Trying to save sea turtles through sound

Nearly every species of sea turtle is endangered. That’s why scientists are trying out some creative solutions to keep turtles from being caught accidentally. 

Imagine a sea turtle swimming around in the ocean. It’s going about it’s usual business and it hears this sound:

The sea turtle’s ears (yes, they have ears) perk up. That’s not a noise it’s used to hearing in the ocean. Now it’s on high alert, looking out for something abnormal in the water — like a fisher’s net.

That’s the idea behind Wendy Dow Piniak’s work at Gettysburg college. She’s an assistant professor in the environmental studies department, and she specializes in sea turtles and sound. She works with researchers from the NOAA National Marine Fisheries Service and students and researchers from Ocean Discovery Institute.

The goal of her team’s research is to develop by catch reduction technology — some sort of device that will prevent the wrong creature from swimming into a net, while still catching the fish that the fisher’s want to catch.

Piniak is focused on sea turtles because nearly every species is endangered. They’re killed by poaching, habitat destruction, and, they are accidentally caught in fishing nets, in which case they get so tangled up that they drown.

Piniak and her student with a sea turtle. (Courtesy of Gettysburg College)

“What we want to do is design some kind of alerting signal that tells the sea turtle that something is out of the ordinary, so pay attention! But we also want it to be a cue that the target catch that the fisherman are trying to catch aren’t alerted by,” Piniak explains.

Her team was focused on sensory cues like light, but Piniak is interested in sound for several reasons: sound travels further in the ocean than light does, plus it can be used in murky, brackish waters.

But sound is also difficult — basically they need to pinpoint a sound that only sea turtles can hear, but other fish can’t hear. Kind of like how a dog can hear super high pitched sounds, but humans can’t.

“People hadn’t really considered acoustic deterrent devices for sea turtles, because most sea turtles and fish hear very similar frequencies,” Piniak says.

Sound under the sea

The fishery that her team works in is in Baja, Mexico. That fishery’s target fish that they want to catch is Halibut, a flatfish that hangs out at the bottom of the ocean. Halibut, like flounder, is called a flatfish, which means it doesn’t have a swim bladder, or an air filled sack that keeps most fish buoyant.

And luckily for Piniak, Halibut have really bad hearing.

Some context here — humans can hear between 20 hertz and 20,000 hertz. Halibut can only hear frequencies up to 200 hertz, which are very low frequencies.

And a sea turtle can hear up to about 1,600 hertz, which is a high frequency.

So as long as they attach a sound to the net called an acoustic deterrent device, that plays a sound between 200 hertz and 1,600 hertz, theoretically only the sea turtles could hear it and be warned, and the halibut would swim into the net just like normal.

They made the device using a recorder that played the sound, an amplifier that made it louder, and then a speaker further down the net to play the noise out into the ocean.

Piniak with acoustic deterrent devices. (Courtesy of Gettysburg College)

Of course, there’s naturally sound in the ocean. It’s called “acoustic habitat” or “sound scape”, and it’s a really important sensory cue for marine animals.

“Animals in the ocean – because sound does travel so far, and you can’t see all the time – often use acoustic cues to communicate with each other, to find food, to navigate, or for general environmental awareness,” Piniak says.

At their study site in Baha, the acoustic habitat sounds like this:

Those tiny clicking noises are from a small crustacean called a snapping shrimp. You can also hear the sounds of the waves crashing on the shore, the boat rocking, and some wind noise.

The experiment 

Piniak and her team put two nets into the water — one net doesn’t make any sound, called the control net, and the other net had the acoustic deterrent devices attached to it. It sounds like this:

The loudest beep is the one closest to the microphone on the net, and the quieter ones are further away from it on the same net.

The two nets are close enough together that they’re in the same environment and could catch the same things, but far enough apart that any marine life near the control net couldn’t hear the sounds from the deterrent devices on the other net.

The team sets their nets in the water. The floating orange boxes are the acoustic deterrent devices. (Courtesy of Gettysburg College)

If a sea turtle hears the noise, it might swim away from it. And even if it doesn’t cause a behavior change, Piniak explains that the weird sound acts as an alarm that something out of the ordinary is happening in the water.

“Over the last two summers, we found that adding the acoustic deterrent devices to nets reduces the amount of turtles you catch by 65%,” Piniak says, “which is huge.”

It’s huge, Piniak says, but there’s still work to be done — the team wants to test the devices in other fisheries that target other kinds of flatfish. They want to study the effect of the devices on the target fish catch, not just the decreased catch of turtles.

“The other thing to think about is we need to be very careful about how much noise we actually put into the environment,” Piniak says. “The ocean’s a really noisy place already due to human caused noise, so we need to think really carefully about the levels of sound we put in, and how much sound we put in.”

The end goal, she says, is to create an acoustic device that fisher’s really want to use.