Do tsunamis affect marine life?

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1:06Millions of people across the Pacific have been told to leave their homes after one of the most powerful earthquakes this century in eastern Russia triggered tsunami warnings. Earlier this year, in July 2025, countries around the world braced themselves for impending disaster. Tsunami waves crashed into buildings. Some were even swept away by the sheer power of the waves. In parts of Russia, waves as big as five or six metres collided with the coastline. But things could have been a lot worse. In the end, despite causing some damage, the tsunami was less severe than expected. And thanks to a successful early warning system, there were hardly any casualties.

1:48Or at least that was the situation on land. But crowd science listener Alvin from the UK has been wondering about the wider picture. I'm very aware that when we have reports about tsunamis that we hear about the devastation effect on land, what I'm completely ignorant about is what's happening below the surface of the water. What is the effect, for example, of a tsunami on fish? What's the effect on the seabed and the plant life there? essentially what's going on. I'm Anand Jagatia, and thanks to listener Alvin, this week on CrowdScience, we're exploring the impact of tsunamis on marine life. It's definitely not something we tend to hear much about in the aftermath of a tsunami.

2:34But before we start thinking about the plants and animals that live in the path of the wave, I want to see a tsunami in action, which is obviously kind of impractical. But there is a place which offers the next best thing, A facility in Indonesia where scientists can make their own tsunamis. Well, now we are in the tsunami wave room facility at my centre. I'm getting a virtual tour of the Tsunami and Disaster Mitigation Research Centre. It was built in the city of Banda Aceh on the northern tip of Sumatra after it was devastated by one of the deadliest tsunamis in history. So this facility is one of the most advanced tsunami testing in Indonesia currently.

3:19This is Professor Sham Sadiq from the Civil Engineering Department at Unishakala University. We've got you on Zoom. We've got a video to see the place. Would you be able to just describe a bit about this centre? Yes, sure. Just to give imagination of the size of the flume. So the length of the flume is 60 metres. it's made of glass but we need to support these walls with steel frame because we are going to contain a water that's equal to 1.2 tonnes of the hydrostatic pressure to the walls. It's a massive structure, 60 metres long and it's sort of a cross between a water park slide, a fish tank and a science experiment basically.

4:05Yeah, some people even say this is a big aquarium but what we have is full of sensors, velocity, wave gauge and even we have sensors to map the particle movement of the water. Okay so really high-tech stuff. How do you actually do that? So this pressure tank can suck in the water inside the flume so we can adjust the speed of the valve then it will create exactly the 2004 tsunami wave and also the 2011 tsunami wave. Right okay so because it's at pressure I guess when you release the valve the water kind of rushes out and then that sort of forces it down the flue. Right yes correct. And here's what the tsunami generator actually sounds like when it's fired up.

5:07That almighty racket is producing an incredibly powerful wave. It's absolutely ferocious, even though it's a massively scaled-down model of a real tsunami. To answer listener Alvin's question, we're going to follow what happens to tsunamis as they form, travel through the ocean and break on the shore, with catastrophic results. Along the way, we'll explore how they impact the marine life in their path. Let's start with what causes a tsunami and how that differs from normal waves, which are caused by the wind. The wind waves are the ones generated by the wind blowing across the surface of the ocean.

5:49So there is a certain depth to which you could go where you wouldn't feel these wind blades above you. At the research centre in Indonesia is Dr David McGovern. He specialises in modelling tsunami defences and leads the Coastal and Ocean Engineering Group at Southbank University. He told me how tsunamis are formed. For the most part they form from undersea earthquakes. So the entire water column has been disturbed by the rupture at the seabed. It's a bit like throwing a stone into a pond, but the stone is sort of thrown from below. There's vertical motion in the seabed and that disturbs the water above it.

6:28The deep ocean is four kilometres deep on average. So that's four kilometres worth of water which has been shifted up. It takes a lot of energy to move that much water. In fact, according to some estimates, the earthquake that triggered the 2004 tsunami released as much energy as 20 ,000 nuclear bombs. So let's see what happens when that sort of force is released into the deep ocean. There is a tremendous amount of energy which is released, but it is spread both in time and in space. This is Emil Okal, an Emeritus Professor of Geological Sciences at Northwestern University in the United States.

