In quest of the splendidly bizarre giants lurking in Earth’s rivers

(The Dialog) – Rivers have been the lifeblood of human civilization all through historical past, and but we all know surprisingly little about what lives in a lot of them – together with the enormous creatures that prowl their depths.

Whereas we all know the most important animal within the ocean is the blue whale and the most important 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 a large freshwater stingray within the distant reaches of the Mekong River.

Weighing an astounding 661 kilos, the stingray surpassed by 15 kilos a large catfish caught in Thailand in 2005 that had beforehand been thought of the unofficial report 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.The world’s largest freshwater fish confirmed to this point is a stingray caught within the Mekong River.

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

Colossal catfish and gargantuan gars

In a brand new ebook, “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 stay solely in rivers and lakes and may develop to greater than 200 kilos or not less than 6 toes (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.

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 surprising 94%, in response to one research of greater than 200 giant freshwater species.

Zeb Hogan, left, tags a 400-pound big catfish that was caught in fishing nets on the Tonle Sap River in Phnom Penh, Nov. 6, 2003. Hogan, a UC Davis researcher, has spent three years monitoring big catfish in Cambodia. “Right here we’ve got a species that runs a danger of being the primary species within the Mekong River to go extinct,” stated Hogan. (AP Picture/Andy Eames)

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 undertaking I’m concerned with known as Wonders of the Mekong, supported by the U.S. Company for Worldwide Growth.

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 through which they stay by retaining their prey populations in test and sustaining biodiversity.

What’s killing off the megafish

The decline of big freshwater fish is because of human impacts, resembling overfishing, dam constructing and local weather change.

Massive fish are disproportionately focused by fishing. Since many of those species are sluggish to mature, they might by no means attain the age to breed. Dam constructing is one other main risk, 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 growing calls to explicitly embody freshwater ecosystems within the 30×30 initiative, a world effort to put aside 30% of land and sea space for conservation by 2030. Up to now, nevertheless, conservation efforts to guard endangered big freshwater fish species are largely regional.

Alligator gars and sturgeon make a comeback

Though the outlook for many big fish stays grim, some species, 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 toes, has lengthy been overharvested by fishers within the Amazon, the place it’s generally known as the Amazonian cod. However stricter fishing rules 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 sport 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 necessary contributor to ecosystem features. Right now, alligator gar populations have bounced again in rivers just like the Trinity in Texas.

An analogous 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 seasonal leisure fishing.

Defending the giants of the Mekong

Again in Cambodia, our Wonders of the Mekong undertaking 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 through which it was sprinkled with flowers earlier than it was let go.

In current months, our undertaking 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.

This April 11, 2022 photograph supplied by the Mississippi Division of Wildlife, Fisheries and Parks and brought in Jackson, Miss. exhibits Eugene Cronley of Brandon and the report setting 131-pound (59.4-kilogram) blue catfish he caught, April 7 within the Mississippi River close to Natchez. (Blythe Summers/Mississippi Division of Wildlife, Fisheries and Parks, through AP)

Whereas big freshwater stingray numbers have plummeted in different components 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 report ray was found. Knowledge collected from that feminine, and reported in a research I co-authored, exhibits it’s staying in a lot the identical location, main researchers to imagine the world could possibly be an necessary 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, not less than for now. On the finish of 2022, the federal government as a substitute put forth a proposal to show the biodiverse stretch of the river, which can be house to a critically endangered inhabitants of Irrawaddy river dolphins, right into a UNESCO World Heritage web site.

Whereas the report stingray is large, it won’t be the most important of this species of ray within the Mekong. Native fishers communicate of rays rising as much as 200 kilos heavier.

It’s additionally doable the enormous stingray just isn’t the most important 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 wonderful creatures to determine how one can higher shield them.”

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