A tank of captive-bred Chinese sturgeons about to be released to the Yangtze River.Credit: Xiao Yijiu/Xinhua via Alamy
Five fish species, including the iconic Chinese sturgeon, have gone extinct, or will soon be extinct, because of dams on the Yangtze River in China, according to a paper released on 10 May in Science Advances1. The findings have reignited a long-running debate among Chinese scientists about the best way to rescue the species in the Yangtze, with some saying that the analysis is flawed.
The Yangtze River is a mighty 6,300-kilometre-long waterway and a global biodiversity hotspot that runs through 11 Chinese provinces. But over the past 50 years, six major hydropower dams and more than 24,000 smaller hydropower stations have been built in the river’s main stream and branches — with even more on the drawing board.
The dams were built to help generate electricity, provide flood protection and make the river easier to navigate. But dams can block migratory fishes and damage their habitat. To mitigate the effects of the dams, fish-rescue programmes have been in place in various forms since 1982, when the first dam was being constructed.
Huang Zhenli, the deputy engineer-in-chief at the China Institute of Water Resources and Hydropower Research in Beijing, and his colleague Li Haiying developed an analytical tool that models the impact of the Yangtze River dams on its fish populations.
They focused on five iconic species: the Chinese sturgeon (Acipenser sinensis), the Yangtze sturgeon (Acipenser dabryanus), the Chinese paddlefish (Psephurus gladius), the Chinese sucker (Myxocyprinus asiaticus) and the largemouth bronze gudgeon (Coreius guichenoti).
By the time of the analysis, the paddlefish was already extinct. The Yangtze sturgeons are being kept alive only through captive-breeding programmes. The Chinese sturgeon is critically endangered. The International Union for Conservation of Nature lists the sucker as vulnerable, and the gudgeon as endangered.
The researchers’ modelling found that all five species will be entirely extinct or extinct in the wild by 2030.
David Dudgeon, a retired freshwater ecologist at the University of Hong Kong, says that the study is helpful in identifying the effect of the dams on the five species, particularly the understudied Chinese sucker. “There is nothing much that surprises me about the conclusions of the study,” he says. “It is good to see a well-integrated investigation of these five species.”
However, not all researchers are convinced by the study. Wei Qiwei, a conservation researcher at the Yangtze River Fisheries Research Institute, Chinese Academy of Fishery Sciences, in Wuhan, says that the authors’ work “deserves to be encouraged”, but disagrees with their conclusions.
Wei — who co-authored a 2020 paper2 that declared the Chinese paddlefish extinct — says the predictions that all species will be extinct or near extinct in by 2030 can’t be relied on because the parameters in the analysis are uncertain and difficult to quantify.
Xie Ping, a freshwater ecologist at the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences in Wuhan, agrees that it might be too soon to draw definitive conclusions from the models’ findings. “More needs to be done to cover more fish species in more geographic regions, so as to validate the effectiveness of the models and to optimize their parameters,” Xie says.
‘Six misjudgements’
The authors blame the dams, and the lack of specialized passageways for migratory fish to bypass the dams — known as fish-ladders — for the five species’ collapse.
“To prevent more migratory fishes from going extinct in China, [its] dam-related fish-rescue programmes must undergo fundamental changes,” Huang says.
As fish numbers continued to decline from the 1980s onwards, China stepped up its efforts to safeguard the ecology and environment of the Yangtze.
In 2021, it commenced a ten-year fishing ban and increased its restocking of the river with young, captive-bred fish.
The Wudongde Hydropower Station on the Jinsha River, an upper stretch of the Yangtze, became operational in 2020 — after the Chinese paddlefish was declared extinct.Credit: CFOTO/Future Publishing via Getty
However, the authors say that it was not enough. They describe “six misjudgements” of these fish-conservation campaigns, including that overfishing is the primary cause of the population declines; and that restocking is a “viable strategy” for mitigating the effects of the dams.
Wei and his team lead the scientific research behind the current conservation plan. He says that the dams’ impacts on fishes exist, but “one cannot ignore other factors”, such as overfishing.
“I believe if the 10-year fishing ban had been introduced to the Yangtze River 30 years earlier, the Chinese paddlefish would not be extinct. Nor would the Chinese sturgeon, the Yangtze sturgeon and the Chinese sucker get so close to extinction,” Wei notes.
As for restocking from captive-bred populations, he describes it as “the most important protection and restoration task” for the Chinese sturgeon and Yangtze sturgeon.
A 2023 study led by IHB researchers3 found that a 2017 pilot fishing ban introduced to the Chishui River — an upstream tributary of the Yangtze — was “an effective measure to facilitate fish resources recovery”.
Steven Cooke, a biologist specializing in fish ecology and conservation at the Carleton University in Ottawa, says that science-based restocking can work “quite well” in cases such as the white sturgeon (Acipenser transmontanus) in North America. “But if the habitat is degraded and fish can’t complete their life cycles, then stocked fish may not survive,” Cooke says.
Dudgeon, meanwhile, regards the paper’s criticism of restocking of the Yangtze as being “well-founded”.
“There is absolutely no evidence that sturgeon restocking has enhanced wild populations, despite the release of millions of cultured juveniles … [and] the fact that the practice has continued for many years,” he says.
Fishway or highway
Xie highlights that, for large and long-lived species such as sturgeons, conservation work is “very hard”.
Chinese sturgeons feed and grow near the sea when they are young and migrate more than 3,200 kilometres up the Yangtze to reproduce. “They spent at least 10 to 20 million years adapting to such a cycle,” Xie says, “They cannot adapt to the huge changes caused by humans within these few decades.”
Xie says that fish ladders might not be enough to save the sturgeons. “Fish passages in Europe and North America are mainly designed for relatively small-sized fishes, such as salmon. But sturgeons are mostly large and need a lot of space to swim in rivers,” Xie says. “Less than 2% of sturgeons are able to successfully navigate through the fish passages in dams,” he says.
Dudgeon says that, even when fish ladders work, the stillness of the water in the dam might not provide adequate cues to guide the fish upstream to complete their migration.
On the downstream journey, both adult and juvenile fish have to find a way to navigate the dam, locate the fish ladder and make a safe descent, he adds.
Dam removals: Rivers on the run
Some countries, such as the United States, France and the United Kingdom, have started to dismantle dams to re-establish migration corridors. When removal is not feasible, or fish ladders are ineffective, Xie and his colleagues suggested in a 2023 paper4 that building river-like side channels around hydropower dams is “the best way” to restore sturgeon migration routes and provide alternative habitats. Successful such cases have been observed in Russia, Canada and the United States, they noted.
Dudgeon says that, with so many complications, improving the situation for fishes in the Yangtze “will be challenging”.
Source: Ecology - nature.com
