Some of the most amazing creatures on Earth hide in rivers and lakes: giant catfish weighing more than 600 pounds, stripes the length of the Volkswagen Beetles, trout six feet long that can swallow an entire mouse.
There are about 200 species of the so-called freshwater megafauna, but compared to their terrestrial and marine counterparts, scientists study them little and little they know the public. And they are disappearing silently.
After an exhaustive survey of the entire Yangtze River basin, researchers declared this month that the Chinese rowing fish was officially extinct. The spatula fish, last seen alive in 2003, could grow up to 23 feet long and once inhabited many of China's rivers, but overfishing and dams decimated their populations.
Rowing can be an omen for many other giant fish. According to research published in August in Global Change Biology, freshwater megafauna has declined by 88 percent worldwide in recent years.
"This study is a first step," said Zeb Hogan, aquatic ecologist at the University of Nevada, Reno, and co-author of the research. "We want to go beyond the simple study of the state of conservation and look for ways to try to improve the situation of these animals."
For the relatively few scientists who focus on freshwater species, the news that the largest are disappearing is no surprise. Since Dr. Hogan began studying giant fish 20 years ago, he has witnessed the decline of many species, and now, the extinction of at least one, the Chinese rowing fish.
"The species that were rare when I started working on them are now critically endangered, and even some of the much more common ones have previously become rare," he said.
In his article, Dr. Hogan and his colleagues defined the freshwater megafauna as any vertebrate animal that spends an essential part of its life in fresh or brackish water and can weigh more than 66 pounds. They identified 207 of such species and combed the scientific literature for at least two population measurements for each, taken at different times.
The researchers found data that met those criteria for only 126 species. His list included mainly fish, but also mammals such as beavers, river dolphins and hippos, as well as cold-blooded creatures such as crocodiles, Giant salamanders and alligator turtles.
If more data were available, "the image would probably get worse," said Sonja Jähnig, an ecologist at the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin and lead author of the study.
According to the researchers' analysis, freshwater megafauna populations experienced an overall decrease of 88 percent between 1970 and 2012. Fish were the most affected, with a decrease of 94 percent. Fish in southern China and southern and southeastern Asia experienced the greatest overall losses, with 99 percent.
"The freshwater megafauna is the equivalent of tigers or pandas," said Ian Harrison, a freshwater scientist at Conservation International who was not involved in the research.
"There is a power in the message that these very charismatic species are extremely threatened, and that the threats they represent concern all species in freshwater systems."
According to the World Wide Fund for Nature, populations of freshwater animals in general are declining at rates more than double what was observed between terrestrial and marine animals.
A multitude of threats cause these declines, including overfishing, pollution, habitat degradation and diversion and water extraction. Dams, however, inflict the deadliest toll on giant fish, many of which are migratory.
According to research published in May, two thirds of the world's main rivers no longer flow freely. Hundreds of dams are proposed or under construction in river basins rich in megafauna, including the Amazon, the Congo and the Mekong.
"We face this challenge of how to balance the conservation of species with the human need for water," said Dr. Harrison. "The effects of climate change will make this challenge even greater."
The authors of the new study emphasize, however, that there are many strategies to ensure that freshwater giants survive, and that some signs of positive change are already present.
"We don't want to send a negative message to the public," said Fengzhi He, an ecologist at the Leibniz Institute of Freshwater and Inland Fisheries Ecology, and lead author of the study.
Conservation initiatives can and do work. People who live around Lake Winnebago in Wisconsin, for example, have tracked the lake's sturgeon population since the 1930s. The lake now houses one of the largest populations of that threatened species in North America.
Arapaima, a 10-foot long South American fish that breathes air, has disappeared from much of the Amazon River basin due to overexploitation. But fishing villages in Brazil that sustainably manage populations have seen the number of arapaima increase up to ten times.
In the United States, the protections provided by the Endangered Species Act have helped stabilize the declining populations of Colorado green sturgeon and pike perch.
Policy makers have also used the Wild and Picturesque Rivers Act to designate certain bodies of water as immaculate. The seven-foot-long green sturgeon on the Rogue River in Oregon is protected by this system, as is the American spatula fish on the Missouri River in Montana.
River restoration and dam elimination projects are also gaining popularity: 1,500 dams have been dismantled in the United States.
However, protections for freshwater bodies generally remain rare. While about 13 percent of the land in the United States is conserved, less than a quarter of 1 percent of its rivers are.
According to John Zablocki, river conservation consultant at Nature Conservancy, part of the problem is that people assume that rivers that cross land protected areas have the same protections by association. In fact, dams are often built inside national parks.
"The rivers are basically the red-haired stepson of the protected areas," he said. "If you look around the world, there are very few examples of rivers that are protected in some lasting way."
To change this, Mr. Zablocki, along with a growing group of scientists and advocates, is pushing for the creation of a global policy framework to protect rivers, something that has been in effect for a long time for marine and land systems. .
Meanwhile, grassroots interventions sometimes force a positive change in the absence of government commitment. Citizens of Bangladesh, New Zealand, Ecuador and several other countries recently obtained legal rights for rivers, which means that courts must treat these bodies of water as living entities.
Large dam projects in the Brazilian Amazon were suspended in 2018 after citizen protests and calls to move towards renewable energy. In 2012, protests in Chile contributed to the decision not to dam the Easter and Baker rivers, and instead install solar and wind farms for energy production.
In fact, as renewable energy prices fall, solar and wind energy are Become viable alternatives for hydroelectric power, especially in developing countries that have not yet divided their rivers with large dams, said Michele Thieme, a leading freshwater scientist at the World Wildlife Fund.
"We see a real opportunity in the developing world to make a leap forward and avoid the mistakes that have been made in other parts of the world," he said.
Cambodia, for example, recently gave the green light to a 60-megawatt solar park, although the country is still considering a large dam on the Mekong River that would block the migration of endangered fish and destroy the critical habitat for Irrawaddy dolphins. in danger of extinction.
While none of these strategies in isolation will save the entire megafauna of freshwater in the world, Dr. Hogan and his colleagues believe that, collectively, they can tip the scales of many species and help preserve freshwater biodiversity.
"These extraordinary fish make our life and experience on Earth more rich and valuable," said Dr. Hogan. "Do we want to live on a planet where we have killed all these incredible animals, or in one where we can find a way to coexist?"