than anything else.

Can it be compared to a human kid eating nothing but mac ‘n’ cheese and cake throughout childhood?

“This is worse,” Kyle says. “Like eating Legos.”

A problem throughout the global ocean

Hawaiian waters aren’t the only garbage-strewn sea turtle habitat. A study published last fall in Global Change Biology estimates half of the world’s sea turtles will ingest plastic at some point in their lives.

Although turtles mistake it for food, plastic ocean debris is more like poison. It acts as a magnet for chemical pollutants, such as pesticides and PCBs, that have been discharged into the ocean. When a baby turtle eats plastic, it consumes those toxins, too. A fish that eats the turtle—and perhaps 10 more like it—gets an even heavier dose, and the toxins continue to build up in the food web.

“Marine litter. Plastic bottles on a beach” by Bo Eide is licensed under CC BY 2.0.

Another serious health concern is what a turtle doesn’t get when it eats plastic. Animals expend energy trying to digest plastic without getting any calories in return. That can lead to developmental delays.

Turtles young and old can also become entangled in plastic trash, like six-pack rings and abandoned fishing nets. Rescuers have retrieved plastic forks and straws from the noses of sea turtles.

Other marine animals are affected, too. Plastic pollution was found in the guts of 97.5 percent of Laysan albatross chicks, according to one study. Recently, 13 sperm whales washed up dead on the German coast of the North Sea; among the contents of their stomachs were fishing nets, pieces of a plastic bucket and the plastic cover of a car engine.

We all play a part

The global scale of the problem means everyone plays a role, says Qamar Schuyler of the University of Queensland, lead author of the study in Global Change Biology. Coastal communities around the globe are grappling with the contradictions of plastic—weighing its convenience against its rising toll on the marine environment.

“Everybody goes to the store and makes a decision about whether they buy something that’s wrapped in plastic,” Qamar says.

In November, California voters will have the chance make a decision that’s bigger than what to cook for dinner. They’ll be asked to decide a referendum that, if approved, would ban single-use plastic bags statewide. Monterey Bay Aquarium urges you to vote YES on Proposition 67 for a plastic-free ocean.

–Lisa Marie Potter

Featured photo: Hawksbill sea turtle. Photo by USFWS.