Deep-Diving Whales Mistake Plastic for Prey

To Whales, Our Plastic Debris Sounds Like Dinner 

We’ve likely all seen videos of how dark the deep ocean is. 

Think of the scenes of deep-water submersibles exploring shipwrecks like the Titanic. It’s pitch-black, and the spotlights on the vessels barely penetrate the darkness to illuminate the wreck. 

That’s because light’s brightness in the ocean decreases with depth. The National Ocean Service reports that while minimal light may be detected as deep as 1,000 meters (3,280 feet), there’s rarely any significant light beyond 200 meters (656 feet). 

And that’s a huge problem if you’re a deep-diving whale looking for food. It’s impossible to see your next meal. 

Scientists assume seals and toothed whales are washing up dead with stomachs full of plastic because they mistake it for food. Sadly, plastic bags and film can look like squid or jellyfish with wavy tentacles, especially if there’s not a lot of light. 

That doesn’t explain, however, why deep-diving whales that use echolocation, like sperm whales and beaked whales, are ingesting plastic. So, marine scientists from Duke University, NC State, UNC-Chapel Hill and the National Oceanic and Atmospheric Administration compared the way sound bounces off plastic floating underwater to the prey that whales usually eat, namely squid and squid beaks. 

Their study found that plastic bags “sound” a lot like dinner to a whale. 

What’s echolocation? 

Echolocation is the ability to observe an environment using sound. 

To echolocate, whales use special organs called dorsal bursae to emit clicks and create echoes that bounce off objects and return information. The whale interprets the strength of the echo to help it find food, avoid objects and position itself in the ocean. The clicks are also used for social interaction. 

Another way to think of those clicks is to listen for echoes when you walk through a tunnel or face a wall and yell. 

But what if prey and plastic sound alike? 

The study found that 100% of the plastics tested—things like plastic bags, rope and bottles harvested from the stomachs of stranded whales—have similar or stronger acoustic target strengths compared to squid. 

That’s right: the sounds bouncing off a plastic bag or bottle sound just like a squid. 

“Assuming these animals are ingesting plastic at depth and not at/near the surface, they are consuming plastic without visually identifying it,” noted a passage in the study. “Deep-diving toothed whales may therefore be misinterpreting acoustic cues when echolocating; presumably plastic’s acoustic signature resembles that of primary prey items, driving plastic consumption.” 

How the study tested plastics 

Researchers collected plastic trash from the beach and strung the items onto a fishing line under a research vessel. The team also strung various sizes of squid along another line. 

All the objects were then blasted with sound waves at sea using an echosounder mounted to the bottom of the boat. An echosounder is a device that uses sound waves to measure water depth or where objects are in the water. The hull-mounted echosounder tested three different sounds at the same frequencies as whale clicks. 

The way an object reflects sound depends on what it is made of or its thickness. And based on the measurements the team recorded, plastic “sounds” the same as whale food. 

Plastic pollution is getting worse 

The findings of this study underscore just how complex and serious plastic pollution is and how it’s getting worse. 

The report says that more than 1,200 marine species are known to ingest plastic waste, which poses a serious threat to the animals’ health. The risks include perforation of tissues, infections, suffocation and gastrointestinal tract blockage, which leads to starvation and death. 

The most common items found are plastic bags, single-use packaging and fishing gear such as nets, ropes and fishing lines. 

It’s a problem that calls for policy actions at the local, national and international level. It also requires all of us to think about how we use and dispose of plastic waste. 

While we know about the physical impact of animals eating plastic, researchers are also looking at the chemical impact. Watch this Sci NC story to learn more.