In the ocean’s abyss, deep-diving whales use echolocation to hunt in pitch dark. Emitting sounds that bounce off objects gives the whales a clear picture of their surroundings.
But such a superpower might come with a downside, a new study reveals. When hit by whalelike sound frequencies, the strength of the echo returned by plastic debris is similar to that of whales’ common prey, researchers from Duke University Marine Lab in Beaufort, N.C., report October 16 in Marine Pollution Bulletin. That may trick sperm, beaked and other deep-diving whales into eating deadly garbage.
“More often than any other species of whale, the ones that are deep diving are the ones that we find with lots of plastic in them,” says Matthew Savoca, a marine ecologist at Stanford University who wasn’t involved in the new study.
Scientists have long wondered whether these whales, some of which dive nearly 3,000 meters, confuse plastic objects for prey because they “sound” similar (SN: 9/23/20). Such mistakes can be deadly, as “the plastic fills up their stomachs [and] intestines and can prevent food from passing,” says Savoca.
Whale biologist Greg Merrill and his colleagues tested the strength of the echoes from nine pieces of plastic junk collected directly from the sea — including ropes, plastic bags, bottles and other items commonly found in the stomachs of stranded whales.
The team also ran tests on Atlantic brief squids (Lolliguncula brevis), which are similar to the cephalopod prey of deep-diving whales but easier to find, as well as on squid beaks found in a stranded sperm whale’s stomach.
The team positioned the plastic items and prey items one at a time on an underwater rig off the N.C. coast and hit them with sounds at three different frequencies: 38 kilohertz, 70 kHz and 120 kHz. Those three covered “a wide range of frequencies that the echolocating whales are using,” Merrill says. A machine then measured the strength of the echo coming back from each item to determine what a whale might perceive.
All of the plastic items returned echoes similarly as strong or sometimes even stronger than those of the prey items, the team found. “That was pretty striking,” Merrill says. This seems to suggest that “these animals have a hard time perceiving the difference between plastic and prey.”
Those results mirror ones presented in June at the International Conference on Underwater Acoustics in Bath, England. In that case, marine biologist Laura Redaelli and her colleagues performed similar experiments in the lab, using a saltwater tank. The strength of the echoes coming back from “natural prey was indeed overlapping with those of multiple plastic items commonly ingested by deep divers,” says Redaelli, of the Marine and Environmental Sciences Centre-Madeira in Portugal.
Those findings, Redaelli adds, are “providing an initial ground to advocate for a change in plastic pollution policies, maybe leading to a modification in their composition to prevent them from being acoustically mistaken for natural preys.”
While deep-diving whales have been around for a long time, plastic garbage reached their world only in the last decades. “These items are fairly new in their environment,” Savoca says. As a next step, he’d be curious to see if, through individual learning or cultural transmission, these animals can “learn their way out of this” or whether “they’re bound by their evolution to sort of fall into this trap.”
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