杏吧原创

鈥淭HEY are the most threatened group of organisms on the planet,鈥� says biologist Ivan Jari膰. 鈥淢ore than 70 per cent of species are critically endangered, some are almost gone.鈥�

He isn鈥檛 talking about the usual suspects: great whales, great apes or the corals of the Great Barrier Reef. He is talking about great fish. Specifically, sturgeons and paddlefish. Together they span 27 species, but 17 are in the most precarious category on the red list of endangered species.

Actually, make that 26 species. Earlier this year, a team including Jari膰 that one of the greatest of them all, the giant Chinese paddlefish, is almost certainly no more. It hasn鈥檛 been seen in the Yangtze river basin since 2003 and a recent exhaustive search failed to find any. 鈥淭he chance it still exists is very, very low,鈥� says Jari膰, who is at the Czech Academy of Sciences Institute of Hydrobiology.

Sturgeons are the hardest hit of a group of animals that rarely make the headlines, even in conservation biology circles, but this group is declining faster than any other. They are collectively known as 鈥渇reshwater megafauna鈥� 鈥� monster fish such as sturgeons, giant catfish, river sharks and rays, along with river dolphins, porpoises, seals, manatees, crocodiles, alligators, snakes, turtles and salamanders.

鈥淭he river megafauna are hidden below the surface of human perception鈥�

All told, there are more than 200 species of freshwater megafauna; most are in deep water and some are probably already doomed to extinction. Yet they are largely overlooked by efforts to save the world鈥檚 biodiversity. 鈥淚t really is a neglected area,鈥� says Sonja J盲hnig a the Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Berlin, Germany.

The Chinese giant paddlefish (Psephurus gladius) would never have made it onto a list of the world鈥檚 most beautiful endangered species. But its demise has turned it into a poster child for the dire conservation status of the world鈥檚 last remaining pool of megafauna.

We are used to thinking of megafaunal extinction as something that happened many thousands of years ago, as humans spread around the world and, probably not coincidentally, ran into the last mammoths, ground sloths, giant flightless birds and many more. Some terrestrial megabeasts survived, but in the past 50,000 years about two-thirds of these species . One place where they survived was in freshwater, probably for the same reason that we ignore them today. 鈥淭hey are hidden below the surface of human perception,鈥� says J盲hnig.

But if we looked, we would be amazed. There are 206 living freshwater megafaunal species 鈥� defined as those that can exceed 30 kilograms, about the size of an adult golden retriever. Three-quarters of the world鈥檚 major river basins are home to at least one, with the Amazon, Congo in Africa, Orinoco in South America, Mekong in South-East Asia and Ganges-Brahmaputra basins especially rich in them.

Many can grow much bigger than the 30 kg lower limit for this oversized club. In 1931, a Chinese biologist claimed that paddlefish in the Yangtze near Nanjing, China, could reach 7 metres in length, although the largest recorded in more recent decades before they vanished are only about half that. That is still a very big animal (see 鈥�The river giants鈥�). The largest freshwater fish in the world is the beluga sturgeon (Huso huso), which is can reach . A small adult whale shark is about the same size.

Final swimmer

The Chinese paddlefish was one of two remaining species of an ancient lineage of fish (Polyodontidae) that evolved in the Jurassic and survived the extinction of the dinosaurs. The last one swimming is now the American paddlefish (Polyodon spathula), which is found in small numbers in the Mississippi river basin.

Its Chinese cousin had been in trouble for years before finally gasping its last. They were once common, with around 25 tonnes caught every year for food. But a in Sichuan province in the summers of 1974 and 1975 found that large specimens were already rare, a fact that was attributed to overfishing.

