Late Night Mystery Calls in the Mariana Archipelago

While analyzing acoustic data from the Mariana Archipelago, Angela Szesciorka (Oregon State University) noticed something funny: a signal unlike any other she'd seen. It showed up over and over, and only seemed to occur at night. In this episode, we talk to Szesciorka about this mystery call and what animal possibly made it.

Associated paper: Angela R. Szesciorka, Jennifer L. K. McCullough, and Erin M. Oleson. "An unknown nocturnal call type in the Mariana Archipelago." JASA Express Letters  3, 011201 (2023). https://doi.org/10.1121/10.0017068

Read more from JASA Express Letters. 

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 Music: 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  00:25

To start this episode, we're going to play a sound clip for you.

Kat Setzer  00:44

That clip comes from an acoustic recorder deployed in the Mariana Archipelago in the Pacific Ocean. Interestingly, though, researchers weren't expecting the sound and didn't know what produced it. Today we're going to talk to Angela Szeciorka, who wrote about this mystery call in the article, "An unknown nocturnal call type in the Mariana Archipelago," which appeared in the January 2023 issue of JASA Express Letters and was featured as an Editor's Pick. Angela, thanks for taking the time to speak with me today. I'm excited to find out more about this mystery call. So just first, tell us a bit about your research background.

Angela Szesciorka  01:16

Sure, yeah. I am currently a research associate in the Marine Mammal Institute at Oregon State University where I study bowhead whale migration and the overlap with vessel traffic in the Pacific Arctic. And I use tags that you would attach onto the back of whales or underwater microphones, which we call hydrophones, to study large whale movement and behavior spanning the tropics to the poles. I have studied humpback whales for my master's research at Moss Landing marine laboratories and blue whales for my PhD at Scripps Institution of Oceanography, but I've also been lucky to have worked with grey, fin, Bryde's, even beaked and sperm whales along the way.

Kat Setzer  01:54

Awesome. So you know all sorts of whales. So what is the Mariana Archipelago, and why were you interested in studying the acoustics in this region? 

Angela Szesciorka  02:01

Yeah, the Mariana Archipelago is a chain of 15 islands. It includes Guam and it stretches in a north-south arc in the northwest Pacific Ocean. It's pretty much equidistant in its location. It's south of Japan, North of Papua New Guinea and east of the Philippines. And probably what most people would recognize is that this area includes the Mariana Trench, which of course James Cameron dove to in 2012. This area also includes two US jurisdictions, a large marine protected area, which is a national monument, and a US Navy testing and training center. And up until 2007, we knew very little about the whales in the Mariana Archipelago except from stranding records, anecdotal reports, and historic whaling records. And the Pacific Island Fisheries Science Center, which is NOAA's Science Center in Hawaii, partnered with the Navy and conducted the first surveys off Guam in 2010. And in 2015, 2018, and 2021, they conducted large vessel visual and acoustic surveys, which they call MACS for Mariana Archipelago Cetacean Surveys. And they did this throughout the entire Mariana Archipelago.

Kat Setzer  03:09

Okay. Okay. So, as you're saying, the Pacific Islands Fisheries Science Center, were doing this monitoring, and they used both visual and acoustic monitoring of the area. What did that actually entail?

Angela Szesciorka  03:21

What that entails is conducting what we call standardized line transect surveys across predetermined tracks to identify and count every whale and every dolphin species that's sighted, and the surveys take about a month to complete. In 2018 and 2021, they also deployed Drifting Acoustic Spar Buoy Recorders, or DASBR for short. And these DASBRs sit straight up in the water column, and they move with the currents, recording all the sounds in the ocean. There's also a lot of smaller components to the surveys. They've had small boats on some cruises that tag and biopsy whales, and they've had three long term acoustic moorings that sit at the bottom of the ocean floor and record sound for an entire year at a time. So a lot of moving pieces, but they're able to collect all this visual and acoustic data.

Kat Setzer  04:09

That's really, really interesting. So what's the benefit of using passive acoustic monitoring in this region?