7:10And as he explains, much of that energy is diffused. These very large amounts of energy, even at the source, they are spread over considerable distances. In rough numbers, the Sumatra Fault, which ruptured in 2004, was about a thousand kilometres long. And also, it is spread over a considerable amount of times. It took about 10 minutes for this rupture to nucleate at the southern end and propagate north along these thousand kilometres. So with all of that energy spread out, what does that mean for tsunami waves? The displacement of the level of the ocean is going to be on the order of a few tens of centimetres, or perhaps in extreme circumstances it can reach one metre.

7:58So in a sense you have a swelling that you can hardly see, that you would not be able to see it with a naked eye as a wavefront. The sheer amount of water in the deep ocean means that even a very violent earthquake only produces a tiny wave on the surface. So if you were a fish swimming around above the epicentre? You're going to ride the wave and you're going to move 10 centimetres up with all your school of fish around you and nothing will happen. The temperature of the water will not change, its chemistry will not change, its salinity will not change, and the plankton that the fish may eat will be carried together with the fish, so they'll be pretty happy.

8:42This is pretty surprising. I was imagining a kind of underwater shockwave blasting outwards from the seafloor, churning up the animals and plants in its wake. And yet, far out in the ocean, all you get is a gentle bobbing up and down by a few centimetres. Although Emil did say that for life on the seabed, the story might be quite different. At the epicentre, you will most probably have a disruption of the seafloor. So you may start mixing the seawater with loose sediment, which will be, in a sense, scraped from the bottom of the ocean and mixed up with the water. And that probably is not going to go very well with the local fish.

9:24As the tsunami moves across the ocean, it goes from some small ripples on the surface to some of the biggest and most destructive waves on the planet. So how does that devastating transformation happen? Here's David McGovern again. The earthquake and that vertical motion of the seabed has moved the entire ocean above it by a few centimetres or whatever. That, even though it doesn't feel like a very big wave on the surface, that obviously is a huge amount of energy which has been stored in that column of water. And then once it gets to the shallower regions, that energy is still there and it must still exist.

10:02We can't sort of disappear the energy. So because it's in shallower water, it starts to slow down because it's interacting with the bed a bit more. There's more friction. There's less space for it to run through. And that same mass is being forced through a smaller area. Once you get to shallower waters, you're starting to compress the area in which the wave can travel. So it grows in height. So these sort of centimetre high waves in the ocean become tens of metres at the shoreline.

10:33As that wave gains momentum, some of the first structures it encounters are coral reefs. And our next guest is someone who has seen the damage that can cause firsthand as part of her research on reefs off the coast of Thailand. My name is Suchana Chavanish, but everyone called me Apple. And now I'm working at Faculty of Science Chulalongkong University. When the Indian Ocean tsunami struck in 2004, Apple was one of the first divers to get into the water and help with the recovery efforts. In that time, I was in Bangkok. But within about three days, I went down to the southern part of Thailand to access the impact from the tsunami.

11:14Could you describe what you saw when you did your first dives? Yeah, my first dive. We saw all the televisions, mattress, all the furnitures. They're supposed to be on the land. Now they end up onto the coral reef and made the coral die. As Apple swam further out, she saw some of the more established corals had taken a battering from the wave. The unique thing that we found is that the brain coral, Massive coral tend to be more susceptible to the tsunami. So what happened is that we saw many of them were overturned. But when we look at the branching coral, it seemed to be fine. So these different shapes of coral, so some that were sort of bulbous shaped like brains, some that were kind of flat on the top like tables, look a bit like big mushrooms, they were badly affected by the tsunami.

12:10but the branching ones or the ones that look like stags, antlers, they seem to be okay. Right. So the force that was created by the tsunami would just sort of ripple through the branches like a tree so it could bend and it wouldn't snap. Right, correct. You are right. So given that some of the corals were damaged by the tsunami, I guess that's also going to have a knock-on effect on the fish and other invertebrates that call those corals home, right? Right. Actually, we also did a survey looking at marine organisms that live or use the coral as their habitat. And we found that the animal that got effect most, probably the very small ones, like a crab or chimps, they use the coral as their home.

13:03So if the color didn't survive, then definitely they have no home for those small creatures like a crab or shrimp. But what about all the other creatures that live and find food or mates in the reef? Well, Apple says that larger animals seem to have been spooked by the tsunami. I remember that it's very quiet. Not many animals, fish were there. But later we figured out that, you know, because those marine animals, they escaped to other places. Two weeks later, I went back there again and I started to see big fish and other big vertebrates, they come back. But many of the smaller fish weren't so lucky.

