- Earth’s freshwater ecosystems are among the most at risk from human-induced threats including overfishing, dam building, pollution and climate change.
- But biologists know relatively little about the animals that live in the murky depths of our rivers and lakes, perhaps least of all about some of their largest inhabitants.
- In a new book, fish conservation biologist Zeb Hogan teams up with journalist Stefan Lovgren to get to the bottom of a curious question: What is the world’s largest freshwater fish?
- An exploration of the world’s freshwater ecosystems from Australia to the Amazon, “Chasing Giants” also looks into the range of threats giant fish face the world over and what scientists, policymakers and the public can do to support their conservation.
In 2005, fishers in northern Thailand captured a Mekong giant catfish of gargantuan proportions. Tipping the scales at 293 kilograms (646 pounds), the critically endangered Pangasianodon gigas raised a question in conservation biologist Zeb Hogan’s mind: Is this the biggest freshwater fish ever caught?
Posing the question in a public announcement following the find, Hogan expected to be inundated with phone calls and emails from fish biologists, researchers and fishers detailing close encounters with leviathans living in the murky depths of our planet’s lakes and rivers. But that never happened. He received no communications.
Probing the question further, Hogan soon realized that nobody had ever looked at the issue on a global scale. Simply, nobody knew the identity of the world’s largest freshwater fish. Hogan, currently a conservation biologist at the University of Reno, Nevada, in the U.S., and co-lead of the USAID-funded Wonders of the Mekong project, was hooked. His interest piqued, he embarked on a decades-long worldwide exploration of the lakes and rivers across the world to uncover the mysteries of their giant inhabitants.
Chasing Giants: In Search of the World’s Largest Freshwater Fish is the story of his discoveries during that search. He and co-author Stefan Lovgren, a journalist, connect freshwater science with adventure, recounting extinctions, rediscoveries of species long thought to be lost, and, of course, close encounters of the aquatic kind with some of the most spectacular fish on Earth.
In the book, Hogan and Lovgren are eager to not only instill wonder about giant fish themselves, but also to inspire action to protect them. During his search, Hogan identified more than two dozen freshwater species that can be termed “megafish” that could grow to lengths of at least 2 meters (6 feet) or reach weights of 90 kg (200 lbs). Although they’re all colossal, the species range widely in biology, shape, behavior and habits, from ponderous and ancient sturgeon to skittish and energetic giant trout.
Never one to rest aboard a boat or on the shore, Hogan often gets in the water with his study subjects. Each of the megafish has a unique personality, he says, which is only really apparent when you’re in their environment, interacting with them on their own terms. In the book, he recounts coming face to face with massive arapaima (Arapaima gigas) and shoals of piranha (family Serrasalmidae) in the Amazon, alligator gar (Atractosteus spatula) in Texas, pigeon-eating wels catfish (Silurus glanis) in France, stingrays (Urogymnus polylepis) in Cambodia, and the gnarled-toothed sawfish (Pristis pristis) in Australia.
The book closes with a scene in a biologically rich stretch of the Mekong River in Cambodia and a record-breaking giant fish find. It provides clear evidence of what can be achieved when communities, scientists and policy experts come together. It’s also a reason for hope in a river system that is still resilient and supporting life despite an onslaught of human-induced threats. “People are recognizing and appreciating the importance of these fish and taking action to protect them,” Hogan says. “But we still have a long way to go.”
Zeb Hogan recently spoke with Mongabay’s Carolyn Cowan ahead of the book’s April 25 release. The interview has been lightly edited for length and clarity.
Mongabay: What made you want to write Chasing Giants?
Zeb Hogan: Megafish are understudied and underappreciated — there are a lot of people that don’t even know they exist — and they’re highly endangered. So I felt it was important to write this book to tell their story so that more people know about them and the habitats in which they live and to boost people’s appreciation for them.
The overall goal of this search for the world’s largest freshwater fish was to find, study and protect these gargantuan species. It all began in 2005 when fishers in northern Thailand captured what was reported at the time to be the world’s largest ever recorded freshwater fish. It was a Mekong giant catfish.
Following that discovery, I asked myself what I thought was a simple question: “Is this really the world’s largest freshwater fish?” I didn’t realize at the time that that question was then going to occupy the next two decades of my life!
