It's well-known that humans are littering Earth's oceans with garbage, including about 8 million additional tons of plastic every year. This is notoriously bad for wildlife, which often mistake the indigestible debris for food. But recent research suggests ocean trash can also sneak back ashore — potentially tricking us into eating it, too.

Most people know better than to eat shards of plastic, but many other animals don't. Aside from seabirds and sea turtles, more than 50 species of marine fish are known to eat plastic in the wild. Scientists are trying to learn more about why they eat it, how it affects them and how it might affect any animals that eat them.

For a study published in 2015, scientists from the University of California-Davis and Indonesia's Hasanuddin University sampled 76 fish from Indonesian seafood markets and 64 from markets in California. About a quarter of those fish had either plastic fragments or textile fibers in their guts, meaning they could potentially pass their misfortune up the food chain to the species responsible for it. That study is one of the first linking ocean trash to the seafood we eat, according to its authors, and illustrates how our actions on land can affect the type of pollution in fish.

"It's interesting that there isn't a big difference in the amount of debris in the fish from each location, but in the type — plastic or fiber," said lead author Chelsea Rochman, an aquatic ecologist at UC-Davis, in a statement. "We think the type of debris in the fish is driven by differences in local waste management."

This plastic fragment was removed from a fish sold at a market in Indonesia. (Photo: Rosalyn Lam/UC-Davis)

In Indonesia, the researchers found bits of trash inside 28 percent of individual fish and in 55 percent of all species they tested. California fish showed similar rates of contamination, with man-made debris in 25 percent of individuals and in 67 percent of species. But all the fragments eaten by Indonesian fish were plastic, while 80 percent of the garbage inside American fish consisted of fibers.

Why the difference? Plastic waste is a global issue, but it's not spread evenly. Some is carried by currents to ocean gyres, forming large "garbage patches," but some also lingers near its point of origin. And as the study's authors note, Indonesia's scarcity of landfills, waste collection and recycling leads many people to discard plastic on beaches or into the ocean — a problem worsened by the country's lack of purified drinking water, which forces residents to rely heavily on bottled water.

"Indonesia has some of the highest marine life richness and biodiversity on Earth, and its coastal regions — mangroves, coral reefs and their beaches — are just awash in debris," said co-author Susan Williams, a marine ecologist at UC-Davis. "You have the best and the worst situation right in front of you in Indonesia."

Students help remove plastic debris from a heavily polluted beach in Indonesia. (Photo: Susan Williams/UC Davis)

Americans can also be reckless with trash, of course, but the study found plastic in only six California fish. That may be partly due to advanced U.S. systems for collecting and recycling plastic, although it doesn't mean the fish were debris-free.

Two-thirds of the California fish contained fibers, and while the authors couldn't determine how many were natural or synthetic, they say the fibers most likely came from clothes. Water used in washing machines empties into wastewater treatment plants, which may not catch tiny fibers before sending sewage effluent out to sea. Just one garment made from synthetic fibers like polyethylene terephthalate (PET) can shed 1,900 fibers every time it's washed, according to a 2011 study.

Anchovies and ocean plastic

A school of anchovies off the coast of Thailand. (Photo: nevodka/Shutterstock)

All contaminants were found in digestive systems of fish, the researchers note, not the meat. Humans are thus more likely to ingest them if fish are eaten whole rather than filleted — a common practice in Indonesia that's mainly limited to small fish like sardines and anchovies in the U.S. But small fish eat ocean plastic, too, and as another recent study found, some anchovies seem to be fooled by its smell.

Using wild-caught northern anchovies, researchers presented the fish with odor solutions made from plastic marine debris or from clean plastic. Anchovies responded to the debris odor as if it signaled food, the researchers report, but didn't respond the same way to clean plastic or in control experiments. The findings suggest plastic debris acquires a particular smell in the ocean that confuses anchovies, according to lead author Matthew Savoca, an ecologist at UC-Davis.

"When plastic floats at sea, its surface gets colonized by algae within days or weeks, a process known as biofouling," Savoca tells the Guardian. "Previous research has shown that this algae produces and emits DMS, an algal-based compound that certain marine animals use to find food. [The research shows] plastic may be more deceptive to fish than previously thought. If plastic both looks and smells like food, it is more difficult for animals like fish to distinguish it as not food."

And even for fish that aren't eaten whole, tiny pieces of ocean plastic are known to harbor a variety of toxins, including chemicals absorbed along the waste stream or from seawater as well as ingredients of the debris itself. Researchers are still investigating whether these toxins can move from plastic to a fish's meat.

Studying fish helps us grasp the broader issue of plastic pollution, but these findings alone shouldn't necessarily ruin seafood for anyone. Future studies may shed more light on the effects of plastic debris, but as Rochman told the San Francisco Chronicle in 2015, the lesson for now is to keep oceans clean, not to fear fish.

"This study doesn't make me afraid of eating fish," she said. "The health benefits outweigh the hazards of my being contaminated with microplastics."

Editor's note: This story has been update since it was first published in September 2015.