GUANGDONG, South China — In Dongshan Lake Park, a patch of relief-giving greenery in downtown Guangzhou, fish loll in a polluted lake, nibbling at waterweed. Gu Dan’gen pulls a wriggling net out of the water and empties its contents into a plastic cooler. A passerby looks on, wide-eyed. “Don’t worry: They’re not for eating,” Gu calls to the woman. She grins. “Of course they aren’t!” she replies. “Look how dirty the water is — those fish would poison you!”

Virtually every fish in the tub is a tilapia, Gu says. Given their faint stripes and long, thin dorsal fins that swoop down their backs like Mohawks, tilapia are relatively easy to identify. But their eye-catching appearance belies the fact that they’re one of China’s most damaging invasive species.

Tilapia are native to Africa, where they have been fished since ancient times. Nowadays, they are also farmed in aquaculture ponds all over the world. Chinese fish farmers began importing tilapia around 40 years ago, raising them in the warm waters of the country’s southern provinces. At the time, farmers appreciated the fish’s rapid reproduction rate and ability to survive in contaminated water — traits that made tilapia easy to breed in large numbers. Officials saw tilapia as a way for the long-undernourished southern population to raise its protein intake.

In 2016, China produced 1.8 million tons of tilapia — almost 3 times the amount for 2001, according to the China Fishery Statistical Yearbook 2017. Around half of them are exported, with the U.S., Mexico, and Russia as the biggest buyers. Domestic consumers, too, continue to savor tilapia’s meaty texture and mild flavor. In southern China, where locals often serve it in clear broth or braised in soy sauce, tilapia even has a nickname: fushouyu, or “the fish of good fortune and longevity.”

But tilapia are no longer confined to Chinese fish farms, wet markets, and dinner tables. Within a couple decades of those first imports, the fish entered southern China’s streams, rivers, and lakes. Once in the wild, the same hardiness that made tilapia a boon to farmers also allowed it to flourish — at the expense of native species.

“Tilapia’s spread [into natural waterways] has been incredibly aggressive,” says Gu, a rangy, tanned researcher at Guangzhou’s Pearl River Fisheries Research Institute who has studied invasive fish species for eight years. “They compete for food and space with native species, churn up the riverbed, and gobble up waterweed. All of this affects the entire water ecosystem.” Even in some parts of their native Africa — Lake Victoria, for example — fast-breeding tilapia have been blamed for accelerating the extinction of hundreds of native fish species.

According to China’s Ministry of Ecology and Environment (MEE), the introduction of alien species into the country’s waterways poses existential threats to 49 endangered kinds of fish — around 17 percent of all species on the MEE’s red list of all endangered inland fish. Other major threats to China’s inland fish populations include habitat loss, overfishing, and hydropower projects. Invasive species threaten biodiversity and ecological stability in a number of different ways: They prey on native species, outcompete them for food and space, destroy the habitats of native fish, or carry diseases that can be fatal to native species.

“As native species suffer significant biodiversity loss due to the worsening aquatic environment and other human impacts, you get these empty ecological niches, which make it easier for alien species to invade and establish themselves,” Gu explains. “These species mostly originate from aquaculture. [They] warn us that if we destroy the environment, then we increase the risk of invasion, which leads, in turn, to further degradation. It’s a vicious cycle.”

The Pearl River, which extends 2,400 kilometers across several southern Chinese provinces, is home to nearly one-third of all the country’s inland fish species, the MEE says. But the region is also one of the worst affected in China by biological invasion. Guangdong, the province where the river empties into the Pacific Ocean, produced 10 percent of China’s farmed freshwater fish in 2016, earning 58 billion yuan ($8.8 billion) in the process, while nationwide, freshwater aquaculture production increased by 39 percent between 2012 and 2016, according to China’s 2017 fishery yearbook. More than one quarter of that output comes from farming non-native fish: Hundreds of alien species have been brought into Guangdong in the past century. Although not all of them pose threats to native species, around a dozen of them have become invasive, including the tilapia, the African sharptooth catfish, and the Mrigal carp.

Gu currently leads a project aiming to catalog the distribution of invasive fish species across the Pearl River region. His tub of park-caught tilapia is destined for a laboratory at his research institute in another part of Guangzhou, where the 32-year-old cuts through each animal’s belly, checks its sex glands, and examines its otoliths — ear bones that grow layer by layer each year, like tree-rings. These help Gu identify the animal’s age, level of sexual maturity, eating patterns, and even whether it was bred on a farm or born in the wild.

As the world’s largest producer and exporter of fish products, China relies on farming alien species in order to meet growing domestic and overseas demand for fish, says Liu Jianyi, a researcher at East China Sea Fisheries Research Institute in Shanghai. “As a country with a large population, food supply is a key concern. These non-native fish grow quickly … so they’re considered ideal for introduction into China,” Liu adds. “But official risk evaluations of the introduction of alien species emphasize financial profits over ecological impact.”