I’ve worked with National Geographic for a long time, and combining storytelling with science to reach people and inspire them to protect the planet is a philosophy that underpins my work with them and also the book: telling the story of these amazing creatures that people don’t know very much about; telling the story of the rivers and lakes where they live; and the story of the people who depend on them for their livelihoods.
Megafish are also underwater and out of sight. The freshwater places they inhabit are a world that people generally don’t have the opportunity to experience themselves. So this book is a way to take people there, to places that are actually close and important to us all — freshwater rivers and lakes — but that people don’t typically immerse themselves in.
When I think of other National Geographic explorers like Bob Ballard, who discovered the Titanic, and Sylvia Earle, who was one of the pioneers of marine exploration, I think as scientists, we want to share our work, share our discoveries with others because if you don’t know that it exists, then you’re not going to value it … so it’s important to find these big fish species to inspire people to make better choices.
While working on this project, I’ve had the chance to travel to 30 different locations around the world. So I was uniquely positioned to tell this important story that brings all my experiences in those places together.
Mongabay: With so many watercourses, on completely separate continents, to choose from, how did you decide where to begin your search?
Zeb Hogan: Since big fish generally need a lot of habitat, the most obvious place to look was the world’s largest rivers and lakes, and that approach generally worked. The Amazon, the Mississippi, the Nile, the Danube and the Yangtze all come to mind. And all those rivers do have fish that are over 6 feet long and weighting more than 200 pounds, which was the criteria for this project.
But once you move beyond the idea that big fish need big rivers and lakes, you begin to find exceptions. The large rivers of Africa don’t actually have many large fish species; and the Amazon, which is the world’s most biodiverse river system in terms of freshwater fish biodiversity, also has fewer big fish than you’d expect.
Talking with local scientists was crucial to find those exceptions, to locate smaller rivers and lakes that actually had big fish. And in that way, the search became more targeted.
Mongabay: Your book really put the giant fish that live in the Mekong River system in Southeast Asia in context, given the multiple environmental pressures the river system is facing these days, not least due to the spate of hydropower dam construction over the past few decades. What is it about the Mekong River that makes it such a hotspot for giant fish?
Zeb Hogan: The Mekong is actually home to more large fish than any river on Earth, and I don’t have a definitive answer for why that is. We know that the Mekong is one of the most productive rivers on Earth, but it’s not the most biodiverse in terms of fish. It’s home to about 1,000 species of freshwater fish, compared to the Amazon, which has 3,000 to 4,000. The Mekong is also among the top 20 largest rivers in the world. It’s home to eight species of megafish that can grow to more than 200 pounds and have evolved to live in the Mekong, which seems to suit these big fish very well. There are giant catfish and giant carp, and then you also have freshwater stingrays, so it’s a diverse and interesting group of big fish living in the Mekong. They seem to be able to find plenty of food in the productive river system and enough varied habitat, from shallow wetlands and flooded forests to deep pools too.
Mongabay: In your search for giant fish over the years, you must have had some up-close encounters and have some insights about these animals in the wild. What experiences from the field stand out for you and why?
Zeb Hogan: Being underwater with the fish in their natural environment has led to my most memorable experiences. Once fish have been captured for monitoring or measuring, they’re no longer displaying their natural behavior, but the times I’ve had the chance to interact with them in their natural habitat on their own terms has been exhilarating. Freshwater fish are sometimes difficult for people to feel a connection with, but being in the water with the wild fish, you can feel a strong connection and feel their majesty and curiosity.
From swimming with many different species, I’ve noticed that each one has its own personality. Giant stingray can be very curious — they’ll lift their snout and sense the world around them almost like they’re sniffing around. Sturgeon have a kind of ancient, plodding manner about them, they seem very at ease when I’ve been in the water with them. On the other hand, giant trout (which are the largest fish in their environment) are very skittish and seem to react fearfully to human presence, perhaps because they can be aggressive and even cannibalistic to one another.
