The Search for the World's Largest Freshwater Fish

Latest update August 24, 2018 Started on May 1, 2005
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A global expedition to find, study, and protect the world's largest and rarest fish before they disappear forever.

May 1, 2005
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In The Field

Floods Aren't Always Bad - The "Flood Pulse" Concept


Mr. Tach Phanara has been monitoring the fish catch at a site outside of Phnom Penh for almost 20 years. Specifically, Phanara is monitoring the downstream dispersal of young fish - over 300 species - that move from the upper Cambodian Mekong to areas downstream of Phnom Penh and around the Tonle Sap Lake.

These young fish are dispersed on the "flood pulse", the annual rise in water level that spreads nutrients and young fish throughout the system. The flood pulse is an important characteristic the Mekong and many other rivers (https://en.wikipedia.org/wiki/Floodpulseconcept).

Populations of two of the species of fish that Phanara is monitoring, the migratory Mekong giant catfish and the striped catfish, have declined significantly in recent years. Some fisheries for the species have disappeared, other have dropped by as much as 98%.

Thank you to Stefan Lovgren who joined me in the field and produced a series of excellent videos about Wonders of the Mekong-supported activities.

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The Importance of the "Flood Pulse" in Cambodia

Thank you to Stefan Lovgren who joined me in the field and produced a series of excellent videos about Wonders of the Mekong-supported activities. This video introduces the work of Mr. Tach Phanara, who has been monitoring the fish catch at a site outside of Phnom Penh for almost 20 years. Specifically, Phanara is monitoring the downstream dispersal of young fish - over 300 species - that move from the upper Cambodian Mekong to areas downstream of Phnom Penh and around the Tonle Sap Lake. These young fish are dispersed on the "flood pulse", the annual rise in water level that spreads nutrients and young fish throughout the system. The flood pulse is an important characteristic the Mekong and many other rivers (https://en.wikipedia.org/wiki/Flood_pulse_concept).

Posted by Monster Fish with Zeb Hogan on Friday, August 24, 2018

It is great to see the history of your work here Zeb. I have been, and I still am a fan of the Monster Fish series. Look forward to seeing your current work and posts. I noticed you use drones, do you use any ROVs in your filming or research? If so, how do they go in the murky rivers?

Southeast Asia May Be Building Too Many Dams Too Fast


Stefan Lovgren, a free-lance writer, joined me in the field last week. Here's his first National Geographic story which focuses on the recent dam failure in Lao PDR and what it means for hydropower development in the region.

https://www.nationalgeographic.com/environment/2018/08/news-southeast-asia-building-dams-floods-climate-change/

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Great article, thanks for sharing! Not a topic I'm well informed about.

Giant Salmon Carp Update


A quick update from the Sesan River in Northeastern Cambodia: we received our first photos back from the fishermen on the Sesan River. They did indeed catch a long-bodied fish with a strange mouth and they sent us photos to prove it (see attached). Unfortunately the fish that they've been catching is not the giant salmon carp. Can anyone identify this fish?

These photos show that fishermen are willing to cooperate - a very positive sign! But it may take some time before fishermen catch our target species. After all, these are Critically Endangered animals that are almost never seen or caught. We always knew this would take patience and plan to continue this work and keeping building our network for the next 24 months.

Better luck next time!

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it looks like: Raiamas guttatus (Day, 1870).

The Giant Freshwater Stingray


Earlier this week, we distributed cameras to fishermen in the Kratie area in the hopes that they would document their catches of giant freshwater stingray. Based on our preliminary observations, the upper Cambodian Mekong seems to be a hotspot for the giant freshwater stingray. But the Mekong is not the only place where the giant freshwater stingray occurs. It is found throughout Southeast Asia. This seems like a good opportunity to provide background information about the giant freshwater stingray, introduce concurrent ongoing research in Thailand, and place the giant freshwater stingray in the context of large stingrays in other parts of the world.

The giant freshwater stingray (Urogymnus polylepis) is one of the world's largest freshwater fish. It can grow to over 500 pounds and reach a length of over 15 feet from nose to tail. Listed as Endangered by IUCN, populations of giant freshwater stingray have suffered from overexploitaiton and pollution. In 2016, as many as 70 giant stingray were killed by an suspected pollution event in the Maeklong River in Thailand (https://www.ecowatch.com/stingray-die-off-2089381293.html)..) Similar die-offs have reported in other rivers, such as the Chao Phraya River that flows through Bangkok.

