Continuous Active Sonar's Impact on Killer Whales

When pulsed active sonar was found to cause mass strandings of whales, researchers turned to the quieter continuous active sonar for underwater monitoring. In this episode, Brian K. Branstetter (Naval Facilities Engineering Systems Command Pacific) shares his work to find out how this sonar affects killer whales.

Associated paper: Brian K. Branstetter, Michael Felice, Todd Robeck, Marla M. Holt, and E. Elizabeth Henderson. "Masking in continuous sonar noise." J. Acoust. Soc. Am. 156 (2024). https://doi.org/10.1121/10.0025855.

Read more from The Journal of the Acoustical Society of America (JASA).
Learn more about Acoustical Society of America Publications.

Music Credit: Min 2019 by minwbu from Pixabay. 

Transcript

Kat Setzer  00:06

Welcome to across acoustics, the official podcast of the Acoustical Society of America's publications office. On this podcast, we will highlight research from our four publications. I'm your host, Kat Setzer, Editorial Associate for the ASA. 

Kat Setzer  00:24

Underwater sonar systems can have a big impact on marine mammals. Today, I'm talking with Brian Branstetter about some of his research related to this issue, which recently appeared in the article, "Auditory masking of tonal and conspecific signals by continuous active sonar, amplitude modulated noise, and Gaussian noise in killer whales (Orcinus orca)," which has recently published in JASA. Thanks for taking the time to speak with me today, Brian. How are you?

Brian Branstetter  00:47

I'm doing fine. How about yourself?

Kat Setzer  00:48

Pretty good. So first, just tell us a bit about your research background.

Brian Branstetter  00:51

All right, I'm a marine biologist. I kind of specialize in bioacoustics. For the last 25 years, I've studied mostly bottlenose dolphins and killer whales. And the topics that I'm interested in are biosonar, basic hearing, the effects of noise on auditory systems, and basically noise in general.

Kat Setzer  01:13

So this research has to do with the impact of sonar on the communication of killer whales. Can you give us some background on killer whales' communication? 

Brian Branstetter  01:21

Yeah, sure. Killer whales are a pretty unique species. They have a cosmopolitan distribution-- they're found throughout the world's oceans. They're an apex predator. They live in these lifelong matrilineal groups, which means both the daughters and the sons will stay with their mothers for the duration of their lives. Since they live in the ocean, vision is limited, especially at nighttime, you can't really see very far, so they use sound for just about everything, including navigation, finding prey, and also coordinating group behavior through communication. They also have these long-term stereotyped calls that can inform a listener of their group identity and how related different groups are. So it's pretty unique. Also the southern resident population, which lives in the Pacific Northwest, these are an endangered population, so they're protected by the Endangered Species Act and the Marine Mammal Protection Act. Right now there's only about 74 individuals. Their population peaked around 1996 with 97 individuals, and there's been a slow decline. So there's a lot of interest in these animals, and especially how noise impacts these animals.

Kat Setzer  02:24

You're literally trying to save the whales, in other words, is what you're saying. 

Brian Branstetter  02:27

Yeah. 

Kat Setzer  02:28

Okay, so how does anthropogenic noise generally impact killer whales, and what are the particular issues that have come up with sonar systems previously?

Brian Branstetter  02:37

So when we talk about anthropogenic noise in the ocean, we usually talk about what are called, "zones of influence." If you can imagine a really loud noise source, and you're far enough away from it, you're not even going to hear it, because it's going to be below the ambient noise. As you move towards that sound source, the first thing that's going to happen is you're going to detect it, and as you continue to move closer to the noise source, it's going to start getting louder, and you're going to encounter auditory masking, which is when one sound interferes with your ability to detect or recognize another sound. If you continue to move closer to that loud sound source, you're going to get things that are called temporary hearing loss. This is what happens when you go to a concert. You lose your hearing for a little bit, then you wake up in the morning and you're fine. And if the sound source is loud enough, the closer you get to it, you might get permanent hearing loss, injury or even death. And in marine mammals, the death is usually caused by a mass stranding. So those are kind of the impacts of anthropogenic noise on marine mammals in general. In our paper, we're just looking at auditory masking. 

Kat Setzer  03:32

Okay, got it. Can you explain what pulsed active sonar is and why it was thought to cause mass strandings? 

