Plastic might not smell like lunch to humans, but after it's marinated in the ocean for a while, it can start to smell that way to seabirds.
A new study found that one of the reasons that some marine animals mistake pieces of plastic trash for food is odor. After floating in the ocean, plastic tends to get covered with algae—and that algae reminds certain species of foods like krill.
"The plastic provides an excellent medium for the algae to grow," says lead author Matthew Savoca, a graduate student at the University of California-Davis. "It's almost like a petri dish for algae in the ocean. Algae grows and concentrates on the plastic, and we found that it gives off this peculiar and very specific scent that certain birds use to find food."
Savoca realized that birds that use a stinky sulfur compound to find food—a smell like rotting eggs—eat plastic five to six times more frequently than those that don't. So he decided to test whether that compound could be found in ocean plastic.
He put plastic beads in the ocean (inside mesh bags, so they could easily be removed and wouldn't add to the problem), waited about a month, and then took them to a food and wine lab that analyzes chemistry. The algae-coated beads were full of the sulfur odor.
Though the study focused on birds, he's now doing similar experiments with fish. Plastic is a problem for marine animals from whales to turtles, and worsening as plastic in the ocean grows. A 2014 study estimated that there are around 5 trillion pieces of plastic in the ocean now; over the next decade, the world will produce as much plastic as it has since the 1950s. Plastic has been found in sea ice and in sea floor sediment.
"It's really all over," Savoca says. "It's an extremely pervasive problem. So figuring out any reason why animals eat it and any way to mitigate that problem I think is important."
As many as 90% of seabirds have eaten plastic; when birds or other marine animals fill up on too much, it can starve and kill them. In the past, researchers have often assumed that birds ate plastic because it looked like food.
"We are potentially just adding a new layer to that story, and saying, hey, it might also smell interesting to some of these species," Savoca says.
Certain species—including the albatross species in the North Pacific famous for bellies full of plastic bottle caps—don't forage for food using the sulfur odor. So smell is likely only part of the explanation, but makes the problem worse for some animals.
It's possible that it also might be solvable, while we try to figure out how to keep plastic out of the ocean in the first place (and remove what's already there).
"It stands to reason that perhaps you could alter the surface of plastic and make it less habitable to algal species that produce this compound," says Savoca.