In 1981, the huge Gezhouba hydroelectric dam was built across the Yangtze, preventing paddlefish from migrating upstream to their ancestral spawning grounds. This seems to have been the last straw for a species already under pressure from overfishing, habitat destruction, shipping and pollution. The 鈥� a 3.6-metre-long female 鈥� was unexpectedly caught in 2003 near Yibin in central China. A rescue programme was launched in 2005, but didn鈥檛 work, and an extensive survey of the entire Yangtze basin in 2017 and 2018 failed to locate a single one of these fish. 鈥淕iven that there hasn鈥檛 been a reliable sighting for so long, there鈥檚 not much hope,鈥� says extinction biologist Dave Roberts of the University of Kent, UK. In all likelihood, the last one actually died between 2005 and 2010 and the species was functionally extinct 鈥� that is, unable to reproduce 鈥� by 1993. There are no specimens in captivity and hence no prospect of a comeback. The scientists who broke the news described the extinction as a 鈥渞eprehensible and irreparable loss鈥�.

It isn鈥檛 the only large animal to have disappeared from the Yangtze in living memory. The baiji, aka the Yangtze river dolphin (Lipotes vexillifer), hasn鈥檛 been sighted since 2002 and is too.

Others are going the same way. The paddlefish鈥檚 close relative, the Chinese sturgeon (Acipenser sinensis), is on the , as are the Chinese alligator (Alligator sinensis), Chinese giant salamander (Andrias davidianus) and the Yangtze giant softshell turtle (Rafetus swinhoei).

Smaller fish are also in trouble. There are ; the paddlefish survey failed to find 140 of them. Most of these are probably highly endangered, according to the team from the Chinese Academy of Fishery Sciences which carried out the survey.

The annihilation of the Yangtze biosphere is entirely down to human activity. Since the 1950s, the 6300-kilometre river 鈥� the longest in Asia 鈥� has undergone explosive development. A third of China鈥檚 population, some 400 million people, live close to what is now the world鈥檚 busiest river; there are more than 40 cities beside it including the vast metropolises of Chongqing, Wuhan, Nanjing and Shanghai. Until the Gezhouba dam opened in 1981, it was free-flowing; other dams including the immense Three Gorges have since been added and or under construction.

And there is more to come. In 2016, the government unveiled the to promote further development. In the face of this immense pressure, the paddlefish survey scientists recently warned that the Yangtze aquatic ecosystem is in danger of collapse.

The Yangtze is just a drop in a very large freshwater ocean. All over the world, freshwater ecosystems and their megafauna are in trouble. According to a , 鈥淭hey are among the most threatened ecosystems globally鈥�.

Some river systems are in even worse shape than the Yangtze. On a measure of threat to biodiversity, the Yangtze scores 0.822. Worst of all is the Danube (0.912), closely followed by the Mississippi (0.900), Shatt al-Arab (0.898) and the Orange (0.858). According to another , even though rivers and lakes (excluding wetlands) cover just 1 per cent of Earth鈥檚 surface, they harbour around a third of all vertebrates and half of fish species. Freshwater vertebrates are declining faster than their terrestrial and marine cousins.

Despite this, freshwater ecosystems are neglected in conservation policy. They really ought to be seen as a separate 鈥� and uniquely vulnerable 鈥� category alongside terrestrial and marine ecosystems, but are usually just lumped in with the former. 鈥淚f you look at how freshwaters are represented in conservation frameworks, they lack their own goals,鈥� says J盲hnig. Freshwater biologists were planning to lobby the 2020 UN biodiversity conference in China for better recognition, but the meeting has been postponed until at least 2021 because of the covid-19 pandemic.

The animals bearing the brunt of this neglect are the 80 or so species of really big fish 鈥� not just sturgeons, but also giant catfish and carp, electric eels, lungfish, freshwater rays, river sharks and more. Since 1970, their populations have declined by 94 per cent on average. Most of them are endangered; some are probably doomed.

Vital yet vulnerable

These aren鈥檛 just aesthetic losses. Ecologists regard megafauna as . Large animals, for example, eat and excrete a lot and so are vital for the nutrient cycling that other, smaller species rely on.

Big fish are especially vulnerable for a number of reasons, says J盲hnig. 鈥淭hey reproduce really late [in life] so they need to have the right conditions for a long time; some need 10 or 15 years to mature. They have relatively few offspring, and they require a big habitat. Many are migratory, so they wander up and down stream and that means if there is a barrier like a dam, they get into trouble.鈥�

鈥淭here are multiple threats,鈥� agrees Jari膰. 鈥淒amming, pollution and big pressure from the human population.鈥� That includes being hunted for food. Overfishing helped to see off the Chinese paddlefish, and has reduced megafish populations in the Mekong 鈥� the most heavily fished river system in the world 鈥� to 鈥渃lose to zero鈥�, says J盲hnig. In this part of Asia, species in the firing line include the Mekong giant catfish (Pangasianodon gigas), giant Siamese carp (Catlocarpio siamensis) and shark catfish (Pangasius sanitwongsei).