Brian Branstetter  03:39

Yeah, so pulsed active sonar describes a wide range of sonar systems that use a short-duration signal, and there's a long, silent gap in between the signals where they're listening for echoes. These tend to be pretty high in amplitude, and they've been implicated for mass strandings of different types of cetaceans, mostly deep-diving cetaceans such as beaked whales. We don't exactly know why they're stranding. We believe that... so these are deep diving animals. They dive down to like 2000 meters, and they can stay under for a couple hours. And the going hypothesis is that pulse active sonar at high amplitudes may frighten these animals, and they swim to the surface rapidly and get decompression sickness, and that's probably why they're beaching, but we don't know for sure. It's hard to test that hypothesis, because once you perform a necropsy on these animals, and if you do find bubbles in their joints or in their tissues, you don't know if it's caused from actual decompression sickness or, you know, they're decaying. So it's it's a little bit difficult to detect that hypothesis.

Kat Setzer  04:40

Okay, so here you focus on continuous active sonar, which is an alternative to pulsed active sonar. How does it work? And how is it different from pulsed active sonar? Why has it been thought to be less problematic?

Brian Branstetter  04:52

So continuous active sonar is lower in amplitude, that's the main feature. It tends to be longer in duration. It's pretty much continuous, like the term says. The continuous active sonar that we use in our experiment is 19 seconds in duration. There's a one second silent interval, which isn't really silent, because there's lots of reverberation that takes place in that. It's a hyperbolic upsweep, starts at 1000 hertz and ramps up to 2000 hertz. The reason why folks use continuous active sonar, as opposed to a pulse active sonar, you're able to track a moving target with much more accuracy because it's continuous, so you're constantly monitoring their echoes. Where in pulse active sonar, you send out a pulse, and you have to wait to get an echo back, and then you send out another pulse, so there's a long time delay between when you can analyze the echoes. So it's basically designed to constantly track moving targets. We think that this may help mitigate some of the more severe effects with pulse active sonar, particularly temporary hearing loss and permanent hearing loss, because it's lower in amplitude, but since it's continuous, it has a higher potential for auditory masking.

Kat Setzer  05:57

Okay, okay, got it. How have the effects of auditory masking on marine mammal communication typically been studied in the laboratory?

Brian Branstetter  06:04

So what we do is we train bottlenose dolphins or killer whales. (At least me. There's other folks that do sea lions and seals.) We train them to take a basic hearing test, and the hearing test is pretty much just the way a human would. You go into a controlled environment that's quiet. If you hear a tone, you'll signal that you heard the tone by either pushing a button or pointing to your ear or raising your hand or saying yes. We do the same thing with killer whales and bottlenose dolphins. What we do is we measure what's called a detection threshold. It's the quietest sound that they can detect 50% of the time. For killer whales in this experiment, the signal that we use was actually a recorded call, recorded in the Puget Sound. It's from J Pod, and it's called an S1 call. So it's an actual call of killer whales, and we use that for the signal and for the noise. We used a continuous active sonar and white noise. 

Kat Setzer  06:55

Okay, got it. So what was the goal of your study? 

Brian Branstetter  06:59

The primary goal was pretty simple. We just wanted to determine whether or not continuous active sonar would be an effective mask or for killer whales, and we did this by comparing detection thresholds for continuous active sonar against detection thresholds in white noise.

Kat Setzer  07:12

So you kind of touched on this before about the hearing test. You mentioned in the article that you used two adult male orcas with "extensive experience participating in hearing tests." So how do you test the hearing of killer whales?

Brian Branstetter  07:25

Again, pretty much like humans, takes a little bit of time to train them up, but you play a sound, and if they hear it, they respond to it. These two killer whales have participated in several other hearing tests before. We tested basic audiograms I think in eight different killer whales that we trained up. These two killer whales also participated in another auditory masking study, where they just detected pure tones in the presence of white noise. It's kind of your standard auditory masking study. Here we're trying to make the situation a little bit more realistic by using noise and signals that they would actually encounter in the wild. So again, it's continuous active sonar for the noise and whale calls for the signal. 

Kat Setzer  08:03

So are they like wearing headphones or anything?

Brian Branstetter  08:06

No, no. What we do is we, they're trained to station on a what's called a stationing device. It keeps them at a fixed distance to the underwater speaker, so we know exactly how loud the signals are, and we do calibrations before and after each one of these hearing tests. So we have you know, precise measurements of how loud these signals and noise are.

