Short Wave - Sea Camp: The Largest Daily Migration On Earth

Episode Date: July 28, 2025

The twilight zone of the ocean is a mysterious place. At 200-1000 meters below the surface, it's a tough place to study. That's why, during World War II, people reading sonograms from this zone were perplexed when it looked as if the ocean floor was moving up. Every day. And then back down again before dawn. In this latest installment of Sea Camp, we explore what this historical mystery has to do with the Earth's ability to cycle and store carbon in the ocean's watery depths.SIGN UP FOR OUR SEA CAMP NEWSLETTER! WE WORKED SO HARD ON IT!Interested in more ocean mysteries? Let us know at [e-mail] to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shorewave from NPR. Hey, Shorewaver is Regina Barber here. And Emily Kwong. Back with our next installment of the Sea Camp series. Yes. Where are we going this week? We're diving into the twilight zone of the ocean. That's 200 to 1,000 meters below the surface,

are about 650 to more than 3,000 feet. And it's where the sunlight is still present, but it's very dim. The twilight zone. I love this name. It makes me think of one of my favorite TV shows. Is it mysterious like the show is? Do do do do do do do do. It is.

It is. Noel Bolin, the marine biologist of the National Oceanic and Atmospheric Administration, who we met last week, says we don't know a lot about the Twilight Zone because of its depth, which is also known as the mesopalgic zone. It is hard for us to access it. As you can imagine, there's great amounts of pressure. There's not a lot of oxygen. And it's deep. So humans aren't exploring down there very often? No.

However, during World War II, militaries were using sonar to look for enemy submarines in the ocean, in the twilight zone, hundreds of meters below the surface. And some of the people looking noticed something kind of odd. The signal coming back was so, it was as if they were bouncing the sound off the bottom of the ocean. But that didn't make any sense because the bottom of the ocean should have been farther away. Mm-hmm. And even weirder.

it seemed like the bottom was moving up. What? Yeah. Scientists would eventually learn that they were seeing the largest synchronous migration on Earth. When a massive amount of Twilight Zone living organisms come up and travel to the shallows to feed under the cover of night's darkness. That is really, really fascinating. Yeah, it happens every day at Twilight in the Twilight zone. But the movement of these organisms is just.

just a huge distance. Noelle said, think of it this way. So that's like saying, oh, Emily, you know, I'm really hungry. We're in Santa Year right now. What are we going to do for dinner? I know. The sun is going down. Let's start walking to Los Angeles for dinner and we need to walk back before the sun comes up. It's that kind of distance. Wow. And this mass movement is called deal vertical migration. And it includes mollisks, shrimps, jellyfish, and Noel's personal favorite. It's called a flashlight fish because one of the places it has some light organs are right in front of its eyes facing forward like they've got headlights. It is critters like these that perform an essential service to not only their ecosystems, but to the entire Earth. Today on the show, how the largest synchronous migration of animals on Earth pulls carbon out of the atmosphere and stores it in the ocean.

I'm Regina Barber and I'm Emily Kwong and you're listening to Shortwave Sea Camp. the Ocean Science series from NPR. So I'm going to take over the episode from here and have with me, Noel Boland from Noah, to talk about the twilight zone of the ocean. Not the television show, though we appreciate its legacy. This ocean zone is very aptly named because it is still very mysterious. And it sounds like a lot of fish live in this zone. How much of the ocean's fish live in the twilight zone?

I love that you use the word mysterious to describe this section of the ocean, because it is mysterious. You know, so like, there's so much ocean. And so for us to be able to take samples of it, observe it, we can't get all of it. And in time, there's no way. So we have to make inferences based on small sample sets. And so what we do know is that two families make up the most abundant in terms of biomass and number of fishes in the world.

And that's where they live. Wow. And so in the Twilight Zone. In the Twilight Zone. And I know critters in the Twilight Zone, they have all kinds of adaptations. Some of them, for instance, have big eyes to try to capitalize on the lack of light. How else have critters adapted? Like, what do they even look like down there?

Yeah, so they're not muscular. They're rather squishy. And so they're not using muscles to deal with this vast distance that they have to travel, which is actually really smart because if you use a lot of muscle to move that kind of distance, you would have to consume so much food to maintain that muscle. Yeah. You were saying earlier, too, that for us, the distance, these organisms travel every day would be like us walking from San Diego to L.A. and back, which I would get so hungry.

Exactly. I mean, the amount of snacks you'd have to carry. It would just be really inconvenient. Yeah. And so what they have inside of them are swim bladders. And they inflate their swim bladder when they need to start moving up. And when it's time to move back down, they deflate that bladder. Oh, my gosh. That's so smart.

I know. So it's not only the fish that are going up and down, and that's happening every day. And not just fish, obviously, lots of organisms. But it's also carbon. Can you explain how the carbon cycle works in the ocean and what role these organisms are playing in it? Sure. So we all contain carbon. We're carbon life forms. We are carbon life forms.

just like these fishes and the other organisms that are moving up and down. Yeah. Well, as they're making their living, just like you and me, as they move up to the surface to eat other organisms that are also carbon lifeworms, they then take that carbon with them,

whatever they've consumed, they take it back down to depth and into the mesopalagic. Then they expel waste that isn't going back up to the surface. It's falling down. Oh, as poo. As poo. And snow and snow, as we call it. Right.

