The Plastic Polluting the Oceans Smells Like Lunch to Marine Life
Think all birds find dinner using their eagle-eye vision?
Not so for many seafaring bird species, which rely on an acute sense of smell to locate swarms of tiny krill and fish swirling in currents on the ocean surface.
“They’re like bloodhounds,” said Gabrielle Nevitt, professor of animal behavior and biological sciences at the University of California, Davis. “A lot of seabirds actually have limited vision, and they really rely on a good sense of smell, rivaling that of dogs, if not better.”
Such birds, including petrels and albatrosses, are the species commonly found with bellies full of tiny bits of plastic. The plastic originates from the 12 million tons of trash bags, cups, straws, and microbeads that are dumped into the ocean every year.
If these birds have such a keen sense of smell, why do they tend to ingest plastic at a rate higher than other birds and marine animals? Is it a case of being bird-brained?
Not according to a study published this week in the journal Science Advances.
Nevitt, UC Davis graduate student Matthew Savoca, and other researchers found that the scent ocean plastics emit mimics the smells the birds have relied on for thousands of years to find food.
“Why do these birds mistake plastic for food?” asked Savoca. “It might not be a mistake; it might be that we’ve altered the environment so much that we’ve messed up their ability to make decisions.”
The problem for seabirds starts at the chemical level. Petrels, shearwaters, and albatrosses spend most of their lives gliding above the ocean, sniffing out a sulfur compound called dimethyl sulfide, or DMS. When krill and small fish munch on algae, DMS is released—acting as a sort of scent alarm for the birds.
The smell triggers their instinct to forage on ocean animals near the surface. What the researchers found was that DMS-emitting algae grows on the bits of plastic debris, and through the course of natural wave movements, it is emitting and mimicking the scent birds seek out to identify food. Attracted by the smell, they eat the plastic.
In the study, the team tested three types of plastic commonly found in ocean gyres—high-density polyethylene, low-density polyethylene, and polypropylene. They put sample beads in Monterey Bay and Bodega Bay in California, sealing the beads in mesh bags that let ocean water pass through. After three weeks, the team retrieved the plastic beads, now coated in algae, and sent them to be chemically analyzed at UC Davis’ Robert Mondavi Institute for Wine and Food Science.
The samples were run through chemical-analyzing machines that vintners and brewers use to dissect flavor compounds in wines and beers.
“It was funny, because people didn’t really give us strange looks when we were in the bay actually placing plastic into the ocean, but we definitely got the side-eye when I told the food chemists in the wine and food lab we were going to put plastic garbage in their analyzers,” Savoca said.
The results were no joke. The analysis showed that after three weeks at sea, all three types of plastic were emitting the scent associated with DMS at levels even the scientists could smell.
“Sometimes you get samples or studies like this, and you can detect chemicals but not at really significant levels,” Savoca said. “But by analyzing them, we were able to show the concentrations were definitely significant.”
The findings could be important for a number of ocean-dwelling animals that rely on scent triggers like DMS to find food, including baleen whales that feed on krill, sea turtles, and some fish species.
“These animals are using their adaptive ability to find food that is vital to their existence,” Nevitt said. “When we see a food additive we know is unhealthy, we can read the label and make a choice. But for these animals, it’s a sensory trap—they’re using their senses to find food, and it’s leading to plastic. It’s not just passively or accidentally ingesting plastic; they’re targeting it.”
For Savoca, the study shows the importance of keeping plastics from reaching the ocean.
“Animals interact with introduced substances in ways we often don’t fully understand, and that’s becoming more evident the more we look at these types of human-influenced issues,” he said.