How Kunpei Kosaka Took a 105.5kg Dogtooth Tuna by Freediving with a Hand Spear: The Spearfisher on the Dangers of the Sea and Instinct

Kosaka started spearfishing in the spring they entered Tokyo University of Marine Science and Technology. Though they had longed for the underwater world since childhood, at the time they could not swim properly. On the first day of university, Kosaka went around the club recruitment booths with a friend but found nothing that caught their interest. Just as they were about to leave, a tanned senior student called out, "How about being a buddy?" Kosaka didn't even know what "buddy" meant, but was drawn in by the senior, who spoke with shining eyes about their activities and how amazing the sea was, and decided to join on the spot.

The weekend after joining, Kosaka went out to sea for the first time and nearly drowned at the water's edge. The senior members took them to a spot about 4–5m deep, but just swimming where their feet couldn't touch the bottom was frightening, and their heart pounded. Losing composure would have looked uncool, so "I was panicking quietly," Kosaka says with a laugh. Back then, a round trip to 5m was all they could manage, and they couldn't stay underwater for even 30 seconds.

The turning point came in the autumn of Kosaka's first year. While swimming, something like a giant carpet fluttered past beneath them. It was a large flounder, about 85cm long. Kosaka frantically aimed for the middle and speared it, and the flounder thrashed violently.

Here Kosaka explains just how weak human strength is underwater. Even if you spear an amberjack of around 3kg at a depth of 15m, if it runs, you get dragged straight down with it. A flounder over 80cm has considerable power, and Kosaka's homemade spear bent badly. The spear eventually came out, but the wounded flounder stopped moving nearby, and the friend Kosaka was buddied with retrieved it. This became the first big fish Kosaka ever landed.

Kosaka recalls that the actual struggle with the flounder lasted less than 10 seconds. Even so, in that moment alone, they were completely serious. Feeling afterward that "in that moment, I was truly alive" was what drew them into the challenge of going up against big fish.

One day in their second year of university, while looking up materials for hand spears during class, Kosaka was captivated by a single photo. Big, glaring eyes, and a face that hardly seemed like a fish's. The shock was so great that Kosaka describes it as "it blew my head off," and they resolved to catch this fish with their own hands at least once in their life. The fish in the photo was a dogtooth tuna.

According to the narration, the dogtooth tuna is a large fish found widely in the warm waters from the Indian Ocean to the Pacific. It swims around rocky reefs and coral reefs facing the open ocean and preys on fish and squid. Most are 1–1.5m, but large ones exceed 2m in length and 100kg in weight. Once it starts running, human strength can't stop it, and it easily drags buoys with enough buoyancy that a person can't sink them underwater, earning it the nickname "the dump truck of the reef." No one had ever taken a dogtooth tuna over 100kg by freediving with a hand spear.

When Kosaka first saw the photo, they had no idea where dogtooth tuna lived or how to catch them. By around their fourth year of university, they were encountering them underwater from time to time, but catching one was still a distant dream. To take one, you need to get within about 1.5m of the spear tip, so merely seeing one far below wasn't enough to do anything.

In their fourth year of university, on a ferry back from a certain island, Kosaka met a person they still call their "mentor." In 2018, the year Kosaka graduated, they pleaded with the mentor to let them join an expedition to waters where dogtooth tuna lived, and took a 40kg dogtooth tuna. Three years had passed since seeing that photo.

Having caught the fish of their dreams, Kosaka was moved to tears. But after a while, upon calming down, Kosaka began to think that they hadn't found the spot themselves; they had merely been brought there by their mentor. That wasn't enough to say they had speared a dogtooth tuna on their own. According to the narration, had it been submitted, the 40kg fish might have been recognized as the spearfishing world record at the time, but Kosaka did not submit it.

So Kosaka set a new goal: to catch one alone, in a place they found themselves, where no one knew there were dogtooth tuna and no one had caught any. Only then, Kosaka thought, could they say "I caught a dogtooth tuna." After that, Kosaka kept diving in seas across many regions while searching for dogtooth tuna habitats on their own.

In 2020, Kosaka began a new challenge with friends: to take a dogtooth tuna of over 100kg by freediving with a hand spear, and to record the process as a documentary film. Every year, the team took two months off work for an expedition, diving while filming.

They caught plenty of dogtooth tuna. There was even an 85kg fish, but none reached 100kg. The footage also captures Kosaka's voice blaming themselves after missing a big chance. Continuing for two months every year, at their own expense and without working during that time, became increasingly hard on their morale too. There was no precedent for anyone taking a fish over 100kg with a hand spear, and they didn't even know whether it was possible. So after talking it over with the team, they decided to set a five-year limit. The fifth and final year was the 2025 expedition.