Researchers in Thailand (Dr. Nantarika Chansue and her team from Chulalongkorn University), supported by a grant from National Geographic, have been studying the giant freshwater stingray in central Thailand for the past several years. Working in partnership with local fishermen, researchers tag and release the giant stingray to better understand their population status and movement patterns. Despite its status as contender for the title of world's largest freshwater fish, very little is known about the ecology of the giant freshwater stingray.

The giant freshwater stingray is one of three species of stingray that grow larger than 200 pounds. Other giants include the freshwater whipray Urogymnus dalyensis in Australia and the short-tailed river stingray Potamotrygon brachyura (which is likely endemic to the Paraná-Paraguay River drainages in southern South America).

All three giant stingrays possess a barb at the base of their tail. This barb, used as a defensive weapon, is reverse serrated and coated in a toxic slime. While potentially very dangerous, humans are unlikely to be hurt by the barb unless they handle or step on a ray. In fact, humans pose a much greater threat to rays (through over-harvest, pollution, and habitat degradation), then rays to humans.

Luckily for the rays, their size, behavior, and distribution makes them difficult catch. A full-size giant stingray can weigh more than 500 pounds and will move immediately to the river bottom when hooked. They can easily bend hooks and break fishing line. The largest stingray can drag boats up and down river, and potentially flip a boat with a quick tug on the line. Thus, a full size stingray is unlikely to be caught unintentionally or as by-catch. They also often occur in large, murky, and remote aquatic habitats. These areas, such as the mainstream Mekong and Parana River, and the crocodile infested wetlands of northern Australia, can be difficult to access and fish.

Video by Stefan Lovgren

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The Giant Freshwater Stingray

The giant freshwater stingray (Urogymnus polylepis) is one of the world's largest freshwater fish. It can grow to over 500 pounds and reach a length of over 15 feet from nose to tail. Listed as Endangered by IUCN, populations of giant freshwater stingray have suffered from overexploitaiton and pollution. In 2016, as many as 70 giant stingray were killed by an suspected pollution event in the Maeklong River in Thailand. Similar die-offs have reported in other rivers, such as the Chao Phraya River that flows through Bangkok. Researchers in Thailand (Dr. Nantarika Chansue and her team from Chulalongkorn University), supported by a grant from National Geographic, have been studying the giant freshwater stingray for the past several years. Working in partnership with local fishermen, researchers tag and release the giant stingray to better understand their popultion status and movement patterns. Despite its status as contender for the title of world's largest freshwater fish, very little is known about the ecology of the giant freshwater stingray. The giant freshwater stingray is one of three species of stingray that grow larger than 200 pounds. Other giants include the freshwater whipray Urogymnus dalyensis in Australia and the short-tailed river stingray Potamotrygon brachyura (which is likely endemic to the Paraná-Paraguay River drainages in southern South America). All three giant stingrays possess a barb at the base of their tail. This barb, used as a defensive weapon, is reverse serrated and coated in a toxic slime. While potentially very dangerous, humans are unlikely to be hurt by the barb unless they handle or step on a ray. In fact, humans pose a much greater threat to rays (through overharvest, pollution, and habitat degradtion), then rays to humans. Luckily for the rays, their size, behavior, and distribution makes them difficult catch. A full-size giant stingray can weigh more than 500 pounds and will move immediately to the river bottom when hooked. They can easily bend hooks and break fishing line. The largest stingray can drag boats up and down river, and potentially flip a boat with a quick tug on the line. Thus, a full size stingray is unlikely to be caught unintentionally or as by-catch. They also often occur in large, murky, and remote aquatic habitats. These areas, such as the mainstream Mekong and Parana River, and the crocodile infested wetlands of northern Australia, can be difficult to access and fish.

Posted by Monster Fish with Zeb Hogan on Friday, August 17, 2018

Extinct or Alive: On the Hunt for the Mekong Giant Salmon Carp


I've been fascinated by the Mekong giant salmon carp (Aaptosyax grypus) since I first saw one in a market in Southern Lao PDR in 1999. It's the only time I've seen the species in over 20 years working on the Mekong. I've made hundreds of visits to local markets and countless checks of fishermen's net, traps, and fishing lines. I am worried that the Mekong giant salmon carp may be extinct, or close to it.

