The discovery of the world’s largest freshwater fish last June in Cambodia – a giant freshwater whipray weighing a dainty 300 kilograms – caused global shock and awe.
A year later, early results from the first empirical study on the record-breaking stingray are filling in the murky details of how these endangered giants live and breed in the Mekong River.
Initial data indicates the section of the Mekong where the massive stingray was found, an area in the northeastern Cambodian province of Stung Treng where the riverbed drops into deep pools, is a habitat that can support the entire life cycle of a ray, from birth to death.
“We were very lucky to tag this world-record ray because it gives us a window into the life, ecology and habitat of this species,” said Zeb Hogan, programme lead for the U.S.-funded conservation project, Wonders of the Mekong. “We are doing this work to learn as much as we can about the species in order to protect it.”
Due to its distinctive habitats and biodiverse waters, this stretch of the Mekong is being considered for UNESCO World Heritage status. At the same time, however, the area is being studied for two new hydropower dams: the 900-megawatt Stung Treng Dam and the 2,300-megawatt Sambor Dam.
The former could guarantee stingray salvation with world heritage status. On the other hand, a growing body of research on dam development suggests the latter will likely cause a local extinction.
The range where the ray was found covers 180 kilometres of the Mekong from the Cambodia-Laos border down to the provincial Cambodian town of Kratie.
This area, representing just 4% of the 4,350-kilometre-long Mekong, is considered a global biodiversity hotspot. It features the Stung Treng Ramsar Site, a designation reserved for wetlands of international importance. As well as the Anlong Kampi conservation area, the potential world heritage site and home of the critically endangered Irrawaddy dolphin.
But even as the Cambodian government pushes for UNESCO recognition, which would increase expectations for the area’s protection, it has also steadily advanced plans for the area’s hydropower development. Both the Sambor and Stung Treng dams would theoretically span the Mekong’s mainstream – a practice controversial with environmental researchers who point to altered water levels, cut-off fish migration routes and blocked flows of vital sediments as reasons to avoid such construction.
Cambodia officially placed a decade-long moratorium on mainstream dam construction in 2020 but is allowing continued research and feasibility studies in Stung Treng, alarming conservationists.
A new paper featuring research from Wonders of the Mekong and published last month – titled “World Heritage, Hydropower and Earth’s Largest Freshwater Fish” – warned that dam construction in this river stretch could be devastating for biodiversity. These concerns were based on the tag data that found the stingray frequented the areas where the two dams have been proposed.
“The risk of extinction of these flagship species calls for attention to this critical juncture in the management of the Mekong’s natural resources and highlights the need to incorporate imperilled species and fishery data into future planning,” stated the paper.
The giant freshwater whipray is considered endangered by the International Union for the Conservation of Nature’s Red List of Threatened Species, which noted a 79% population decline in the past century.
The northern half of the Mekong in Cambodia is estimated to have lost 50-95% of its stingrays, while the southern half is believed to have lost 30-50%.
These concerns are backed by a second study released earlier this year – titled “Fish biodiversity declines with dam development in the lower Mekong Basin” – that studied seven years of fish monitoring data on Cambodia’s major rivers.
The findings showed that hydropower dams had caused significant declines in biodiversity and abundance in two of the Mekong’s largest tributaries in Cambodia, the Sesan and Sreypok. On the other hand, fish species were found to be thriving in the Sekong, which is one of the last free-flowing rivers in the lower Mekong basin.
With nearly 130 commissioned dams proposed across the basin, the looming crossroads in Stung Treng presents an existential debate between development and conservation mirrored throughout the Mekong.
The record-breaking stingray isn’t the first endangered regional resident to break the scales – it edged out the previous largest-ever freshwater fish, a 293-kilogram Mekong giant catfish, caught in Thailand in 2005.
The fisherman who netted the Moby Dick of stingrays last year notified Wonders of the Mekong, which rushed researchers to Stung Treng to outfit the stingray with an acoustic tag before releasing it. This is the first time one of these rays has been tagged in Cambodian conservation history.
Teresa Campbell, a contributing researcher to Wonders of the Mekong, is compiling this tag data with all known sightings of stingrays in the region.
Campbell said this information, upgraded with new acoustic technology, will lay a more expansive foundation of knowledge. Previous models of fish tags could only collect two data points – when the fish was released and when it was captured. Now, Campbell explained these acoustic tags paired with receiver arrays “allow us to track the stingrays in finer, detailed movements” by collecting multiple data points.
“No data like this has ever been collected before in Cambodia,” Campbell said. “The fact that Cambodia is doing this with such big individuals means we have the potential to learn so much valuable information, which is a game changer for preserving this species.”
Additional results from the acoustic tags showed the record-breaking female stingray hasn’t swam far from where it was captured.
Hogan – who recently published a book, Chasing Monsters: The Search for the World’s Largest Freshwater Fish – originally assumed that such a big fish would need space to swim or migrate like other megafish in the river.
“They seem to be staying in a remarkably small area of the river given their large size,” Hogan said. “It is good because that small stretch of river can be more easily protected and so there can be these pockets of self-sustaining populations.”
The review Campbell is working on found that the endangered species is facing a myriad of threats across its known range, including “fishing, bycatch, bottom trawls, pollution, and habitat destruction and fragmentation”.
She said only further research will confirm if this stretch of the Mekong in Cambodia is particularly significant for the species’ survival, but early signs point to that.
“It does seem like a special area for them but we need to learn so much more in order to confirm,” Campbell said.
The longer-term goal of this acoustic tag data is to build a species conservation action plan for the stingray, according to Hogan. A key step is bringing together researchers and conservationists for an international workshop, which Hogan hopes can be hosted in Cambodia in September.
“The Mekong is a transboundary river shared by six countries, all six countries bear responsibility for its survival,” Hogan said. “In a way, the stingrays are caught in the middle of a larger debate about the fate of the lower Mekong basin.”