Plastic pollution sounds like food for scuba diving whales

To whales hunting with sound waves in the lightless depths of the ocean, a broken plastic balloon and a delicious squid seem remarkably similar, according to a new study that subjected some plastic beach litter to underwater acoustic tests .

“These acoustic signatures are similar, and this could be a reason why 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 investigation.

“One hundred percent of marine plastic debris tested has similar or stronger acoustic strengths compared to whale prey,” the authors report in a paper appearing online at Marine Pollution Bulletin.

To find food in the dark, deep-diving whales such as sperm whales, pygmy sperm whales, and beaked whales send out clicks and hums from a vocal cord-like structure near their holes. These sounds are transmitted to the surrounding water by the bulbous, oil-filled “melon” structure above their mouths.

Sounds bouncing off objects in the water are received by fat-filled sensory organs in their lower jaws and sent to the inner ear and then to the brain for interpretation. The system has served them well for at least 25 million years.

But ocean plastics, such as shopping bags, ropes and bottles, are a growing problem and are commonly found in the guts of stranded whales and other animals. Merrill wanted to test the acoustic signatures of the materials to see if the whales were being confused by the plastics.

After gathering some typical, barnacle-encrusted plastic trash from the beaches of Beaufort and nearby Atlantic Beach, North Carolina, the researchers put it under the sonar transponder of Duke Marine Lab’s R/V Shearwater to test- the

“It was plastic bags, balloons, things that are commonly seen in the stomachs of stranded whales,” Merrill said.

They made an H-shaped platform out of fishing line with weights on the bottom to hold the samples four to five meters below the transponder, which is located on the underside of one of the catamaran’s keels. Acoustic tests were then performed at three different sonar frequencies, 38, 70 and 120 kilohertz, which span the range of “clicks” used by different species of scuba diving whales.

For comparison, they also tested real (but dead) squid and pieces of squid beak that had been retrieved from the stomach of a dead sperm whale. Trash almost always sounded like food, especially plastic film and plastic fragments, two particularly noisy items most often found in dead whales.

“There are hundreds of types of plastic, and different material properties such as polymer (chemical) composition, additives, shape, size, age/weathering and degree of contamination likely play a role in the frequency-specific responses observed,” the authors. report

It might be possible to redesign some plastics to not have an acoustic signature, Merrill said. “But I don’t think that’s really a viable option, because then if the net and fishing line are invisible, those are things that the whales get tangled up in as well. So we don’t want them to be unable to identify those things.” .