Or they, and or they've eaten at the surface and eventually they die. Maybe they die at depth. And that brings the carbon back down. Yeah. That's right. And so this is a very simplistic way, of course, without going through actual equations. I mean, what's incredible about this carbon is it's coming from the atmosphere of CO2, right? And then it's getting absorbed by phytoplankton, which are eaten by zooplankton, which are eaten by larger predators.

the carbon just keeps sinking down. But silly question, how does the carbon that's then in the bottom of the ocean come back up? And do we even want it to? So some of the carbon that will come back up will come back up in the form of animals that are consuming detritus or consuming other organisms that have fallen, you know, maybe a falling body or something. They'll eat that carbon. And then one way is that if it's a fish and it's going to spawn and release. eggs, those eggs float right back up to the surface. Over time, it takes time for that to happen. Mining is another way that a lot of people are talking about right now. When we are scooping things up

and bringing them up to the surface, forcing them up to the surface, we are releasing the carbon that has been sequestered or fallen and staying at the bottom. Yeah. You know, I think when it comes to mining and logging and all these practices that serve our human needs, we forget that what we're ultimately moving around a lot of the time is carbon, and we're playing a role in the carbon cycle in that way. That's right. Is there currently fishing and mining that touches the Twilight Zone? Yes.

Fishing in the Twilight Zone or Obisoplogic is not the most efficient use of resources like fuel, ship time, because it's so expansive. Yeah. But when you are headed to the bottom of the ocean to maybe mine for minerals, You drop your equipment from the ship through the mesoplagic or twilight zone. And then you scoop things up and you bring them up. Yeah, through the zone. Sediments through the zone.

You're disturbing that habitat because naturally things are falling. You're filtering stuff. And then you're discharging the stuff that you don't want. And it's clouds and plumes that are going to move through the sunlight zone and the twilight zone. So you're upsetting those environments. How is climate change impacting the twilight zone? One example is that we are getting the expansion of low oxygen concentrations that exist in pockets in the twilight zone are expanding. And only specialized organisms in these zones can inhabit them with success.

And it's not the majority of them. As the water warms, there's less oxygen available. And so you get lower oxygen concentrations, and that makes a hostile living environment for a lot of organisms. And these low oxygen areas in the Twilight Zone are expanding. And we call it shoaling when they move towards the surface. Wow. And that's a bad thing for the organisms that cannot inhabit those regions. It's almost like a kind of habitat loss. Like I'm picturing almost as if desert conditions.

are spreading throughout the ocean, except deserts are great. It's just the concept that regions where there isn't what life needs, in this case oxygen, are just growing. And so things are getting pushed to the margins because of it. That's right. Yeah. That's right. Wow, that's really hard. Well, given how important the Twilight Zone is to the health of the planet, the health of humans, what research do you think is essential right now for people to be doing on the Twilight Zone? I think continuing research understanding distribution and abundance of the different organisms and how they work together and how they interact. This is research that's happening and it's ongoing.

Something that I think is really important in terms of research is finding out the best ways to communicate the findings of these results and how they impact humans so that more people, know about it and more people can care and then have the chance to make changes. How is that hope of yours is complicated in this reality with so many funding cuts to NOAA, affecting research and also public engagement? I mean, you said it. Yeah. That's it.

If we can't get our science out, if we can't do our science, we can't get the word out, then I think what ends up happening is we all start moving in the dark. I'm scared that we're going to lose a lot of information that's in the works. And then we just don't know what the right and left hand are doing. I think some people view scientific research on something like a little mesoplagic fish is just, you know, an inflated privilege that you just get to look at these little fishes that don't matter for anything. But they do. They matter for our ability to understand.

how our habitats are linked. And I believe there's room for all of us. We just have to change the way we do some things, some big changes. But, golly, it's the thing I say to my kid every morning when I drop them off for school. You can do anything. Noel Bolin is a fisheries biologist at Noah. Thank you for coming on the show and giving so much of your time to help all of us understand the Twilight Zone and just the ocean for the Sea Camp series. It's been really great to talk to you.

Thank you. I really enjoyed the assembly. This has been an absolute privilege and joy. Thank you. Short waivers, you do not want to miss out on the Seacamp newsletter, because if you sign up, maybe we'll do it again. This newsletter dives deep into each ocean zone. It's got comics. It's got games. It's got ocean research. What else could you want? And if you want to support our show, share it with a friend. Follow us on your podcast platform and know we are grateful for your support. This episode was produced by Burley McCoy. It was edited by our showrunner, Rebecca Ramirez, and fact-checked by Tyler Jones. The audio engineer was co- Takasugi Chernoan. Beth Donovan is our senior director, and Colin Campbell is our senior vice president of podcasting strategy.

I'm Emily Kwong. Thank you for listening to Shortwave Sea Camp from NPR.