The Wonders of the Mekong project works with fishermen in Cambodia who inform the team when they catch an endangered fish so that researchers can tag and release the fish to learn more about its movement patterns. We typically stay with the fish for a while to check it is not injured or tired from being caught in the net. We take the fish into the middle of the river and a lot of the time I’ll get in the water and dive down with the fish to get it a little deeper. The water in the Mekong is very muddy and so it gets dark very quickly as you descend, the world goes black and your ears pop from the pressure. You’re entering the fish’s world that’s completely unfamiliar to us humans, even though you’re less than 10 feet [3 m] from the surface.
For me personally, all it takes to fall in love with each species is learning more about them; to be in the river where they live and to talk to the local people — the fishermen and the scientists — who know about them.
Mongabay: There were megafish extinctions during the period you were searching for the world’s largest freshwater megafish. In 2020, scientists declared the Chinese paddlefish (Psephurus gladius) extinct. Can you tell us your reflections on that loss and about your experience searching for it in the Yangtze River?
Zeb Hogan: In 2007, I went to China to meet with Dr. Qiwei Wei, a paddlefish expert who was doing Chinese paddlefish surveys on the Yangtze River system. We didn’t find it, and what we didn’t realize at the time was that it could’ve already been extinct way back then.
The Chinese paddlefish was very distinctive and unique and one of only two species of paddlefish in the world (the other being in North America). It was a large and unusual-looking predator that could grow to over 7 meters (23 ft) long. The loss of one of only two species of paddlefish in the world during the life of this project was sobering, and one of our goals of our search and this book is to stop the extinction of other species.
As the first of these big megafish species to go extinct, the Chinese paddlefish was a particularly sad example because there were none in captivity and no breeding program for them, so their loss from the wild meant the species was permanently lost. It served as a wake-up call. People really took notice and asked, “How could we let one of the largest fish on Earth just disappear on our watch?”
Mongabay: How much trouble are giant freshwater fish in?
Zeb Hogan: There are between 30 and 40 species of freshwater fish that grow to over 6 feet or weigh more than 200 pounds. Seventy percent on them are at risk of extinction, and many are critically endangered. Overfishing, habitat fragmentation and water pollution are the three key risks that they face.
Overfishing is a big threat since these species are slow to mature, so when they’re harvested at high rates, they don’t have a chance to get big enough to begin reproducing. Habitat fragmentation and degradation is also a big issue because so many of these large fish are migratory. Migrations are crucial to trigger reproduction and for young to disperse downstream as part of their life cycle. In some parts of the world, water quality and issues with pollution are problems. Some species are very sensitive to water pollution: there’s been big die-offs of giant freshwater stingrays, for instance, in the Mae Klong and Chao Phraya rivers in Thailand due to poor water quality.
Overfishing alone rarely causes extinctions. Habitat fragmentation and modification of river flow tends to be what ultimately causes extinctions because these factors change the habitat in such a way that the fish can no longer complete their life cycle. For instance, when you dam a river, it can alter the conditions so that they’re no longer conducive to fish spawning, so you end up with old fish in the system, but the population is not being replenished with young fish, and over time, obviously, that will cause the extinction of the species. That’s what happened to the Chinese paddlefish: dams like the Three Gorges Dam blocked the species from accessing critical spawning habitat and altered the water conditions. When you have such wholesale changes in river systems, then species start to disappear.
Mongabay: What species of giant freshwater fish are most in peril of extinction?
Zeb Hogan: Several species of megafish are on the brink of extinction right now, and it’s difficult to know what to do to save them. Obviously, we have to keep rivers free-flowing and we have to keep the water clean and free of pollutants. The giant salmon carp [Aaptosyax grypus], a predatory species that can reach a length of 6 feet and is endemic to the Mekong River, springs to mind. It was thought extinct until late in 2022 when it was rediscovered. There’s no breeding program for it and there are none in captivity. So if it disappears from the river, it’s gone.
It’s difficult to know what to do to save the species since we don’t know how many individuals are left and they’re so difficult to find. As a result, we have very little information about them, which makes it challenging to galvanize governments and the public toward the kind of conservation action that’s needed to ensure the species’ survival.
The individual giant salmon carp that was found in 2022 was mature, and, as is the case with so many river systems that have been dammed, juveniles of the species are rarely encountered. So it’s clear some adults are hanging on, but their life cycle has perhaps been so disrupted by dams that it’s affecting their ability to reproduce.