As part of the USAID-funded Wonders of the Mekong Project, we began surveys for endangered Mekong fish in early 2018. One of the species that we included in our survey is the Mekong giant salmon carp - "pa sanak / sana" in Lao language. Some fishermen did report the species in their catches, although most reports were of juvenile fish and it's unclear whether or not the fishermen were actually referring to Aaptosyax or a different, but similar looking species. We did not see the fish ourselves during the surveys.

The next step was to visit the fishermen again during the season when they report catching the fish, and attempt to collect a live specimen, DNA, or take a photo. While we did not see any live or dead specimens on this trip, we did provide the fishermen with cameras in the hopes that they will be able to get a photo of this very unusual, and very rare, fish.

Aaptosyax grypus is only found in the Mekong River. It's a predatory carp that can reach lengths of over four feet and weigh over 50 pounds. Once widespread, it may now be gone from Thailand, and populations elsewhere have dropped to alarming levels. IUCN categorizes the species as Critically Endangered, and no scientist that I've met has seen the species in about 10 years. I hope that the giant salmon carp still out there, somehere, and that it's not too late to find solutions to bolster populations and avoid extinction.

Video by Stefan Lovgren Photo courtesy Chaiwut Grudpan

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Extinct or Alive? The Search for the Mekong Giant Salmon Carp

I've been fascinated by the Mekong giant salmon carp (Aaptosyax grypus) since I first saw one in a market in Southern Lao PDR in 1999. It's the only time I've seen the species, in 20 years working on the Mekong. I've made hundreds of visits to local markets and countless checks of fishermen's net, traps, and fishing lines. I am worried that the Mekong giant salmon carp may be extinct, or close to it. As part of the USAID-funded Wonders of the Mekong Project, we began surveys for endangered Mekong fish in early 2018. One of the species that we included in our survey is the Mekong giant salmon carp - "pa sanak / sana" in Lao language. Some fishermen did report the species in their catches, although most reports were of juvenile fish and it's unclear whether or not the fishermen were actually refering to Aaptosyax or a different, but similar looking species. We did not see the fish ourselves during the surveys. The next step was to visit the fishermen again during the season when they report catching the fish, and attempt to collect a live specimen, DNA, or take a photo. While we did not see any live or dead specimens on this trip, we did provide the fishermen with cameras in the hopes that they will be able to get a photo of this very unusual, and very rare, fish. Aaptosyax grypus is only found in the Mekong River. It's a predatory carp that can reach lengths of over four feet and weigh over 50 pounds. Once widespread, it may now be gone from Thailand, and populations elsewhere have dropped to alarming levels. IUCN categorizes the species as Critically Endangered, and no scientist that I've met has seen the species in about 10 years. I hope that the giant salmon carp still out there, somehere, and that it's not too late to find solutions to bolster populations and avoid extinction.

Posted by Monster Fish with Zeb Hogan on Thursday, August 16, 2018

Drones - and a 150 pound "Dog-eating" Catfish - on the Srepok River, Cambodia


Now on the Srepok River, we test out the drone. The drone is a great way for us to get a bird's eye view of the river and see how people connect with it. The Srepok itself is huge, muddy, and fast flowing. Along its banks are neat wooden houses, on stilts, surrounded by gardens, coconut palms, cows, chickens, pigs, ducks, and rice fields. From the air its easy to see how important the river is to people here - and how connected the pace and rhythms of local life is to the pace and rhythms of the river. The Mekong isn't a single place or single river, it's a living, connected system that supports 60+ million people in six countries. Keeping it healthy is important for people, like the fishermen we are meeting on this trip, and for the almost 1000+ species of fish that call the Mekong home.

Our team is visiting the Srepok to meet with fishermen to gather knowledge about rare species. Here, we are lucky enough to meet with an expert fisherman who has been collecting fisheries data for the Cambodian Fisheries Administration and the Mekong River Commission for several years. The long-term data the fisherman has been collecting are invaluable to those who want to understand the ecology and fisheries of the river in this area. The fishermen, 73 years old, and has been fishing since he was just a boy. He knows as much about this section of the Srepok River and its fish as anyone on Earth.

