For the first time, plastic particles have been found in the stomachs of tuna and other fish that are a staple of the human diet.

More than 18 percent of sampled bluefin, albacore, and swordfish caught in the Mediterranean Sea and tested in 2012 and 2013 carried levels of plastic pollution in their bodies, according to a study published in the journal Marine Pollution Bulletin.

All three species migrate between the Mediterranean and the Atlantic Ocean, so these plastic particles could make their way onto the plates of American consumers. The plastics found in the fish contained phthalates, nonylphenol, bisphenol A, brominated flame retardants, and other chemicals that previous research has linked to endocrine disruption, low reproductive rates, and other health risks.

A 2010 study by French and Belgian marine biologists estimated that 250 billion pieces of microscopic plastic were floating in the Mediterranean. A 2014 expedition by Gabriel Gorsky of Pierre-et-Marie Curie University found that “there is not one parcel of the Mediterranean Sea that is devoid of plastic or plastic fragments.” Another study published last year estimated that all of the world’s oceans combined carry more than 5 trillion pieces of plastic pollution.

The current study of large pelagic fish (which live in the open sea, away from the shores or the bottom of the ocean) examined 56 swordfish, 36 bluefin, and 31 albacore that had been caught in the Mediterranean. Of those fish, seven swordfish, 11 bluefin, and four albacore contained plastics in their stomachs.

The plastics varied in size from large pieces more than 25 millimeters wide to microplastics smaller than 5 millimeters. The swordfish were more likely to have ingested large fragments of plastic, while the albacore ingested mostly microplastics.

Most of the pieces were white or transparent, while some “yellowish” plastics were found in the stomachs of the swordfish and bluefin.

As large, “top of the food chain” predators, the fish could have picked up plastic that had first been eaten by smaller fish; a study published last year found that Mediterranean bogue, an important prey species for swordfish, ingest large quantities of microplastics. The researchers, from the Institute for Environmental Protection and Research in Italy, wrote that other plastics could have been ingested while the tuna chased schools of prey fish into shallow waters, where floating plastics are more abundant.

Although this study did not examine the effects of all this plastic, the authors did pose questions about how the toxic chemicals they contained could affect the health of the three fish species or the humans that ate them.

“Data continues to mount about the pressing problem that plastics pose for our ocean environment,” said Alison Chase, oceans program senior policy analyst for the Natural Resources Defense Council. “Plastic litter simply doesn’t belong in the ocean. It often looks like food to animals, may contain and soak up toxins like PCBs and pesticides in marine sediments, and can potentially be passed on to people when we eat seafood. We need to stop treating our oceans like a trash can and reduce the amount of plastic we produce and that reaches our waters.”