This story was initially printed on The Dialog and seems right here underneath a Artistic Commons license.

Rivers have been the lifeblood of human civilization all through historical past, and but we all know surprisingly little about what lives in lots of them—together with the large creatures that prowl their depths.

Whereas we all know the most important animal within the ocean is the blue whale and the biggest marine fish is the whale shark, the id of the world’s largest freshwater fish species lengthy remained a thriller.

Till 2022, that’s, when fishers in Cambodia caught an enormous freshwater stingray within the distant reaches of the Mekong River.

Weighing an astounding 661 kilos, the stingray surpassed by 15 kilos an enormous catfish caught in Thailand in 2005 that had beforehand been thought of the unofficial file holder.

The invention marked a milestone in fish biologist Zeb Hogan’s greater than two-decade quest to check and shield big freshwater fish. As a gaggle, these megafish are among the many most endangered animals on the planet.

Earlier than releasing the feminine ray again into the river, Hogan’s analysis staff put an acoustic tracker on her. She has been sending again clues about stingrays’ elusive habits ever since.

In a brand new guide, Chasing Giants: In Search of the World’s Largest Freshwater Fish, Hogan and I inform the troubling story of the 30 or so fish species that dwell completely in rivers and lakes and might develop to greater than 200 kilos or at the very least 6 ft (1.8 meters) lengthy.

Discovered on all continents besides Antarctica, they’re a splendidly bizarre bunch of creatures, from colossal catfish and carp to gargantuan gars.

The Mekong’s big catfish can develop to lengths and weights a lot bigger than these of the people catching them. Zeb Hogan

However freshwater vertebrate populations have declined over the previous 5 a long time at twice the speed skilled by species inside terrestrial or marine ecosystems. Megafish numbers specifically fell by a stunning 94 p.c, based on one research of greater than 200 giant freshwater species.

One of many largest species, the Chinese language paddlefish, is believed to have gone extinct someday within the 2000s. “It is a fish that had been on Earth for greater than 100 million years earlier than disappearing in a flash,” says Hogan, who used to host Nationwide Geographic’s “Monster Fish” tv present and now leads a College of Nevada, Reno, analysis venture I’m concerned with referred to as Wonders of the Mekong, supported by the U.S. Company for Worldwide Improvement.

The significance of very giant fish to freshwater ecosystems has been woefully understudied. Many big freshwater fish are apex predators that may have profound results on the ecosystems by which they dwell by retaining their prey populations in test and sustaining biodiversity.

The decline of big freshwater fish is because of human impacts, comparable to overfishing, dam constructing and local weather change. Giant fish are disproportionately focused by fishing. Since many of those species are gradual to mature, they could by no means attain the age to breed. Dam constructing is one other main menace, as a result of giant fish typically have to make lengthy migrations to finish their life cycles, and a brand new dam can block their migration paths.

Within the Mekong, the place extra big fish species are discovered than in every other river, local weather change is inflicting extra extreme droughts and disrupting the monsoon seasons that govern the river’s important flood regime.

There are indicators that curiosity in freshwater species is gaining momentum, together with rising calls to explicitly embody freshwater ecosystems within the 30×30 initiative, a worldwide effort to put aside 30 p.c of land and sea space for conservation by 2030. To date, nonetheless, conservation efforts to guard endangered big freshwater fish species are largely regional.

Arapaima, present in South America, are sometimes over 200 kilos and might develop a lot bigger. Zeb Hogan

Though the outlook for many big fish stays grim, some species, just like the air-breathing arapaima in South America, could also be bucking the development.

The arapaima, a torpedo-shaped big that may develop to lengths of greater than 12 ft, has lengthy been overharvested by fishers within the Amazon, the place it’s often known as the Amazonian cod. However stricter fishing laws launched by Indigenous communities seem to have led to populations’ rebounding in lots of locations.

In the USA, the alligator gar, one other air-breathing megafish, was as soon as largely thought of a “trash fish” thought to devour recreation fish, so it was systematically exterminated from a lot of its southern vary. However then scientists started to check the species and located it was an vital contributor to ecosystem capabilities. As we speak, alligator gar populations have bounced again in rivers just like the Trinity in Texas.

An identical case includes the lake sturgeon, one of many few true freshwater sturgeons, whose populations in Wisconsin have benefited from long-term conservation efforts and science-driven administration that features strictly regulated seasonalrecreational fishing.

Again in Cambodia, our Wonders of the Mekong venture is elevating public consciousness in regards to the plight of the megafish, and we’re working intently with native fishers to encourage them to guard threatened species. In an instance of these efforts paying off, fishers in early 2023 caught a Mekong big catfish weighing greater than 200 kilos. As an alternative of killing it and promoting the meat for a large revenue, the fishers determined to launch the fish in an elaborate ceremony by which it was sprinkled with flowers earlier than it was let go.

In current months, our venture has additionally reintroduced into the Mekong uncommon big catfish that have been captured as younger fish and raised in Cambodia, and big barb, one other critically endangered megafish species that traditionally has grown to 600 kilos.

Large freshwater stingrays, like this one, can breathe air. Stefan Lovgren

Whereas big freshwater stingray numbers have plummeted in different elements of its native Southeast Asian vary, the inhabitants seems to be comparatively sturdy within the higher stretches of the Mekong River in Cambodia the place the file ray was found. Information collected from that feminine, and reported in a research I coauthored, exhibits it’s staying in a lot the identical location, main researchers to imagine the realm could possibly be an vital refuge for the stingrays and presumably different megafish.

Lengthy-standing plans by the Cambodian authorities to construct two giant dams on this part of the river seem to have been scrapped, at the very least for now. On the finish of 2022, the federal government as an alternative put forth a proposal to show the biodiverse stretch of the river, which can be dwelling to a critically endangered inhabitants of Irrawaddy river dolphins, right into a UNESCO World Heritage website.

Whereas the file stingray is large, it won’t be the biggest of this species of ray within the Mekong. Native fishers converse of rays rising as much as 200 kilos heavier.

It’s additionally attainable the large stingray will not be the biggest freshwater fish species. Analysis on the arapaima, for instance, suggests it may develop as large, and even larger, in locations like Guyana. However, as Hogan says, “It’s not about discovering the most important fish. It’s about studying extra about these superb creatures to determine the best way to higher shield them.”

Stefan Lovgren is a analysis scientist in river ecosystems on the College of Nevada, Reno.