Plastic isn’t food, but fish seem to eat it anyway—to the confusion of many researchers. As the 9.5 billion tons of plastic the world has produced since the 1950s makes its way into the world’s rivers, lakes, and oceans, the animal consumption of such waste has become a big problem. Yes, a lot of that plastic is tiny—roughly the size of fish food—so it’s not a stretch to think that fish are simply confusing these morsels with plankton. But the fish that are apparently attracted to plastic are usually picky eaters.

“I've heard both scientists and nonscientists say that there's so much plastic out there that these animals sort of bump into it and consume it without thinking,” says Matthew Savoca, a California Sea Grant State Fellow at the National Oceanic and Atmospheric Administration (NOAA) Southwest Fisheries Science Center. “But people who know about foraging behavior and study this sort of thing know that couldn't be further from the truth.”

Savoca co-authored a recent study in the journal Proceedings of the Royal Society B that seeks to better understand why fish chow down on literal junk food. According to their findings, it may come down to smell. The work was influenced in part by a study Savoca worked on last year on marine birds.

In that study, researchers came to a malodorous conclusion by diving into sea bird data. They discovered that birds attracted to the smell of algae—specifically a compound called dimethyl sulfide—eat a lot more plastic than species that don’t use those odor compounds to find food.

“Most of the animals that are eating plastic in the greatest quantities are what we call dietary specialists, they're really looking for very specific types of food,” says Savoca.

The compound, which Savoca says smells like rotting seaweed, often emerges when algae cells walls die—like when zooplankton such as krill eat them. For some animals, the scent serves as a pungent signal that the plankton they love to eat are in the neighborhood.

Plastic doesn’t smell like dimethyl sulfide on its own (rotting seaweed wouldn't sell many water bottles) but algae can infuse the material with this oceanic aroma. Algae like to attach to hard, smooth surfaces, and they float up near the air to collect sunlight—so they often hitch a ride on our garbage.

“It's like setting up a Petri dish for algae in the ocean,” says Savoca.

In the new study, Savoca essentially wanted to ask an animal whether or not it cared for the smell of marine plastic debris. To do that, he turned to anchovies. They are known to eat plastic in the wild, and their relatively small size makes them easy to study.

It may not seem intuitive, but fish do use their sense of smell to help work their way around aqueous environments. Fish take in smells through tiny holes on their head that look like nostrils, called nares. Odors help fish avoid predators, find their way home, and, of course, search for food, just like a sense of smell helps animals on land. Anchovies generally go sniffing for zooplankton. They ignore pretty much everything else—dirt, rocks, fish parts—that they might come across.

But not plastic.