Overfishing has also been the scourge of sturgeons (see 鈥�Caviar catastrophe鈥�). But despite this generally gloomy picture, there are glimmers of hope. The near-certain extinct status of the Chinese paddlefish, on top of the earlier loss of the river dolphin there and the generally rotten state of the Yangtze, seems to have focused the minds of the Chinese government, says Jari膰, who works closely with freshwater biologists in the country.

鈥淭he paddlefish was a wake-up call in China,鈥� says Jari膰. After this, he says, there was a meeting between researchers working on fish in the Yangtze, the International Union for Conservation of Nature, conservation group WWF and the government to develop some new actions. 鈥淭here seems to be a will to protect the Yangtze鈥檚 fish stocks and ecosystems,鈥� he says. 鈥淪o there could be positive effects from this.鈥� One immediate action was a in 332 designated areas, which will be extended to the whole river and all of its tributaries next year. That is a start, says Jari膰, but fishing bans don鈥檛 always work and can make matters worse. 鈥淚f there is not good control, people just switch to illegal fishing, and then there鈥檚 even less control.鈥�

And fishing bans don鈥檛 address the bigger issue of dams. New ones usually have fish gates to allow migratory species to swim freely up and down stream, but they rarely work as well as advertised, says Jari膰. 鈥淓ven if they are constructed, they are usually not as efficient as we want, some migration occurs through them, but not as much as is needed.鈥�

Dams remain an issue for river animals worldwide. A found that only 37 per cent of those over 1000 kilometres are completely free-flowing. These are restricted to the Amazon and Congo basins and remote regions of the Arctic. 鈥淚t鈥檚 pretty unrealistic to say don鈥檛 build any dams,鈥� says J盲hnig. 鈥淏ut if we really want to build them 鈥� which is really a questionable thing, they have so many negative effects 鈥� can we move them to a certain part where maybe biodiversity won鈥檛 be that much affected?鈥�

Despite these obstacles, efforts to save the megafish are under way. In Europe, for example, sturgeon restoration projects are gathering momentum. The last remaining wild population of the European sturgeon 鈥� which 150 years ago was common in every major river system connected to the North Atlantic, including those in the UK 鈥� is clinging on in the Gironde river in France. Ditto the Adriatic sturgeon in the Po in Italy. Captive breeding programmes are attempting to restore those populations to healthier numbers, while programmes are also under way to to rivers in Poland and Germany. Whether these can succeed isn鈥檛 clear, says Jari膰. 鈥淲e don鈥檛 know if they can establish stable populations, the rivers are still heavily fished, polluted and degraded.鈥�

鈥淭he animals bearing the brunt of this neglect are the 80 or so species of really big fish鈥�

In the Americas, the picture is brighter. 鈥淭he Amazon river basin is reasonably fine still,鈥� says J盲hnig. On the Juru谩 river, which feeds into the Amazon in Brazil, sustainable fishing projects have even allowed the pirarucu, an air-breathing megafish, to make a .

In North America, strict conservation measures have allowed other species to turn the corner. The Alabama sturgeon is probably lost and gone forever, but the lake and Atlantic sturgeons are showing signs of recovery. 鈥淣one of them are really doing well,鈥� says Jari膰. 鈥淭hey are still critically endangered, but many populations have a positive trend.鈥�

In the Mississippi, meanwhile, another critically endangered megafish seems to have dodged extinction and has a shot at recovery. The US river is even more degraded than the Yangtze, but somehow its own weird fish, the American paddlefish, is doing relatively well. The loss of its Chinese cousin remains a reprehensible and irreparable loss. But maybe after 50,000 years of big animal extinctions at the hands of one uniquely destructive megafauna, we are finally learning to look after the giants that remain.