He tells us the story of a Critically Endangered Chao Phraya catfish (Pangasius sanitwongei), also known as the "dog-eating" catfish, that he and his father caught about 60 years ago. They caught the fish using live bait on a large hook - the fish weighed over 150 pounds and was so large they could not bring it into the boat. They had to partially sink their fishing boat to get the massive fish to the bank, then drained their boat and killed the fish. It was the largest fish that the fishermen had seen in six decades of fishing.

The Chao Phraya catfish can grow to almost 10 feet long and weigh as much as 600 pounds. It is now listed as Critically Endangered by IUCN. Once a semi-regular catch at certain spots along the Mekong, populations have crashed an estimated 95-99% and adult fish are extremely rare now. The last confirmed catch that I've heard about was reported from below the Xayaburi Dam in 2015 or 2016. We know nothing about the current population size or distribution of the fish, very little about its ecology, and even less about the steps needed to take to bring the species back from the brink of extinction.

Video by Stefan Lovgren

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By Boat, By Drone, By Air: There is More Than One Way to Learn About and Appreciate the Mekong River

Now on the Srepok River, we test out the drone. The drone is a great way for us to get a bird's eye view of the river and see how people connect with it. The Srepok itself is huge, muddy, and fast flowing. Along its banks are neat wooden houses, on stilts, surrounded by gardens, coconut palms, cows, chickens, pigs, ducks, and rice fields. From the air its easy to see how important the river is to people's lives here - and how connected the pace and rhythms of local life is to the pace and rhythms of the river. The Mekong isn't a single place or single river, it's a living, connected system that supports 60+ million people in six countries. Keeping it healthy is important for people, like the fishermen we are meeting on this trip, and for the almost 1000+ species of fish that call the Mekong home. Our team is visiting the Srepok to meet with fishermen to gather knowledge about rare species. Here, we are lucky enough to meet with an expert fisherman who has been collecting fisheries data for the Cambodian Fisheries Administration and the Mekong River Commission for several years. The long-term data the fisherman has been collecting are invaluable to those that want to understand the ecology and fisheries of the river in this area. The fishermen, 73 years old, and has been fishing since he was just a boy. He knows as much about this section of the Srepok River and its fish as anyone on Earth. He tells us the story of a Critically Endangered Chao Phraya catfish (Pangasius sanitwongei) that he and his father caught about 60 years ago. They caught the fish using live bait on a large hook - the fish weighed over 150 pounds and was so large they could not bring it into the boat. They had to partically sink their fishing boat to get the massive fish to the bank, then drained their boat and killed the fish. It was the largest fish that the fishermen had seen in six decades of fishing. The Chao Phraya catfish is now listed as Critically Endangered by IUCN. Once a semi-regular catch at certain spots along the Mekong, populations have crashed an estimated 95-99% and adult fish are exteremly rare now. The last confirmed catch that I've heard about was reported from below the Xayaburi Dam in 2015 or 2016. We know nothing about the current population size or distribution of the fish, very little about its ecology, and even less about the steps needed to take to bring the species back from the brink of extinction.

Posted by Monster Fish with Zeb Hogan on Thursday, August 16, 2018

Local Markets as Sources of Information about Rare and Endangered Fish


Around 6am, I head to the Stung Treng fish market. Local markets can be some of the best spots to gather information about rare species. What's sold changes from day to day and month to month. Once I find an interesting fish, I talk with the fish seller to see if he or she will provide me with information about how and where the fish was caught. On this particular visit, one woman was selling three large river catfish Pangasianodon hypophthalmus. They looked like wild fish, but I'm not sure when or where they were caught.

It was at a market like this in Lao PDR where I saw my first and only giant salmon carp (Aaptosyax grypus), a Critically Endangered predatory carp that can grow to almost five feet in length. That was in 1999.

Video by Stefan Lovgren

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Looking for Rare Fish in Local Markets in Northeastern Cambodia

Local markets can be some of the best spots to gather information about rare species. What's sold changes from day to day and month to month. Once I find an interesting fish, I talk with the fish seller to see if they will provide me with information about how and where the fish was caught. On this particular visit, one woman was selling three large river catfish Pangasianodon hypophthalmus. They looked like wild fish, but I'm not sure where they were caught.

