Before she helped to release a 181-kilogramme giant stingray back into the Mekong River in May, Chea Seila had only seen parts of the pancaked-shape fish before -- sliced and being sold at local markets in Cambodia.
"Wow, it was so big," exclaimed Chea Seila, a conservationist who works as project coordinator with the Wonders of the Mekong project, recounting the May 5 release during a webinar run by the US-based Stimson Center later that month.
"I'd never seen a fish like that," she said. "I'd just seen small pieces of giant stingrays at the market, along with all the other big fish. But I'd never seen a live one."
The release of this endangered species, which was incidentally caught by fishermen in a remote part of the northeastern Cambodian province of Stung Treng, made headlines not only in Cambodia and around the world.
The 3.9-metre-long fish was caught in the waters near Koh Preah village, a small island in the Mekong that has nearly 28m-deep pools nearby.
The event drew attention to the bounty that makes the Mekong River invaluable to Southeast Asia and the world. But it was also a reminder of what could be lost amid a range of threats including dam development, overfishing, habitat loss, pollution and degradation, and climate change.
As the villagers and researchers released the stingray, the commune chief told the children who were watching on that "this is the giant stingray, which should be protected so your children can see it in the Mekong", Chea Seila recalled.
Stingrays, among many megafauna in the Mekong, live in the pitch-black waters of deep pools that descend up to 80m in a thriving subaquatic world unseen by human beings. It is here where species big and small spawn, and where one of the world's biggest animal migrations occur each year, a phenomenon that is intertwined with the seasonal flows of the Mekong river system.
Beyond a depth of 10m, "it's completely dark, totally silent and it's a world totally apart from the world that we live in", said Zeb Hogan, director of the Wonders of the Mekong project and a research biologist at the University of Reno in Nevada, United States.
"This area is the last place in the world where all of these megafauna, these endangered aquatic megafauna, are found together," Prof Hogan said. "This is an incredibly important and special area."
These pools, which are the deepest parts of the Mekong, are described as the last habitat of the world's largest freshwater fish, many of them endangered. Apart from the giant stingray, the giant barb, giant softshell turtle, the Mekong giant catfish and the last population of the critically endangered Irrawaddy dolphins are found here. The river is home to some 1,000 species.
Weeks before the stingray's release, Prof Hogan's team had been on an expedition that used unmanned submersibles to study the largely unexplored deep pools of the Mekong, in the area between Kratie and Stung Treng provinces towards Cambodia's border with Laos.
What National Geographic describes as "one of the most incredible animal migrations on the planet" begins in late September or early October, as the rains end and floodwaters of the swollen Tonle Sap lake -- which bring water and fish to Cambodia and Vietnam through the Tonle Sap River and then the Mekong -- ease.
With big species like giant catfish and giant barb leading in a migration that experts say takes four to six months, huge numbers of fish travel 1,000km or so from the Tonle Sap river near Phnom Penh, up the Mekong and to the deep pools in the Cambodian-Lao border.
Spawning occurs in these pools starting from May or at the start of the rainy season, a process that sees the production of at least 200 billion small fish.
This natural cycle is crucial to the Mekong river system, which feeds over 300 million people and provides livelihoods to millions. Local fishers have long known the river's rhythm, even its migration and breeding patterns.
Next to where the Wonders of the Mekong team were based for this year's expedition was a fish reserve the local community set up near the deepest spot of the Mekong in this part of Stung Treng.
For years now, fishing with commercial or bigger gear has been closed from the start of the spawning season in May.
It was the locals, who were part of the research, who called Chea Seila after finding the giant stingray had been caught.
"Trust has to be with us and also with the fishermen," she said. "It is very important."
Discussions about the management of fish species have built relationships that make for a joint research and protection effort.
"The fisherman (who caught the giant ray) was proud of himself, and how he had supported its release back into the river, because we do research with them," Chea Seila said.
In 2021, Chea Seila recalls, she got a call from a fisherman who had caught five giant fish he wanted to release back into the river. This is different from the past, she says, when locals tended to keep captures of megafauna to themselves, and usually chop them up to sell at the market.
But much more is needed to look after Mekong fisheries, since the nearly 5,000km river runs through six countries from the Tibetan plateau in China to Vietnam's Mekong Delta.
Two challenges are often discussed in relation to the Mekong -- around dam construction in different parts of the river and fishing pressure, including declining fish catches and overfishing.
Changes in the hydrological flow are disruptive to Mekong fisheries because they are highly migratory and their life cycles depend on the river's natural rhythms. They are very vulnerable to habitat fragmentation, including that caused by hydropower projects.
For instance, Prof Hogan explains, Mekong catfish used to migrate up through the Khone Falls further north in Laos, "but migration is now problematic because of a dam at the falls". The Don Sahong dam is in the Hou Sahong channel of the Mekong mainstream in Laos' Khone Falls, home to several fish-migration pathways near the Lao-Cambodia border.
"Some species, like catfish or eel, migrate about about 1,000km to get to the spawning grounds, so hydropower can block their migration routes," said Vi Vu An, a freshwater fish ecologist with the Inland Fisheries Research Group.
"If something happens with the hydrological regime, they might not know when they migrate and where they migrate."
Giant species do not recover easily. "They live very long, like up to a decade or century, and they produce very few eggs. The stingray, I think, give birth to one cub per season," he added.
Reports of declining fish catches in the Mekong region are common. In interviews with Mekong Delta fishermen that started six years ago, more than 90% said their catches have been going down and attributed this to "illegal fishing or fishing pressure", Vi Vu An explains. Mekong fishers report catching 2-3kg a day, down from 10kg a day in the 1990s, Prof Hogan says.
There have been several reminders of the threats to the Mekong, in events quite unlike the happy tale of the giant stingray's release.
A wounded Irrawaddy dolphin was found dead on May 12 downstream of the Kampi deep pool in Kratie, the third adult death reported in the first five months of 2022. The deaths of two calves in Cambodia were also reported this year.
On Feb 15, the last Irrawaddy dolphin that had lived in a transboundary pool at the Lao-Cambodia border was found dead off Kaoh Lngo Island, in the Preah Romkel commune in Stung Treng, with injuries from having been caught in fishing nets. "This death most likely represents a national-level extinction for Laos," World Wildlife Fund-Laos said. The Irrawaddy dolphin was declared functionally extinct in Laos in 2016, when only three were left.
In 2020, Cambodian fisheries officials reported the Irrawaddy dolphin population at 89 swimming in a 180km stretch of the Mekong in the country's northeast. ©Reporting ASEAN
Johanna Son is editor/founder of the Reporting ASEAN issues.