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World’s largest freshwater fish found, released in the Cambodian Mekong River by Global Water Center team

What has long been one of the great wildlife mysteries – the identity of the world’s largest freshwater fish – appears to have been solved last week as fishers in northern Cambodia, working with an international team of scientists including from the University of Nevada, Reno’s Wonders of the Mekong research team, discovered a 661-pound (300 kilos) giant freshwater stingray near a remote island in the Mekong River.

The startling size of the endangered fish, whose weight was confirmed by scientists as twice that of an average lowland gorilla, makes it larger than the 646-pound (293 kilos) Mekong giant catfish caught in Thailand in 2005, which was the previous record- holder for largest freshwater fish on Earth. Freshwater fish are defined as those that have spent their entire lives in freshwater, as opposed to giant marine species like tuna and marlin, or fish that migrate between fresh and saltwater like the huge beluga sturgeon.

The record-breaking stingray, which measured over 13 feet (almost four meters) from snout to tail, was hooked by a fisher from Koh Preah island south of the town of Stung Treng, in the Mekong River as it runs through northern Cambodia. Recognizing the importance of his catch from him, the fisher quickly contacted a team from the USAID-funded Wonders of the Mekong research project to help release the ray, an endangered species, back into the river.

Scientists measure the world’s largest freshwater fish.

“This historic event highlights the success of USAID’s Wonders of the Mekong project,” USAID/Cambodia Acting Mission Director Hanh Nguyen said. “Cambodia is blessed with incredible biodiversity and we are proud of the team’s efforts to promote sustainable management and raise public awareness on the important role of the Mekong River.”

For Zeb Hogan, a fish biologist in the College of Science’s Department of Biology, who leads the Wonders of the Mekong, the stingray find is evidence that the natural world can still yield new and extraordinary discoveries, and that many of the largest aquatic creatures remain woefully understudied.

“In 20 years of researching giant fish in rivers and lakes on six continents, this is the largest freshwater fish that we’ve encountered or that’s been documented anywhere worldwide,” Hogan, who is also the host of National Geographic’s “Monster Fish” television series, said. “This is an absolutely astonishing discovery, and justifies efforts to better understand the mysteries surrounding this species and the incredible stretch of river where it lives.”

The huge ray was fitted with an acoustic tag, technology that will enable biologists to learn more about the secretive creature’s elusive behavior, a first for a stingray in Cambodia.

acoustic tag being placed on stingray
Stingray being fitted with an acoustic tag to track its movements in the river.

“Tracking the fish and identifying critical habitats will help the Wonders of the Mekong project develop additional environmental safeguards for the river and its communities,” Sudeep Chandra, co-director of Wonders of the Mekong and also the director of the University of Nevada, Reno’s Global Water Center, said.

In collaboration with the Cambodian Fisheries Administration, the Wonders of the Mekong project established a network of fishers who agreed to report catches of giant and endangered fish, including stingrays. Last month, fishers in Koh Preah reported to the team that they had caught a 400-pound giant female stingray, which the research team then helped release safely into the depths of the river.

“Fishers now cooperate with our project when they find giant stingrays so that we can tag and release them,” Chea Seila, program manager for Wonders of the Mekong, said. “These successful releases illustrate the importance of partnerships with local communities. Together, we all have an important role to play in fisheries monitoring and conservation.”

For the world record fish, fishers called to inform the team that a “much bigger” ray had been caught during the night of June 13. The stingray was safely returned to the river, it appeared strong and healthy as it quickly descended into the murky depths of the Mekong.

close up of stingray
Fishers named the stingray Boramy, or full moon in English, as she was released as the full moon was rising at dusk.

A crowd that included international scientists, Cambodian fisheries officials, and community fisheries members gathered for the release of the large female stingray, which they named “Boramy,” or “full moon” in the Khmer language, because the round-shaped fish was released at dusk with the moon shining on the horizon.

“The discovery of this world record stingray indicates the special opportunity we have in Cambodia to protect this species and its core habitat,” said His Excellency Poum Sotha, Delegate of the Royal Government, Director General of the Fisheries Administration. “In partnership with the Wonders of the Mekong project, and together with other countries in the Lower Mekong Basin, the Inland Fisheries Research and Development Institute of Cambodian Fisheries Administration will host a meeting to map out a regional species conservation action plan and solidify safeguards for the river, wildlife, fisheries and local communities.”

“Research is vital to solving today’s conservation and development challenges,” Mary Melnyk, who leads USAID Asia’s Environment Division, said. “Wonders of the Mekong research will be used to identify how best to safeguard the species and productivity of the Mekong River as it’s affected by both climate change and growing demands for infrastructure development.”

This hotspot has conservation measures in place, including community fish reserves, fishing restrictions in freshwater dolphin habitat and river guards. But the stingrays are also vulnerable to changes throughout the Mekong river basin such as pollution, habitat fragmentation and unsustainable hydropower development. In Thailand, populations have plummeted with mass mortality events due to water pollution and disrupted life cycles from river alterations.

Worldwide, more than 30 species of freshwater fish grow to over six feet long and more than 200 pounds. They are a diverse array of remarkable freshwater creatures, from air-breathing 500-pound plus arapaima to 325-pound alligator gar and carp as big as grizzly bears. There is cause for concern, as recent studies show that global populations of giant freshwater species have plummeted by almost 90% in the past four decades—twice as much as the loss of vertebrate populations on land or in the oceans.

“This fish record highlights the important connection between people, fish, and freshwater. It is a signal to us to protect our rivers and lakes. Large fish are bellwethers of the health of freshwater ecosystems worldwide – the fish are sending us a message and we need to listen to it.” Hogan said.

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