Eating plastic debris is a major problem for hundreds of species of marine animals, from tiny zooplankton to giant baleen whales. But very little is known about how so many animals mistake plastic for their natural prey.
A new study from the University of California, Davis, shows that marine plastic debris emits the odor of a sulfurous compound that some seabirds use to find food. The study, published in Science Advances, helps explain why plastic consumption is more common in some seabird species, and why it’s important to consider the animals’ point of view in questions like this.
Plastic debris is found in ocean environments worldwide. A 2014 global analysis reported a quarter of a billion metric tons of plastic floating in the world’s oceans. More than 200 species of fish, marine mammals, sea turtles, and seabirds are known to ingest plastic at sea.
“In the short-term, ingesting plastic makes the animal feel satiated, so they don’t eat,” says lead author Matthew Savoca. “However, plastic provides no nutritional value, so they starve if they eat a great deal of plastic as a component of their diet.
What’s worse, plastic has some nasty chemicals associated with it, and it also adsorbs toxins from the water. It can even block the gut or teat the intestines. There’s nothing good that comes from eating plastic, and yet animals do it so much.”
In ocean ecosystems, a sulfurous compound called dimethyl sulfide (DMS) is produced by marine algae when they are being eaten by animals like krill. Krill are a favorite meal of seabirds, so the odor of DMS draws them in to forage.
Some of the seabirds attracted to DMS are the tube-nosed seabirds (order: Procellariiforms), which includes albatrosses, petrels, and shearwaters. Tube-nosed seabirds have a keen sense of smell, which they use to hunt over vast expanses of open ocean. They are also among the birds most severely affected by plastic ingestion.
Savoca and his colleagues analyzed available data on plastic ingestion in seabirds. “After we looked at all this bird data, we found something really shocking,” says Savoca. “The species of bird that use this scent to forage are five to six times more likely to eat plastic than the species that do not use DMS to forage.”
To see if the odor of plastic ocean debris was confusing seabirds, Savoca and his colleagues deployed plastic beads in mesh bags at two locations off the California coast. They collected the plastic after three weeks marinating in the ocean and compared their odor profile to that of clean plastic beads.
They analyzed the plastics at the UC Davis Robert Mondavi Institute for Wine and Food Science, where researchers are more accustomed to testing wines and whiskeys than plastic trash.
The researchers found DMS coating the ocean-exposed plastics, but not the clean ones, in concentrations that seabirds can detect.
The results suggest that patches of floating plastic trash might act as olfactory traps for susceptible marine wildlife such as seabirds. Those species that have evolved to use DMS as a sign of food can be tricked by marine plastic debris that also emits DMS.
Savoca says this finding might apply to other marine animals, like fish, sea turtles, and marine mammals.
“Understanding, from the animal’s perspective, why they might be confusing plastic for food could allow us to make better predictions as to which species are more likely to eat plastic,” he says.
Reference: Savoca, M. S., Wohlfeil, M. E., Ebeler, S. E., and Nevitt, G. A. (2016). Marine plastic trash emits a keystone infochemical for olfactory foraging seabirds. Science Advances 2:e1600395.