Chasing Giants: The Struggle to Protect the World’s Largest Freshwater Fish

Tuesday - August 15 2023, 22:15 UTC - 1 year ago

The identity of the world’s largest freshwater fish species long remained a mystery until 2022, when a giant freshwater stingray was caught in the Mekong River, surpassing the previous record holder. The discovery marked a milestone in fish biologist Zeb Hogan's two-decade quest to study and protect giant freshwater fish, which are among the most endangered animals on the planet. The decline of these creatures is due to human activities such as overfishing, dam building and climate change.

Rivers have been the lifeblood of human civilization throughout history, and yet we know surprisingly little about what lives in many of them – including the giant creatures that prowl their depths.

While we know the biggest animal in the ocean is the blue whale and the largest marine fish is the whale shark, the identity of the world’s largest freshwater fish species long remained a mystery.

Until 2022, that is, when fishers in Cambodia caught a giant freshwater stingray in the remote reaches of the Mekong River. Weighing an astounding 661 pounds, the stingray surpassed by 15 pounds a giant catfish caught in Thailand in 2005 that had previously been considered the unofficial record holder.

The discovery marked a milestone in fish biologist Zeb Hogan’s more than two-decade quest to study and protect giant freshwater fish. As a group, these megafish are among the most endangered animals on the planet.

Before releasing the female ray back into the river, Hogan’s research team put an acoustic tracker on her. She has been sending back clues about stingrays’ elusive behavior ever since.

Colossal catfish and gargantuan gars .

In a new book, "Chasing Giants: In Search of the World’s Largest Freshwater Fish," Hogan and I tell the troubling story of the 30 or so fish species that live exclusively in rivers and lakes and can grow to more than 200 pounds or at least 6 feet (1.8 meters) long.

Found on all continents except Antarctica, they are a wonderfully weird bunch of creatures, from colossal catfish and carp to gargantuan gars.

But freshwater vertebrate populations have declined over the past five decades at twice the rate experienced by species within terrestrial or marine ecosystems. Megafish numbers in particular fell by a shocking 94%, according to one study of more than 200 large freshwater species.

One of the largest species, the Chinese paddlefish, is believed to have gone extinct sometime in the 2000s. "This is a fish that had been on Earth for more than 100 million years before disappearing in a flash," says Hogan, who used to host National Geographic’s "Monster Fish" television show and now leads a University of Nevada, Reno, research project I am involved with called Wonders of the Mekong, supported by the U.S. Agency for International Development.

The importance of very large fish to freshwater ecosystems has been woefully understudied. Many giant freshwater fish are apex predators that can have profound effects on the ecosystems in which they live by keeping their prey populations in check and maintaining biodiversity.

What’s killing off the megafish .

The decline of giant freshwater fish is due to human impacts, such as overfishing, dam building and climate change.

Large fish are disproportionately targeted by fishing. Since many of these species are slow to mature, they may never reach the age to reproduce. Dam building is another major threat, because large fish often need to make long migrations to complete their life cycles, and a new dam can block their migration paths.

In the Mekong, where more giant fish species are found than in any other river, climate change is causing more severe droughts and flooding that are straining fish communities.