"These acoustic signatures are similar, and this might be a reason that these animals are driven to consume plastic instead of, or in addition to, their prey," said Duke University graduate student Greg Merrill, who led the research.
"One hundred percent of plastic marine debris tested have either similar or stronger acoustic target strengths compared with that of whale prey items," the authors report in a paper appearing online in Marine Pollution Bulletin.
To find food in the dark, deep-diving whales, like sperm whales, pygmy sperm whales and goose-beaked whales, send out clicks and buzzes from a vocal cord-like structure near their blowholes. These sounds are transmitted to the surrounding water by the bulbous oil-filled 'melon' structure above their mouths. Sounds that bounce back from objects in the water are received by fat-filled sensing organs in their lower jaws and sent to the inner ear and then the brain for interpretation. The system has been serving them well for at least 25 million years.
But ocean plastics such as shopping bags, ropes and bottles are a growing problem and are routinely found in the guts of stranded whales and other animals. Merrill wanted to test the acoustic signatures of the materials to see whether the whales were being confused by plastics.
After rounding up some typical, and barnacle-encrusted, plastic trash from beaches in Beaufort and nearby Atlantic Beach, N.C., the researchers put them under the sonar transponder on the Duke Marine Lab's ship R/V Shearwater to test them. "It was plastic bags, balloons, things that are commonly observed in the stomachs of stranded whales," Merrill said.
They made an H-shaped rig of fishing line with weights at the bottom to hold the samples four to five meters below the transponder, which is on the bottom of one of the catamaran ship's keels. Acoustic testing was then done at three different sonar frequencies, 38, 70 and 120 kilohertz, which spans the range of 'clicks' that different species of deep-diving whales employ.
For comparison, they also tested real (but dead) squid and pieces of squid beak that had been recovered from the stomach of a dead sperm whale.
The trash almost always sounded like food, especially plastic films and fragments of plastic, two particularly noisy items that are most often found in dead whales.
"There are hundreds of types of plastic, and the various material properties including polymer (chemical) composition, additives, shape, size, age/weathering, and degree of fouling likely play a role in the frequency-specific responses observed," the authors report.
Perhaps it would be possible to re-engineer some plastics to not have an acoustic signature, Merrill said. "But I don't think that's really a viable option, because then, if fishing net and fishing line are invisible, those are things that whales get entangled with, too. So we don't want them to not be able to identify those things."
Merrill worked on this project with scientists at neighboring marine labs operated by NOAA, NC State University and UNC -- Chapel Hill.