We've long known that the fish we eat are exposed to toxic chemicals in the rivers, bays and oceans they inhabit. The substance that's gotten the most attention — because it has shown up at disturbingly high levels in some fish — is mercury.

 

But mercury is just one of a slew of synthetic and organic pollutants that fish can ingest and absorb into their tissue. Sometimes it's because we're dumping chemicals right into the ocean. But as a study published recently in Nature, Scientific Reports helps illuminate, sometimes fish get chemicals from the plastic debris they ingest.

 

For many years, scientists have known that chemicals will move up the food chain as predators absorb the chemicals consumed by their prey. That's why the biggest, fattiest fish, like tuna and swordfish, tend to have the highest levels of mercury, polychlorinated biphenyls (PCBs) and other dioxins. (And that's concerning, given that canned tuna was the second most popular fish consumed in the U.S. in 2012, according to the National Fisheries Institute.)

 

What scientists didn't know was exactly what role plastics played in transferring these chemicals into the food chain. To find out, Chelsea Rochman, a postdoctoral researcher at the University of California, and her co-authors fed medaka, a fish species often used in experiments, three different diets.

 

One group of medaka got regular fish food, one group got a diet that was 10 percent "clean" plastic (with no pollutants) and a third group got a diet with 10 percent plastic that had been soaking in the San Diego Bay for several months. When they tested the fish two months later, they found that the ones on the marine plastic diet had much higher levels of persistent organic pollutants.