Angela Szesciorka  04:14

Well, first of all, this is a pretty middle-of-nowhere place. So it's really hard to get there to survey every two years or so. So passive acoustic monitoring is great. The moored hydrophones can sit out for an entire year or so at a time just passively recording. It can record animals that stay underwater for hours at a time. So they're only surfacing for brief moments and they're really hard to miss during visual surveys. And also the vessels are only covering one track line at a time and the area for the full survey is so large. And finally, the drifting buoys that stay out for the duration of the cruise can give you a better sense of who can be found throughout the entire area during that time. And then again, the moored hydrophones are sitting on the ocean floor, so they give you even more information for who is there and when, for even over a longer period of time.

Kat Setzer  05:01

Okay, that's awesome. So you get so much data from them, it sounds like. Speaking of data, how did you collect data for this study?

Angela Szesciorka  05:08

I was actually brought in as a contractor with the Pacific Island Fisheries Science Center in 2021 to help test and compare automatic detectors to try to identify beaked and sperm whale echolocation clicks from the data they collected on these cruises. And while I was doing that, the team actually asked me to look through the low-frequency acoustic data from the 30 DASBRs, those drifting buoys that they had deployed in 2018 and 2021, to see if I could find another mysterious sound that was first recorded in the Mariana Trench in 2014. And this was called the "Western Pacific Bio Twang." It's a super funky call; it lasts only three seconds, and it's got five different parts, starting at really low frequencies starting at 30 hertz, and then it ends with this metallic sounding part that goes all the way up to 8000 Hertz. 

Kat Setzer  05:57

Oh, wow. So as the title of the article mentions, there was one mystery call amidst all your data, how did you find this mystery call and what's so unique about it compared to the other acoustic data from the region?

Angela Szesciorka  06:09

Well, luckily, it was one mystery call amongst the data, but it didn't happen just one time, that would have been really easy to write off as some sort of aberration or physical process or machine noise. I ended up finding almost 18,000 of these half-second, low-frequency 500 hertz mystery calls. 

Kat Setzer  06:25

Oh, my goodness.

Angela Szesciorka  06:26

 Yeah. So we're, we look at the acoustic data as spectrograms. This is just a visual representation of sound; it's a lot faster than listening to the sound, and it's kind of like looking at music notes. And so I was already going through the acoustic data looking for these other Western Pacific Bio Twangs. And as I'm going through, you know, I'm really thorough, I never want to miss one sound when I'm going through acoustic data. So I'm staring really hard at these data. And I'm seeing a ton of the Western Pacific Bio Twang, which was great. But then I started to see this other signal. And probably for the first ten or so I, it didn't click in my head. So I probably ignored it. But then I started to see it again and again. And I said, "Wait a minute, this is something repeating over time, and it's clearly biological." And so I started documenting each one I saw. And in some cases, they were clearly two to even four distinct individuals. All the calls for one individual were very similar in time and space. And then all the other calls were just, you know, ever so slightly at a different frequency or timing. So it was really clear that these were two or more individuals. And it wasn't until after all the calls were documented that I saw most of these calls were happening at night, so that was super interesting, too.

Kat Setzer  07:35

Okay, so you're hearing some nighttime conversation. So there were quite a few creatures that the call could belong to. How did you narrow down what you think made this vocalization?

Angela Szesciorka  07:45

It's really tricky to ultimately say what this animal is. We figured it could be fish, sea turtle, toothed whale, or baleen whale. So those were the four main likely culprits based on the frequency range at which the animals could call it. So we did step through each one as logically as possible, because it's so low frequency, and there were no other high frequency components, we felt comfortable saying that it was not a toothed whale, so not sperm or beaked whales or anything like that. We looked at all the published papers on sea turtle vocalizations underwater (of which there is only one paper, and it's about green sea turtles), and it didn't match any of the 11 sounds that they recorded from the turtles. So obviously, that doesn't 100% rule out turtles, but it wasn't matching what the literature said. What led us also to believe that it was not fish and then also not turtles, was that the call spanned many habitats and many geomorphological regions. They were found throughout the entire Mariana Archipelago, including near the trenches and sea mounts, so it had to be an animal that could move over great distances. And fish that usually sing are associated with near-shore kelp or reef habitats. And fish also tend to chorus and we never saw chorusing in the data either. When fish chorus, you can't even see individual calls. And like I said, I was able to identify 18,000 calls, and they never overlapped. 