Farmed fish invade natural waterways in a number of ways. Sometimes, poorly maintained drainage systems on Chinese fish farms carry individual animals into nearby rivers and lakes through outlet pipes. Other fish are deemed too small to be profitable by fish farmers, who simply dump them into local water sources. And still more escape when flooding causes aquaculture enclosures to overflow into natural water sources — a major concern, since Guangzhou is among the world’s cities most threatened by rising sea levels.

Further risks come from the growing trend of farming exotic fish species for sale to owners of private aquariums. According to a 2015 report by Xiong Wen, a lecturer at Guangdong Ocean University, 349 species of aquarium fish have been introduced into China. And in 2016, more than 4 billion freshwater aquarium fish were farmed nationwide, a rise of 15 percent on the previous year, according to the country’s fishery yearbook. Many such fish are bought through poorly monitored online sellers and end up in rivers and lakes after their owners abandon them, Xiong says.

In Guangdong, fishers have already witnessed the invasion of the suckermouth catfish, an aggressive species often kept in aquariums because they eat tank silt and help to keep the water clean. In the wild, the catfish eat the juvenile fish of other species. At a reservoir near the city of Zhanjiang, there are so many suckermouths that official catch-and-kill schemes since 2015 have disposed of 50 tons of the fish per year, on average.

State-sponsored efforts to restore freshwater ecosystems are frequently stymied by the very officials who run them. It is common practice to release non-native species of juvenile fish into the wild in an effort to heal damaged underwater habitats. But sometimes, these fish go from saviors to invaders. In western China’s highland lakes — unique ecosystems that evolved largely in isolation — the introduction of alien fish has pushed native fauna to the edge of extinction. A similar cause of invasion is the Buddhist practice of fangsheng, in which religious adherents buy living animals and release them into the wild.

When he’s not netting his own fish, Gu, the Pearl River researcher, seeks out tilapia at local fish markets. In Qingyuan, a city of around 4 million people in northern Guangdong, fishers gather along river banks before sunrise and later sell their catches in waterside parks. Gu wanders through the early-morning throng, occasionally stopping to ask a fisher about the price of different specimens, or weighing them on electronic scales.

On one side of the path, 61-year-old fisher Li Yaoguang is hawking his morning catch, about one quarter of which is tilapia. It’s a normal proportion these days, but Li remembers a time when tilapia were a rarity. “There were no tilapia around here before the 1990s,” he says, adding that the population seems to have grown rapidly in the last 15 years. Tilapia are now so common that Li prefers to sell native fish with higher economic value, such as the spotted longbarbel catfish. But increasingly, he is finding fewer and fewer native fish to go around.

Gu worries that the overfishing of native species is only accelerating the tilapia’s rise. “Fishers earn more from catching native fish than from catching tilapia, so they prioritize native species to maintain their incomes,” he says. “This piles more pressure onto native species, leaving us with fewer local fish and more alien species. It’s a dead loop.” His research team has developed a poison designed to kill tilapia, but because its additional ecological and health impacts remain unclear, Gu has not yet received permission to deploy it. Even if he had, a tilapia cull would likely draw strong resistance from local fishers. “They’ve now got an economic stake … in catching and selling the fish,” he says.

Liu Huanzhang, a researcher at the Institute of Hydrobiology, a Wuhan-based research organization associated with the Chinese Academy of Sciences, says that by the time fishers and scientists realize there’s an invasion going on, it’s usually too late to fix it. “Compared to invasive plants and insects, invasive fish are harder to identify,” Liu says, adding that inland invasions tend to interfere with human agricultural production in much more visible ways than waterborne ones. “By the time people notice them, [invasive fish] have already had a huge impact and are very difficult to control.”

China has a number of laws and regulations designed to protect native animal populations. A 1992 quarantine law controls the entry and exit of potentially harmful plants and animals in the country, and guidelines from 2009 aim to manage the multiplication and release of aquatic organisms. But neither dwells much on what to do once invasive species have established themselves in great numbers.

Xiong, the author of the 2015 report on invasive aquarium species, says that more and more non-native freshwater fish will enter China’s waterways in the absence of robust national laws and specialist monitoring networks. “We must conduct more research to evaluate the risk of fish invasion and design policies accordingly,” Xiong tells Sixth Tone, “and probably should forbid certain areas from introducing alien species altogether.”

Gu, meanwhile, says that policymakers must urgently prioritize habitat conservation, in order to stabilize the aquatic environment and build resistance to biological invasion. “Fish are the top underwater species, and the slightest change in one part [of the local ecology] can affect the situation as a whole,” he says. “The impacts of biological invasion on ecosystems are long-term and irreversible.”

Editor: Matthew Walsh.

(Header image: Fishers prepare for a morning fish market in Guangdong province, Sept. 13, 2018. Li You/Sixth Tone)