It's well-known that humans are polluting Earth's oceans with garbage, including about 8 million tons of plastic every year. This is notoriously bad for wildlife, which often mistake the indigestible debris for food. But according to a new study, ocean trash is also sneaking back ashore — potentially tricking us into eating it, too.

Most people know better than to eat shards of plastic, but this trash is subtler than that. While lots of marine debris washes onto beaches around the world, some is returning to land in disguise, researchers report, by hiding inside fish.

PHOTOS TO INSPIRE: 30 of the most beautiful places on Earth

That's according to a team of scientists from the University of California-Davis and Hasanuddin University in Indonesia, who 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 pass their misfortune back up the food chain to the species responsible for it.

Published in the journal Scientific Reports, this study is one of the first to directly link man-made ocean trash to the seafood we eat, the authors say. It also sheds light on how our actions on land affect the type of pollution found inside 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," says 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."

plastic fragment from fishThis 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," says 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."

Indonesia beach cleanupStudents 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.

All contaminants were found in digestive systems of fish, the researchers point out, 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. Yet even 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. The researchers are still investigating whether these toxins can transfer to a fish's meat.

Despite that possibility, this research shouldn't necessarily ruin seafood for anyone. Studying fish helps us understand the broader issue of pollution — whether it's mercury from power plants or specks of microplastic. Future studies may offer more details about how ocean garbage affects seafood, but as Rochman tells the San Francisco Chronicle, the lesson for now is to keep oceans clean, not to fear fish.

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

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.