May 2025, the final day of the last expedition. Rough seas were forecast from the next day, so only two hours in the morning remained. Two hours allows about six dives. Kosaka thought of these as the last six.

Up until the previous day, there had been too many sharks in the water to spear a dogtooth tuna. The big dogtooth tuna stay at the very bottom of the school, so to reach them, you have to line up beside sharks about 3m long and dive underneath them. But on that day, of all days, there wasn't a single shark.

On the second or third dive, looking at the school, Kosaka saw one fish that was clearly out of scale with the rest. The moment Kosaka saw it, they were certain: "That's definitely the one." Avoiding unnecessary movements so the fish would drift toward them, Kosaka held their breath and waited.

But then, doubt crept in. Having thought about dogtooth tuna "24 hours a day, 365 days a year," Kosaka had an image of the ideal fish they wanted to land someday: an old female covered in scars, with a dinosaur-like face, a "super cool" fish. The one in front of them was the exact opposite, with a smooth face like a "female with beautiful skin." Once the spear is released, success or failure, the day's attempt is over. Kosaka hesitated, wondering if this was really the fish they wanted as the last one of five years.

While hesitating, Kosaka was carried away from the fish by a fast current. Then the silhouette seen from behind turned out to be, in a different sense, exactly the shape Kosaka had always wanted to catch: a body type they call "a bomb." Thinking "it's got to be that one," Kosaka changed their mind and chased it underwater.

The distance closed only a centimeter at a time. The harder Kosaka kicked, the more they could sense the tension rising in the school as it detected something following. At the point where one more kick would make it flee, there was only one place to aim: the back. Kosaka had shot at that spot many times before and failed, but thought the person they were now might be able to pull it off, and released the spear.

Normally, a speared dogtooth tuna runs for the seabed. But this time, even after surfacing, there was no tension on the rope. Thinking the spear had pulled out, Kosaka looked down to see the dogtooth tuna swimming in the school with the spear still in it. Kosaka felt they had shot an incredible fish. After a fierce struggle, Kosaka gradually hauled in the rope and finally brought it to the surface. About 2m long, weighing 105.5kg. According to the narration, the five years amounted to about 2,800 dives, and the success came within the truly final six.

Facing the fish, Kosaka says all kinds of emotions welled up. Over a lifetime of making choices every day, if even one had been different, they would never have arrived at this moment. Thinking that, Kosaka felt gratitude toward every event and every person up to that point, and cried.

To take a giant fish with a spear, you need to get within about 1m. But Kosaka says fish detect your presence from subtle changes in the water before a human can even see them. By land-based human standards, fish eyesight is said to be weak. On the other hand, just as people in a city can tell to some extent the direction and distance of a sound and what happened without looking, fish sensitively pick up changes in water pressure and flow through their lateral line. Kosaka's sense is that, also using smell and sight, fish perceive the underwater world at about the same resolution humans perceive the world on land. So if you approach holding something long like a spear, you're noticed immediately.

Kosaka therefore approaches without thrusting the spear forward, keeping it as close to the body as possible. At that point, the fish is basically watching Kosaka, and their eyes meet. When Kosaka decides where to shoot and pulls back the rubber, the spear and rubber rub together, making a faint "squeak." When the spear goes forward, the dogtooth tuna's eyes, which had been locked on Kosaka's, roll toward the spear tip. That's the moment it detects the spear tip through its lateral line. If you can release the spear before that signal reaches the brain and the nerve impulse makes the tail move, it will hit where you aimed. That is how Kosaka explains it.

To reach that single instant, you have to keep waiting underwater for a long time without breathing. Kosaka's records are a depth of 68m freediving and a static breath-hold of 6 minutes 45 seconds on land.

According to Kosaka, the reason people unaccustomed to breath-holding quickly feel discomfort is carbon dioxide. When you hold your breath, oxygen is used up and carbon dioxide is produced. As it dissolves in the blood and makes it more acidic, the body senses this and tries to make you breathe again. But what determines whether you black out is the amount of oxygen in your body. At first, oxygen hasn't dropped that much, but carbon dioxide builds up and only the discomfort grows stronger. The low-oxygen world lies beyond that, so you need to get used to that initial discomfort to some degree, Kosaka says.