Posted by Monster Fish with Zeb Hogan on Wednesday, August 15, 2018

Amazing photos. Thank you so much for sharing these.

Building Trust with Fishermen to Reduce Illegal Harvest and Trade of the World's Largest Carp Species


A few days ago, I met with Mr. Phan Sok Phoen on his floating house on the Tonle Sap Lake. In 2017, Phoen caught two Critically Endangered giant barb in his fishing nets. The giant barb, Catlocarpio siamensis, can grow to over 400 pounds and is the national fish of Cambodia.

Despite the fact that the two giant barb were probably worth more than $20,000 on the black market, Phoen decided to follow the law and release the fish back into the lake. His story, and the larger issues at play, are chronicled in an excellent article by Rachel Nuwer in National Geographic Magazine:

https://www.nationalgeographic.com/animals/2018/07/illegal-giant-fish-cambodia-vietnam-cuisine-delicacy-wildlife-watch/

I met with Phoen to thank him for releasing the endangered fish, I was also hoping to see the photos and video that he took during the release to confirm the size of the fish and the location where the fish were caught. I also gave Phoen a camera to take photos of any additional fish he may catch in 2018. After my conservation with him, he indicated that he will continue to release any endangered fish that he catches in his nets.

On my way to Phoen's house I also had a chance to see first hand the Tonle Sap Lake's famous water snakes. As recently as 2007, the Tonle Sap Lake was thought to be the site of the world's largest aquatic snake harvest - an estimated 7,000,000 snakes were harvested from the lake each year. While no one seems to know for sure how many snakes are harvested now, we met one family who estimates they catch 100-200 snakes a day in August and 400-600 snakes at the peak harvest period later in the year. With millions of people living around the Tonle Sap Lake, it easy to imagine a current harvest that's similar or perhaps greater than it was in 2007.

Video by Stefan Lovgren

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Fisherman Releases Two 100kg+ Giant Barb on the Tonle Sap Lake

A few days ago, I met with Mr. Phan Sok Phoen on his floating house on the Tonle Sap Lake. In 2017, Phoen caught two Critically Endangered giant barb in his fishing nets. The giant barb, Catlocarpio siamensis, can grow to over 400 pounds and is the national fish of Cambodia. Despite the fact that the two giant barb were probably worth more than $20,000 on the black market, Phoen decided to follow the law and release the fish back into the lake. His story, and the larger issues at play, are chronicled in an excellent article by Rachel Nuwer in National Geographic Magazine: https://www.nationalgeographic.com/animals/2018/07/illegal-giant-fish-cambodia-vietnam-cuisine-delicacy-wildlife-watch/ I met with Phoen to thank him for releasing the endangered fish, I was also hoping to see the photos and video that he took during the release to confirm the size of the fish and the location where the fish were caught. I also gave Phoen a camera to take photos of any additional fish he may catch in 2018. After my conservation with him, he indicated that he will continue to release any endangered fish that he catches in his nets. On my way to Phoen's house, I also had a chance to see first hand the Tonle Sap Lake's famous water snakes. As recently as 2007, the Tonle Sap Lake was thought to be the site of the world's largest aquatic snake harvest - an estimated 7,000,000 snakes were harvested from the lake each year. While no one seems to know for sure how many snakes are harvested now, we met one family who estimates they catch 100-200 snakes a day in August and 400-600 snakes at the peak harvest period later in the year. With millions of people living around the Tonle Sap Lake, it easy to imagine a current harvest that's similar or perhaps greater than it was in 2007.

Posted by Monster Fish with Zeb Hogan on Wednesday, August 15, 2018

Fishermen as Citizen Scientists: Giant Freshwater Stingray


As I head upstream from Phnom Penh, my first stop is near the small town of Kratie (a spot famous for its freshwater dolphins). I meet with the wife of a fisherman who occasionally catches giant freshwater stingray. The giant freshwater stingray is certainly a contender for the title of the world's largest freshwater fish, with documented catches of around 500 pounds. The giant freshwater stingray is one of three species of freshwater stingray (giant freshwater stingray, Australian whipray, and short-tailed river ray) that are among the top 10 world's largest freshwater fish. I've asked the family to help me document the frequency and size of these catches using an old-fashioned technique: an instant camera. Not all fishermen have cameras or camera phones, so I'm hoping that the novelty of an instant camera will be motivation to document their catches. I'll visit this family every few months to see what they've caught and if they were able to take a photo to document the catch.