13:48We could see they end up actually on the beach because, you know, they tried to escape during the tsunami. I think because they're too small, they could not navigate themselves out to the ocean. and that's why they end up dying on the beach. So far, we've followed a tsunami on its journey from the deep ocean into the shallows, which culminates in a towering volume of water crashing onto land. And after it breaks, the reverse becomes true, as the wave drags everything on land into the sea. And we'll be exploring the long-lasting effects of that next. This is summer at its peak, Whole Foods Market Summer Fruit Fest is your invitation to eat the season.

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15:29You're listening to CrowdScience from the BBC World Service, and today we're answering Alvin's question about how tsunamis affect marine life. Perhaps counterintuitively, it seems the closer you are to the epicentre, the safer you are. Fish in the deep ocean may barely notice anything, while animals at the shoreline, like turtles that lay eggs on the beach, would have been physically obliterated by the waves. But what happens in the weeks, months and even years after impact? Our next story is from a fishing community in Japan that felt the force of a tsunami long after the seas had calmed. At least a thousand people are feared dead after the biggest earthquake ever recorded in Japan.

16:13The quake unleashed a ferocious tsunami. The sea is claiming the land. Cars, ships, buildings seized by a wall of water. which is sweeping away everything in its path. News of the disaster soon reached Hiroshi Sato, a diving instructor who grew up in northeast Japan and was living in Thailand.

16:40When I first heard about the tsunami on the radio, I didn't realise that it was in Japan. I thought that it was happening off Sumatra, and then I was told that it had hit my hometown. down. Hiroshi grew up on the San Riku coast, 300 picturesque kilometres of bays, cliffs and coves that's heavily reliant on the fishing industry. The tsunami tore fishing boats into pieces, smashed up docks and dragged nets out to sea. Many people lost their lives, others were missing, and the wreckage was everywhere in the water.

17:17I didn't think I'd be able to go back to Japan as soon as the disaster happened. And I was told by my wife and family not to return because of the trouble at the nuclear power plant, and it would be too dangerous. But in Thailand, he was living alongside survivors of the 2004 Indian Ocean tsunami. They pushed him to go home and use his diving skills to help the recovery effort, even giving him money to get back to Japan. When he eventually got into the water, he could barely see.

17:51The water visibility was poor, zero metres. We have dive computers, and even when we brought them close to our faces, we couldn't see much. That's how poor the visibility was. So Hiroshi, even though it might have been hard to tell at the time because of this poor visibility, what do we know now about how the tsunami impacted the fish in the area and what effect did that have on the fishermen who lived there?

18:23They had nothing to eat and also we didn't have much to eat either. So there was a species, it's a type of flatfish that you often see in the ports close to the shore. We thought, why not eat some of this fish that had been caught before the tsunami and see what it tastes like. So we did. But they stank of gasoline so much that we just couldn't eat them. And that just showed us how devastating the impact of the tsunami and the disaster was on species of fish. So for approximately three months, the visibility of the water remained at zero. We couldn't see any form of marine life and our biggest task at the time was to try and put ropes around anything that we could find and get it out of the water.

19:05Hiroshi was soon joined by others who wanted to help. My mother is Japanese and I was living in Tokyo at the time and found out about Sanliko volunteer divers. Bonnie Waycott is a keen diver, as well as being a journalist who specialises in the fishing industry. She met Hiroshi as they worked on the recovery effort. So I started to work with perhaps nine or ten, maybe more, other divers and a handful of fishermen. And I think one of the things I was most impressed by was the determination, the team effort, the closeness. Already I could see this relationship had been built between the divers and the fishermen.

19:46Two very different occupations had come together and they were working hard to do some good. Yeah, and I suppose I've never sort of seen firsthand a situation like this, which we do often see on the news. So it must have been quite something anyway to see this stuff in real life. It was. It felt really, I actually felt really small. One of the things that Hiroshi did when we weren't diving was to take us around to the areas on land that had been affected. And I saw with my own eyes the images that we'd seen on the news. So houses that were completely destroyed, cars turned upside down, all these mountains and mountains of rubble.

20:30And I remember it really hit me just how big the impact of this disaster really was, that this is the power of Mother Nature. And this is the earth destroying an area that's so vast, like in mere seconds. And that really hit home to me, I think, was just how big this disaster had been. And the same level of destruction could be seen in the ocean too. In the water, I saw cars, houses, boats, fishing gear, everyday items like handbags, shoes. I also saw the impact of landslides and so trees and branches and leaves and so on had all been washed into the ocean. So it was this image of chaos. Slowly, Bonnie, Hiroshi and the ever-expanding group of Sanriku volunteer divers cleared away the rubble and debris.