As the discomfort grows further, movements trying to breathe appear in the chest and abdomen, regardless of your will. These are contractions. They are often described as spasms of the diaphragm, but Kosaka believes that, strictly speaking, they are the intercostal muscles trying to open. Even when the intercostals open, your breath is held so the lungs can't expand, and instead the diaphragm is pushed up. Kosaka's view is that this is the phenomenon called contractions. On land it's one thing, but if contractions come when water pressure has compressed your lungs, negative pressure develops inside the lungs, which can damage the alveoli. It's also an unpleasant sensation, so normally it's best to stop around the time contractions start, Kosaka says. If you keep enduring, your blood oxygen finally begins to drop. The narration adds that, unlike the discomfort caused by carbon dioxide, low oxygen leads directly to loss of consciousness—a blackout.

Kosaka cites sea lions and sperm whales as examples. Sea lions swim underwater for long periods looking for fish, then return to land. Sperm whales are said to dive to depths of about 3,000m and stay under for one to two hours. They don't have gills; like humans, they are mammals that breathe with lungs and dive by holding their breath. How can they do it? Kosaka's explanation is that mammals are creatures that came from the sea, and the instincts from that time still remain. That is the mammalian dive reflex (Mammalian Dive Reflex, which Kosaka and others call MDR), and Kosaka says everyone's body has it.

According to the narration, when this reflex kicks in, the heart rate drops first, reducing oxygen consumption. Blood vessels in the extremities such as the hands and feet constrict, and the limited blood and oxygen are prioritized for the brain and heart. Diving deeper triggers a blood shift, in which blood from the extremities moves into the chest, supporting the lungs from the inside as they are compressed by water pressure and enabling deeper dives.

What Kosaka finds interesting is that this reflex doesn't appear if you can't relax because of tension or fear. It only works well once you let your body go slack. Kosaka describes a sensation of their outline blurring from the extremities inward, like melting into the sea, with their head becoming a little hazy. Once you get used to the sea, you can dive to about 40m without much trouble, Kosaka added with a laugh.

On the moments when death feels closest, Kosaka first notes that sharks and low-oxygen blackouts are obvious threats that are easy to imagine. Having said that, the threats Kosaka named were waves and currents. A current of about 2km/h seems slow if you think of it as walking speed. But when you're floating in the sea wearing a wetsuit and fins, continuing to swim against that current is quite difficult.

When things go wrong underwater, you can't call for help or ask someone like you can on land. Kosaka says it's precisely at times like these that they feel a definite sense, and loneliness, that they have to deal with it alone and make it back alive. If you panic when you might not make it back, you die. During four years at university, Kosaka was taught by senior members that losing your head is the most dangerous thing, and that panic is the worst. What Kosaka practices is observing and assessing their own condition. If they notice they're out of breath, their blood is rushing to their head, or their breathing is getting fast, they deliberately take deep breaths to calm down.

Kosaka rejects the view, voiced by some overseas influencers and others, that "sharks are friends." Sharks are carnivores, and their temperament varies completely by individual and species. For example, Kosaka's view is that young sharks under 2m are more aggressive, while sharks of 3–4m are more cautious and calm. Even so, depending on the shark's mood, you could get hurt, so Kosaka doesn't want to approach them carelessly or provoke them unnecessarily.

But even if you don't approach them, sharks sometimes come to you. It happened when Kosaka landed a big fish over 2m that they had pursued for years. Usually in those waters, sharks steal the fish you spear, but on this day the fish came up smoothly, and there were few sharks, so Kosaka thought they were lucky. But before they knew it, a huge number of sharks had gathered. Kosaka says sharks apparently have senses so sharp they can smell a single drop of blood in a swimming pool. When a large fish thrashes in front of them, their senses are so overwhelmed they lose control of their behavior and enter a "feeding frenzy," trying to bite anything moving nearby. A shark on the verge of that state charged up from below and chased Kosaka from behind with its mouth open as Kosaka swam on the surface. Kosaka felt it wouldn't have been surprising to lose a leg or an arm before making it back to the boat.

Somehow Kosaka made it back, handed the spear to the captain, and climbed aboard. Just when all that was left was to haul the fish in and bring it up, a pitch-black mass shot up from below, sending up a column of water. Kosaka reflects that perhaps people long ago saw things like this and thought they were umibozu, sea monsters.