Kat Setzer  09:06

Okay. 

Angela Szesciorka  09:06

We also sent it around to our colleagues that study fish in the region. So we felt like based on all that, it seemed like it had to be a baleen whale. So that got us to whale. And then you we knew the time of year, this was May through July. We know the call repertoires for many other species. So we were able to rule out humpback whales, Sei whales, fin whales. and blue whales. Now they could be minke whales, but they are really rare in the region, and the calls are not like any other minke whale call. And then finally, a really interesting fact was that the calls sometimes co-occurred with that Western Pacific biotwang that I was also documenting. And there is growing evidence that the Western Pacific biotwang is actually made by Bryde's whales, and they are present year round in the Mariana archipelago. And there were also more visual sightings of Bryde's whales in 2018, which coincided with an increase in this number of mystery calls that I found. So at the end of the day, knowing that we could be proved wrong one day, we hypothesize that the calls belong to Bryd'es whales. And I really do hope that we find out one day whether or not that's true. 

Kat Setzer  10:11

Okay, that's cool. So what's the significance of finding a Bryde's whale call or  baleen whale call unlike the other vocalizations found up to this point?

Angela Szesciorka  10:19

Well, what it reminds me is that we still have so much to learn about the ocean and the animals that live there. We can only speak to things we have only seen and heard, right? So we forget that just because we haven't seen it, doesn't mean it's not there, doesn't mean it's not real. For example, we haven't seen many whales mate or give birth, but obviously, it's happening. So there's just so much out there we have yet to document, right? And then the other thing that this really reinforces for me is that there's no substitute, unfortunately, for slogging through data manually. We're, you know, collecting so much acoustic data. These days, the storage rooms at Scripps Institution of Oceanography are filled with hard drives, and the new approach in big data analysis is machine learning and artificial intelligence. And I 100% agree that is the way to go for some things. But if you don't dig into your data with your own eyes and ears, you never know what you might find. 

Kat Setzer  11:09

Right, right. And sometimes machine learning, just like you said, can't catch things that you would notice with your eyes. So what are the next steps for the research? Or perhaps more importantly, how do you confirm what animal actually made the sound?

Angela Szesciorka  11:21

So one way we could identify the species would be to get visual and acoustic evidence at the exact same time knowing that other species are not present. This can be done with visual surveys paired with either a towed array or sonobuoys, and they rely on real-time acoustic data, and they send it back to the vessel. And another even better way, perhaps, would be to attach an acoustic tag onto a whale. That way you could record any sound that that specific animal made. We've also been encouraging our colleagues in the bioacoustics world to keep an eye out in their datasets for this signal. And we did have one colleague reach out with a very similar recording but at higher frequencies, in 2009, in the Mozambique Channel, so that's west of Madagascar, and off Reunion Island, which is east of Madagascar, and Bryde's whales have been cited in that region. So you never know. It could be the same signal. 

Kat Setzer  12:12

That's so fun! Yeah, it's really interesting, since you know, now that folks are able to share data more easily between researchers, it's interesting to see those kinds of connections showing up. 

Angela Szesciorka  12:24

Right. It's like a scavenger hunt. 

Kat Setzer  12:26

Yeah, right? Right.  It's also neat that our abilities to monitor underwater soundscape has advanced so much right and allows us to find out more about species that we haven't been able to know about as much before. This sounds like a very intriguing discovery for you. And thanks for taking the time to speak with me about your research.

Angela Szesciorka  12:45

Thank you so much for having me. I really appreciate it.

Kat Setzer  12:47

You're welcome. Thank you for tuning into Across Acoustics. If you'd like to hear more interviews from our authors about their research, please Subscribe and find us on your preferred podcast platform.