Hundreds of marine species are known to eat plastic – including those that regularly end up on our dinner plates. But why? It now seems that ocean-borne plastic has a smell that marine animals find appealing.
Matthew Savoca at the NOAA Southwest Fisheries Science Center in Monterey, California, explored the dietary preferences of marine life while he was a researcher at the University of California, Davis. He and his colleagues exposed schools of anchovies to seawater that contained odours from plastic. To make this, they left plastic beads in the ocean for three weeks, then stirred the beads into seawater samples before filtering them out – leaving just the associated odour chemicals.
In the ocean, plastic quickly becomes covered with a layer of algae that releases smelly sulphur compounds. Foraging fish such as anchovies, which feed on algae-munching marine crustaceans called krill, are thought to use these compounds to help them locate their prey.
When analysing videos of the anchovies, the researchers noticed that the fish reacted to the fouled plastic solutions as if they were their crustacean prey. The decision to use solutions that smelled of plastic rather than actual pieces of plastic meant the fish weren’t responding to visual cues; the fish must have smelled the odours. They did not respond to clean plastic.
The work builds on Savoca’s earlier research, which suggested that similar sulphurous odours lure tube-nosed seabirds – which are also krill-feeders – into eating plastic.
Fish aren’t stupid
“This problem of animals eating plastic has not been investigated as completely as it should have been,” says Savoca. Seabirds and fish aren’t stupid, he says, and generally they are good at locating the correct prey. Savoca thinks we might have overlooked the role of odour because we are such visual animals. “That’s how we perceive the world most readily.”
This explains why, for example, it is often suggested that marine animals mistake plastic bags for jellyfish, or plastic pellets for fish eggs. “This is mostly anecdotal,” says Chelsea Rochman at the University of Toronto. She thinks Savoca’s experiments provide the most rigorous explanation yet for the high frequency of plastic consumption among marine species.
This dietary confusion is a problem for fish, but it could also be an issue for animals higher up the food chain, including us.
“There’s no doubt that we eat microplastics when we eat seafood,” says Rochman. What we don’t know, she says, is how much of the chemicals leaching out of the plastic then enter our systems – and whether it matters if they do.
An FAO report released last week highlights how much we don’t know about plastic consumption by animals and the possible impact on human health. While Savoca’s work doesn’t provide solutions, he feels it is important to establish why animals are eating plastics in the first place.
“For any problem, if you want to effectively mitigate it or solve it in any way, you have to fully understand what’s going on,” he says.
Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2017.1000
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