Video by Stefan Lovgren

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Fishermen as Citizen Scientists: Giant Freshwater Stingray

For the past week I have been traveling around Cambodia, meeting with expert fishermen who I believe can help me document the existence and distribution of the largest and rarest fish in Cambodia. As I head upstream from Phnom Penh, my first stop is in Kratie (a spot that's famous for freshwater dolphins) where I meet with the wife of a fisherman that occasionally catches giant freshwater stingray. I've asked the family to help me document the frequency and size of these catches using an old-fashioned technique: an instant camera. Not all fishermen have cameras or camera phones so I'm hoping that the novelty of an instant camera will be motivation to document their catches. Do you think it will work?

Posted by Monster Fish with Zeb Hogan on Tuesday, August 14, 2018
Preparation

Trillions of Fish on the Move


My first stop before I leave Phnom Penh: join Cambodian Fisheries Administration biologist Mr. Tach Phanara. Every year, at the height of the flood season, Mr. Tach Phanara pilots our tiny Wonders of the Mekong project boat out into the vast expanse of the Mekong River. At this time of year, the Mekong transforms from a slowly flowing, green tinged river to a sediment-filled Monster. Water levels can rise by more than 10 meters - one of the largest dry to wet season swings of any river - inundating much of central Cambodia and southern Vietnam. The rising waters result in strange physical and ecological phenomena, such as the change of direction of the flow of the Tonle Sap River, and also cue a mysterious ecological miracle: the dispersal of trillions of baby fish that move downstream on the flood pulse.

Phanara makes his way out to a fisherman, who has set a strange looking net in the murky water, not far from the river's edge. Fishermen have been using this technique - a giant cone shaped net as long as a boat with super fine mesh that resembles the cheese cloth you might use to make tea or strain broth in the kitchen - for as long as anyone can remember. And their target, while not the 10 foot long catfish and carp that make the Mekong famous, are equally important. In fact, Phanara and the fisherman are after fish that are as small as the giant catfish is large - nearly microscopic larval fish that are traveling downstream after record rainfall in the upper watershed, riding the swift flowing currents of the swollen, swirling Mekong River.

The diverse assemblage of fish, over 300 different species, take unfamiliar forms, crazy shapes and sizes all manner of mouths, bug-eyes, and long whiskers. Originating from critical fish spawning habitat in the remote and rugged island strewn river stretches and 200 feet deep pools upstream, these are baby, goblin-like fish that then travel downstream in the trillions. Silent and unseen, they disperse with the floodwaters across much of the country, and over time, as they grow into full-size edible fish, they power Cambodia's natural resource based culture and economy. It's been like this for a thousand years: pounding rain and typhoon storms pummeling the region, soaking the region like a huge sponge, and triggering rising waters, spawning fish, and a downstream dispersal of young of a magnitude unlike anything seen elsewhere in Asia or the world.

While this physical and ecological miracle has occurred for thousands of years, Phanara worries about the Mekong's future. One by one, dams are being built upstream - dozens (hundreds of you count smaller weirs and barrages on tributaries) and with each dam the flow of the Mekong is altered, migration and spawning cues become muddled, and baby fish are blocked from their epic downstream dispersal. The largest of these dams - the newly constructed Xayaburi and Don Sahong in Laos, and the Lower Season 2 in Cambodia, have been built smack dab in the middle of core spawning, migration, and dispersal routes. For this reason, the Lower Sesan 2 alone has been predicted to eventually result in the loss of hundreds of thousands of tons of harvestable fish. But as potentially detrimental as existing dams may be to fisheries, the looming threat is of mainstream dams on the upper Cambodian Mekong, a rugged stretch of river pock-marked with thousands of rocky islands and characterized by dozens of deep dry-season pools. It's this area a last refuge for the Mekong's remaining freshwater dolphins and it is the core, critical spawning habitat for most of Cambodia's migratory fish. It is the connection of this core area and downstream floodplains, and the natural river hydrology of the rainy season high water, that keeps the entire region and its people vibrant, healthy, and well-fed.