One ray of hope is that although the Mekong is undergoing a lot of changes, a lot of dams are going in and conditions are changing pretty rapidly, there’s still enough of the lower Mekong River that’s free-flowing that the giant salmon carp could hang on for a little while longer until we can study it more and understand how best to save it.
Mongabay: Why should we make an effort to safeguard giant fish species?
Zeb Hogan: Megafish can be seen as umbrella species, and they’re indicators of ecosystem health — by protecting them, then we’ll also be keeping the rivers and lakes in which they live healthy. In order to protect big fish, you have to safeguard the freshwater ecosystem as a whole. That implies there will also be healthy populations of smaller fish that support other wildlife and fisheries upon which many millions of people depend. So by protecting the big fish, we also preserve aquatic biodiversity.
In the Mekong, for instance, Mekong giant catfish use the same migration corridors as hundreds of other species of fish. By keeping the Mekong giant catfish habitat healthy and keeping the river free-flowing in those areas that are important for the species’ life cycle, then all those other fish species are also sustained, some of which are commercially important that are critical to people’s nutrition and livelihoods in the region.
Many of these big fish species also have cultural importance and they instill a sense of wonder and pride, which can have knock on benefits when it comes to conservation action.
Mongabay: Is there any good news? Are any of these species doing better than you’d thought?
Zeb Hogan: There are a few messages of hope. One is what’s happened with other species. Take sharks, for instance. Several decades ago, public perception of sharks was very poor. Sharks were viewed as a menace and scary and not worthy of protection. But today, the three largest species of sharks have international protection, there’s amazing research being done on them, and there’s widespread appreciation for how important sharks are as amazing animals that are essential components of our oceans. That shift in the perception of sharks is hopeful to me because I feel like we’re early on in a similar trajectory with freshwater megafish. We’re beginning to learn about them, there’s more attention around them, there’s growing appreciation for them, and I hope that ultimately what happened for sharks can also happen for these giant freshwater fish.
It’s an uphill battle, though, because freshwater is an inherently local issue. So starting to view what’s happening in our freshwaters around the world as a global issue is important. Goals like the 30 by 30 initiative [a plan to conserve 30% of Earth’s land and sea areas by 2030 through “area-based conservation measures” like protected national parks] can help in unifying all of us toward a common goal.
There are also many smaller-scale stories of hope. Fishing clubs are protecting locally occurring species through sustainable management, for instance, lake sturgeon in North America are doing well through this approach in some places. Also, fishermen in the Mekong are now recognizing critically endangered Mekong giant catfish as rare and under threat and releasing them back into the wild if they catch them. All of this shows that people are recognizing and appreciating the importance of these fish and taking action to protect them. People are trying to change the trajectory, to try to slow the losses to zero and then to improve the situation for these species, through habitat restoration and fisheries protections. But we still have a long way to go.
Mongabay: What are the main things that need to happen for giant fish species to thrive and persist into the future?
Zeb Hogan: One of the main things that needs to happen for these species to survive is maintaining as much free-flowing stretches of river as possible around the world. The concept of protecting 30% of habitats by 2030 is a good goal. If 30% of rivers remain free-flowing then there will be habitat refuges available for the fish. We’re seeing in freshwater ecosystems that if we protect areas of aquatic habitat for biodiversity, we see fisheries benefits outside of those protection zones because those core protected areas are sustaining fish populations that feed into nearby areas. So protecting 30% of freshwater by 2030 is a great goal that has real potential to slow or stop the extinction of many of these large species.
For species on the brink of extinction, trying to establish captive populations and figuring out how to breed them in captivity could be one emergency measure that might prevent extinction. But we don’t want these giant fish species to just survive in captivity. The idea is to keep rivers and lakes healthy enough to support wild populations. Besides keeping rivers free-flowing, avoiding overharvesting and making sure we continue to gather more knowledge about these species are vital.
Banner Image: Through their book, Zeb Hogan (pictured here on the right with fishers and a Mekong giant catfish) and co-author Stefan Lovgren are eager to inspire action to protect the world’s largest freshwater fish. Image courtesy of Zeb Hogan.
Carolyn Cowan is a staff writer for Mongabay. Follow her on Twitter @CarolynCowan11
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