Even so, unable to accept having the fish they had long targeted stolen by sharks, Kosaka dove again and speared another fish of the same species. Thinking the mistake last time had been holding the rope attached to the fish, this time Kosaka let the fish run and tried to return to the boat alone. But as soon as Kosaka entered the area where the remains were drifting, sharks began closing in, and about 20m below was a 4m-class bull shark that hadn't been there before. Kosaka realized that in the roughly 20 minutes since the earlier fish was torn apart there, sharks from all over the area had gathered at the smell of blood. Kosaka made it back to the boat after going through the same ordeal again, but the fish they had let run was eaten in the end too.

Angry, Kosaka jumped in a third time. Before they had even done anything, a tornado of sharks had formed below. They didn't attack all at once, but one by one, they drifted up from below and then disappeared. When Kosaka sensed they were coming to check them out, they came to their senses, thought that going any further would truly get them killed, and pulled out.

Because adrenaline was flowing at the time, Kosaka was able to dive repeatedly, but it became a trauma for several years afterward. Diving in offshore spots like the one where they had speared a fish and been attacked by sharks, their breathing would naturally quicken and a cold sweat would break out on their back. If there was even a hint of sharks, they couldn't concentrate on diving at all.

But Kosaka decided it was wrong to be afraid because sharks had been there, or to keep being afraid that they might appear when they weren't there. Kosaka gradually began observing shark behavior again, trying to understand what an individual shark was thinking about them. Kosaka is still "not okay," but the places where they spear dogtooth tuna have a great many sharks, so there's no choice but to find a way to manage. The further one goes into places people don't venture in search of big fish, the more these things come with the territory, and Kosaka accepts them as part of their own spearfishing.

Kosaka also described several memorable scenes encountered underwater. When drifting with the current, Kosaka sometimes ends up blocking the path of a school of dogtooth tuna swimming up the current. Then a large individual in the school opens its mouth, makes a "boof" sound, and tells Kosaka to move.

Once, while floating in search of dogtooth tuna, a sailfish appeared below. Kosaka thought, "It's a dragon." Its dorsal fin was large and ragged at the tip, and its movements were very big. That fish was about 2m 85cm. Sailfish displayed in aquariums look like beautiful blue and silver fish, but underwater it was the color of a 10-yen coin, with purple lightning-like patterns running across it. After a sailfish speared by one of Kosaka's companions ran, it leapt from the surface and charged toward them at high speed. In front was cameraman Muro, heading out to film, and Kosaka thought, "Muro's dead," but the sailfish leapt again in front of Muro. It was a scene where you could tell it was angry, as if they had angered a god, Kosaka says.

Another was a scene with giant trevally (GT). According to the narration, the GT is the largest member of the jack family and can grow up to 1.8m long and 80kg. Kosaka usually encounters GTs in rocky areas where the seabed is visible. But this time, in a deep, empty spot, a single jet-black male GT came swimming up the current. Behind it was a huge shadow, and on closer look, it was a wild whale shark. Even more surprising, about 10 GTs were clinging tightly to the whale shark, rubbing their bodies against it. Large GTs, true to their Japanese name ronin (masterless samurai), are usually alone or in pairs. Kosaka occasionally sees schools of small GTs around 1m, but says it was the first time they had seen large GTs that looked to be 1m-plus-several-tens-of-centimeters in a school, let alone attached to a whale shark. Kosaka doesn't know what they were doing either.

With fishing rods and speargun available, why take on giant fish with a hand spear? Kosaka says it feels like technological advances keep tilting the balance of power between fish and humans further in humans' favor. Kosaka adds that this is purely their own view for themselves, and they don't mean to criticize people who do it differently.

If you're going to hunt a fish's life for your own selfish reasons, it's better that you also run a greater risk of being hunted. That, Kosaka says, is the reason for using a hand spear. After shooting a fish, if your hand doesn't come free of the rubber loop, you get dragged down and can't come back up. Since they are taking a life, Kosaka also accepts danger in the same sea.

Asked why they keep diving to such lengths, Kosaka answers that it's "kind of like an addiction." Kosaka says they long ago passed the stage of liking to dive or wanting to dive, and it's as if they dive to keep themselves together.

The theme of Kosaka's spearfishing is challenging the limits of spearfishing for big fish. Big fish are few in number and powerful, so both your equipment and your way of fighting are tested. Since no one else does it with a hand spear, much remains unknown. Kosaka says they want to keep pushing the limits of their own style of spearfishing.

Finally, asked "What is spearfishing to Kunpei Kosaka?", Kosaka answered, "I don't know." It would be nice to say something cool, but maybe they dive because they don't know—they still don't know. Kosaka also added that they feel they might stop diving once they find the answer.