21:26And soon they began to think about how to get the fishing industry up and running again. The solution involved yet more diving, but this time to plant seeds underwater by hand.

21:41In order to revive aquaculture, we gathered seedlings and planted them from scratch as a way of trying to get the farming industry going again. When the tsunami came, the water had mixed with lots of different things and there was all sorts of nutrients. So for species like seaweed, that was actually a good thing because they grew well. And also baby sea urchins that eat the seaweed were washed away. So for the seaweeds, it was an ideal environment and we saw that they grew back quite quickly. While the seaweed was able to take advantage of the sediment that was stirred up by the tsunami, other plants, like seagrass, suffered.

22:17There's a species called eelgrass, which is like a grass that lives on land, and it has roots. And those roots have been completely pulled up and washed away by the tsunami. There was only about a fifth of seagrass left compared to before the disaster, and that was more difficult to revive. And so several species of fish disappeared quite quickly after the tsunami because they weren't so good at swimming and tended to shelter among that seagrass that was lost. So Hiroshi, I can see that in your background on the call is a photo of one of the bays near where you live, and it's a beautiful picture, clear blue water, white sand and a green forest.

23:00How does it compare to when you first went into the water in 2011 and couldn't see your computer in front of your face? It's a completely different world. And this morning I actually went diving in the area that you can see on the screen right behind me and it looks so different now. This area immediately after the earthquake was filled with rubble but we worked really hard to remove all that rubble. And now this is the result. And one of the things that I actually saw on the dive this morning was just how much the seagrass has changed in terms of the area that it's covering. So according to the latest figures, in 2012 there was about 680 square metres, but now in October 2025 the area has expanded out to around 15 ,000 square metres.

23:45So that's a 22-fold increase, and that's a hugely positive sign that things are really recovering. So thanks to Hiroshi, Bonnie and all the volunteers who helped pull the wreckage from the water, the San Riku coast now looks much clearer and the sea life there has made a strong recovery. But not all of the debris from land sinks to the bottom of the ocean after a tsunami. Some of it floats a lot further afield. Opposite the San Riku coast, nearly 7 ,500 kilometres across the Pacific Ocean, is the United States. And 15 months after the Japanese tsunami, a huge surprise washed up in Oregon State.

24:29A large concrete dock from the fishing town of Misawa, Japan, washed up onto Agate Beach, which is near Newport, Oregon. And this dock was about 70 tons in weight and approximately 9 feet tall. So it's a fairly large structure that washed up fully intact. This is Samuel Chan, a professor at Oregon State University and a specialist in invasive species. He told me that after the dock, wreckage from Japan just kept coming. One of the items that washed up was a fairly large wooden temple gate, and it was still red in colour. Then Samuel got wind of a rather unique marine migration that was soon to become local legend.

25:18One of the most iconic organisms that washed up was a fishing boat. And remarkably, in the back of this fishing boat, which was actually partially submerged, were five fish. And these fish were called straight beak fish. And roughly about maybe six inches in length. So fairly large fish that actually had survived this long journey across the Pacific. And this boat was traced back to Japan. And so we knew when that boat was actually lost due to the tsunami. and we knew roughly had drifted over two years in the ocean. But these five fish were actually living in almost like a basin or a cavity in this boat that resembled much like a marine reef.

26:07That is, it had algae, it had barnacles, it had a whole suite of organisms, which was almost like habitat that these fish are normally used to. And these fish were able to survive by eating the organisms that had grown into the hull of this ship. It was an amazing find, and also a potentially alarming one. They're called striped beakfish because they're actually very powerful beaks that are really meant for crushing shellfish. Clams, barnacles, snails, and mussels. And this would be quite harmful to the organisms here on the Pacific coast of North America because these shellfish are not adapted to that particular species of fish.

26:53Tsunamis are pretty common in this part of the world, so isn't it surprising that these fish haven't washed up before? Because the amount of infrastructure built by humans was fairly small along the coast until the Industrial Revolution occurred. Marine organisms drifting across the ocean, their likelihood for survival was small. Even a log, for example, or a coconut would not have provided enough surface area for organisms to colonize and survive long trans-Pacific journeys. and we're talking about a lot of debris. It was estimated that about 5 million tons of debris was initially washed up from 2011 tsunami off Japan.