Phanara's trip to this site has a second purpose - he is also collecting larval (baby) river catfish and Mekong giant catfish. Both species have been categorized by IUCN as endangered species, and Phanara will transport a few thousand of the fish to the local research station, where they will be raised and then released into aquatic protected areas for conservation purposes. In all of Phanara's monitoring and collection last year, he only found one single Mekong giant catfish, which has become our project mascot and nicknamed "Wonder". Wonder the giant catfish will be released into the wild next year.

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Trillions of Fish on the Move on the Mekong River

At this time of year, the Mekong transforms from a slowly flowing, green tinged river to a sediment-filled Monster. Water levels can rise by as much as 10 meters - the largest dry to wet season swing of any river - inundating much of central Cambodia and southern Vietnam. The rising waters result in strange physical and ecological phenomena, such as the changing in direction of the flow of the Tonle Sap River, and also a mysterious ecological miracle: trillions of baby fish travelling downstream on the flood pulse. The diverse assemblage of fish, nearly a hundred different species, take unfamiliar forms, crazy shapes and sizes all manner of mouths, bug-eyes, and long whiskers. Originating from critical fish spawning habitat in the remote and rugged island strewn river stretches and 200 feet deep pools upstream, these are baby, goblin-like fish that then travel downstream in the trillions. Silent and unseen, they disperse with the floodwaters across much of the country, and over time, as they grow into full-size edible fish, they power Cambodia's natural resource based culture and economy. It's been like this for a thousand years: pounding rain and typhoon storms pummeling the region, soaking the region like a huge sponge, and triggering rising waters, spawning fish, and a downstream dispersal of young of a magnitude unlike anything seen elsewhere in Asia or the world.

Posted by Monster Fish with Zeb Hogan on Tuesday, August 14, 2018

Rainy Season Surveys for Mekong Ghosts


The research team set out this week to gather information about Mekong "ghosts", the rarest of the rare of large-bodied Mekong fish species. An old fisherman, fishing the river every day for 60 years, may catch only one of these creatures in his lifetime. These Critically Endangered fish are extremely difficult to find in the wild, and so our team is developing new approaches to locating and documenting the existence of these fish. Most of them, like the giant barb, the giant stingray, the Chao Phraya catfish, and the Mekong giant catfish, are only seen once or twice a year. The most elusive species, the giant salmon carp, has not been seen by project-affiliated scientists in over 10 years. Could this unique predatory carp be extinct? We start our search in Phnom Penh, with a small team of biologist from the Cambodian Inland Research and Development Institute. We will be meeting fishermen in the field who we hope will be able to provide proof (photos, DNA samples, live specimens) of these extremely rare fish.

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Expedition Background

On May 1, 2005, the fishermen of Hat Khrai village, Thailand caught and killed at 646 pound Mekong giant catfish. The fishermen didn’t know it at the time, but they had just broken the record for the largest freshwater fish in the world. Or had they? This expedition seeks to answer that simple question: what is the world's largest freshwater fish?


Sponsored by National Geographic and the University of Nevada, this expedition, spanning six continents, focuses on finding, studying, and protecting the world’s largest freshwater fish. It has been an adventure that has stretched across the globe, from the most remote locations on Earth into our own backyards, and it has turned into a race against the clock to find and protect these ancient leviathans before they disappear forever.

The results so far have been at times inspiring, often depressing, and always new and exciting. We've found that beneath the surface of the world’s rivers and lakes swim dozens of species of mysterious aquatic giants, the real-life Loch Ness monsters the freshwater world. There are roughly thirty species that grow to over six feet long or weigh more than 200 pounds. They are a diverse assemblage of poorly understood creatures, from gargantuan gars to sumo-sized sting-rays. These enormous fish – some of which have been around for hundreds of millions of years – play critical roles in aquatic environments. Yet their numbers are dwindling. Threatened by overfishing, habitat loss, dams, pollution and climate change, more than 70 percent of the world’s freshwater megafish are today at risk of extinction.

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Feel free to email us if you have any questions. openexplorer@natgeo.com