Kat Setzer  08:25

Okay, and then what do they do to indicate that they've heard something?

Brian Branstetter  08:30

So these whales produce what's called a raspberry sound. It's, kind of sounds like a pppppbbbbbbbtthhhh.

Kat Setzer  08:38

Oh, okay, it's like human raspberries. 

Brian Branstetter  08:40

Yeah, exactly. So they're trained to produce a raspberry sound anytime they hear a tone, so it's an acoustic indicator, and it also produces bubbles, so we can actually see a visual signal that the sound took place. And that's kind of important in these masking studies, because sometimes there's a lot of noise in the background that we're putting into their pool, but we can actually see a visual indicator as well as the acoustic indicator that they responded to it. And if they don't hear anything, they're also trained to just remain silent.

Kat Setzer  09:03

Okay, got it, got it. How did the orcas end up performing during the hearing tests?

Brian Branstetter  09:09

So what we found, the main takeaway from the experiment, was that continuous active sonar is a pretty significant masker, and it was more difficult for them to detect their calls in continuous active sonar than it was in white noise, which was a little bit surprising. We think there's a couple of reasons for that continuous active sonar. Even though the signal that we use was an upsweep from 1000 hertz to 2000 hertz, it does have a significant amount of higher frequencies due to distortion products, and we think this is also masking some of the higher frequencies of the call as well, and also, we think there might be some informational masking taking place. What that means is, we think the animal could actually hear their call, but was mistaking it for the continuous active sonar, because there's a lot of features that are similar. They both have sort of like a tonal quality to them. So we think the animal was hearing it but confusing it with the continuous active sonar, and that was one of the features that led to higher thresholds.

Kat Setzer  10:07

Okay, okay. Can anything be done to lessen the impact of masking from continuous active sonar?

Brian Branstetter  10:14

Potentially, there's a couple things that you could do. One is to maybe insert silent intervals between bouts of continuous active sonar. This might create a window so the animals can communicate. I don't know if that would be, you know, conducive to whatever world navies are trying to do, but that's one way you could help the animals communicate. Just produce some silent intervals. Another thing you can do is understand where the animals are and when they're going to be there. So if you're doing some type of training, you can time your training when the animals aren't going to be there. And there are some programs out there that are doing this. One of them is called the Marine Species Monitoring Program, where they're actually going out in the ocean, and they're doing research to try to find out where these animals are and when they're there so they can kind of train around the animals. And of course, more research. The more we know about sonar design and animal hearing, we can better help protect these animals.

Kat Setzer  11:02

Amazing. So actually, that segues really well. What are the next steps for this research?

Brian Branstetter  11:07

What I would like to do is test their directional hearing. So most marine mammals have very directional hearing. What that means is they can hear sounds directly in front of them very well, while sounds to the periphery or behind them are much quieter, if you can imagine multiple sound sources out there and masking taking place right now, all the models assume that sounds are going to be equally loud coming from any direction, which isn't the case. If an animal is pointing at a sound, it's going to be much louder than if the animal is not pointing at the sound. So it'd be interesting to look at directional hearing and how that affects masking. Also understanding killer whale mitigation behaviors or coping mechanisms. They already do things right now to cope with noise. If there's a noisy environment, they tend to produce louder calls to mitigate the noise, they might have preferential calls, which might be easier to detect, and that really hasn't been studied. 

Kat Setzer  11:59

Okay, okay. Do you have any closing thoughts?

Brian Branstetter  12:03

Yeah, I guess so. So right now, there's only about 74 Southern Resident killer whales left, and anthropogenic noise has been identified as one of the major impacts of these animals. And the more we know about how noise affects these animals, the better we can protect them.

Kat Setzer  12:19

That makes a lot of sense. I hope that you can learn more to help protect them more. It seems like there are so many ways that sonar can impact marine life, even it's not as extreme as mass strandings, like with the pulsed active sonar. 

Brian Branstetter  12:31

Yeah. 

Kat Setzer  12:32

It'll be really interesting to see what techniques can be used to lessen the impact as we begin to understand how animals like orcas perceive these sounds. Thank you again for taking the time to speak with me today, and good luck with this research. 

Brian Branstetter  12:43

Thankyou. Thanks for having me.

Kat Setzer  12:46

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