Thailand's Notorious Invasive Fish: A Silent Threat to Livelihoods and Ecosystems

The menu at Kor-Tae seafood restaurant, in Thailand’s Samut Prakan province, is filled with Thai classics – from tom yum talay, a fragrant hot and sour soup, to spicy larb salads. But the restaurant’s chef is also experimenting with a more controversial ingredient: blackchin tilapia.

“People are hesitant, but once they try it – [they say] it’s delicious,” says owner Adisorn Jamsuksaward, who has been offering the non-native fish free of charge to friends who request it.

Cooking is a way to make use of some of the alien species that has infiltrated his pond, but Adisorn admits it is unlikely to ever be a bestseller. Blackchin tilapia is probably Thailand’s most notorious fish.

Native to west Africa, the fish was first reported to be spreading in Thailand in 2011, in Samut Songkhram, south of the capital Bangkok. Since then, it has proved unstoppable, upending the livelihoods of shrimp farmers and coastal fishers in the province. It has spread further across central and southern areas, infiltrating at least 19 provinces, according to reports – from the canals of Bangkok to the coastal waters of the tourism hotspot Pattaya. It’s feared the fish could travel further, crossing Thailand’s borders.

The invasion in Samut Songkhram was so severe that local fisher Wallop Khunjaen was forced to abandon shrimp farming altogether. In the space of two months, blackchin tilapia ate nearly all of the million baby shrimp in his pond. “They ate everything. They ate the shrimp, they even ate the crab,” says Wallop. Native fish have become less common locally, and he has stopped seeing some species, such as fiddler crab.

“I told [government officials], if the blackchin tilapia had feet, they would climb up your fisheries department office,” Wallop says.

Thai authorities have increased control measures over recent years: releasing Asian sea bass as a potential predator; developing blackchin tilapia that will produce sterile offspring; and paying people to fish for it. Thousands of tonnes have been removed, according to officials.

Experts say it is too late to eradicate the fish, however. “We have gone far beyond the point that we can return,” says Thotsapol Chaianunporn, assistant professor at the department of environmental science at Khon Kaen University.

Blackchin tilapia reproduces rapidly and juvenile fish have a high survival rate, allowing it to quickly reassert itself after control measures are taken. It is also an adaptable fish, and can thrive in either brackish or freshwater conditions.

The most sustainable option, Thotsapol says, is to find economic uses for blackchin tilapia, either making products such as animal feed – or eating it.

Fish invasions have become a growing global threat, often linked to aquaculture or the aquarium trade.

Researchers have developed new ways to monitor fishes’ presence – from the analysis of water samples for environmental DNA, known as eDNA, to underwater facial recognition cameras that can identify species, and acoustic tracking to monitor fish movements.

Sampling eDNA is the most powerful monitoring tool, says Dean Jerry, distinguished professor in aquaculture genetics at James Cook University in Singapore. All organisms, at all times, shed DNA, he says: “Fish are particularly big shedders.”

Jerry likens a sample of water to a “DNA soup”, which researchers can use to detect specific species, or – through a “meta-barcoding” approach – to reveal the entire communities of fish, or other organisms, present. Such tools are most useful when introduced early.

The source of the invasion in Thailand is disputed. Fishers have filed a lawsuit against the agribusiness giant Charoen Pokphand Foods Plc, accusing it of introducing the fish. It legally imported 2,000 blackchin tilapia for breeding research in 2010, but says all imported fish were destroyed and rejects the claims. In a statement the company said its facility operated “under a closed-system protocol” and pointed to research, funded by its parent conglomerate Charoen Pokphand Group, which concluded the invasion was not the result of a “single introduction”.

The full impact on ecosystems is still unknown. Aside from outcompeting native species for food and habitat, it is feared blackchin tilapia may also degrade water, as females initiate courtship by digging pits in the sand, increasing sedimentation. “When thousands of females dig holes, it’s going to affect the benthic invertebrate, or even the fish down there,” Thotsapol says, referring to the spineless organisms that live at the bottom of a water body. It could also affect the photosynthesis of underwater plants.

Blackchin tilapia also feed on zooplankton, which plays an important role as it feeds on algae, keeping blooms in check, Thotsapol says.

Countries such as Australia have introduced poison to kill invasive fish – doing so after larger native wildlife has been relocated and sections of the waterway dammed. Restaurant owner Adisorn wishes this were possible in Thailand.

Some blackchin tilapia products have been developed, including fermented fish sauce, and it is used for animal feed. But fishers say the fish, present in such vast quantities, is not profitable.

Few in Thailand, which prides itself as the kitchen of the world, want it to eat it. Tongta Samtia, 61, a trader at a fish market in Samut Prakan, tried to sell blackchin tilapia but says nobody bought it. “They don’t know how to eat it,” she says. It’s not a beautiful fish, she adds.

Besides, no one wants to eat it every day, adds Thanandon Charoenhiransaku, a fishing-net seller and fisher in Bangkok, who emphasises the scale of the crisis.

“When you cast the net three times, you can easily get 20 to 30kg of blackchin tilapia – that’s how much is in our canals,” he says.

He only hopes it won’t spread any further – or to Thailand’s neighbours.

“It’s a silent threat,” he adds. “You don’t notice until it’s there.”