Puchong, Malaysia – On a recent Sunday morning, about a dozen men with fishing nets skirted the rubbish-strewn banks of the Klang River just outside the Malaysian capital of Kuala Lumpur.

Surveying the river, the men cast their nets into the polluted water. The nets billowed open and sunk quickly under the weight of metal chains.

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From where they stood on the riverbank, they started to pull in their nets, already filled with dozens of squirming black-bodied catfish.

“You don’t see any other fish. Only these,” said Mohamad Haziq A Rahman, the leader of Malaysia’s “foreign fish hunter squad”, as they emptied their catch of wriggling suckermouth catfish into piles, away from the river.

None of the fish caught that morning were sold at nearby markets or food stalls. The sole purpose of the expedition was to cull suckermouth catfish, one among a growing number of invasive species that have in recent decades dominated freshwater habitats across Southeast Asia.

Once brought in for commercial or hobbyist reasons, invasive fish are not only threatening to edge native species out of the food chain in Malaysia and elsewhere, but they also spread diseases and cause great damage to local environments.

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Invasive fish are a problem the world over, but experts say the issue is keenly felt in mega-biodiverse Malaysia.

“More than 80 percent of rivers in the Klang Valley have been invaded by foreign fish species, which can cause the extinction of the rivers’ indigenous aquatic life,” said Dr Kalithasan Kailasam, a river expert with the Malaysia-based Global Environment Centre.

“It’s growing in almost all other main rivers in Malaysia,” said Kailasam, explaining how species such as the suckermouth have the potential to quickly reproduce and survive in dirty water, leaving local fish on the losing side.

Aside from the suckermouth, Malaysia’s waterways are now threatened by species such as the aggressive peacock bass, Javanese carp and redtail catfish, he said.

While the full extent of the problem is not yet known, Malaysia’s fisheries department, after a four-year study until 2024, found invasive species in 39 areas across nearly every state in peninsular Malaysia and on the island of Labuan, including in dams, lakes and major rivers.

Alarmed by the threat, a small group of citizens banded together to fight the aquatic invaders.

Led by Haziq, they are working to reclaim Malaysia’s rivers one fin at a time.

Invasive fish invasion

The citizen fish hunters’ quest to fight invasive species started during the country’s COVID-19 lockdowns, when Haziq, a former healthcare consultant, turned to fishing as a pastime in a river near his house in central Selangor state. He found every fish he caught was of the suckermouth variety, also known as the “pleco” or “ikan bandaraya” – which translates as the “janitor fish” in Malay and is favoured by hobbyists to keep aquariums clean, as the suckermouth feeds on algae, leftover food and dead fish.

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Native to South America, varieties of the suckermouth have also been introduced into waterways in the United States, Bangladesh, and Sri Lanka, often when owners dump them into rivers, canals, dams or free them after they grow too large for their aquarium tanks.

Because of their thick, scaly skin, suckermouths are usually avoided by even larger predators in Malaysia, and can grow to about half a metre (1.6ft) in length.

As bottom feeders, the catfish have been known to eat the eggs of other species and destroy their nesting sites. Catfish also burrow into riverbanks to nest, causing them to erode and collapse, which is a serious environmental issue in flood-prone Malaysia where year-end monsoon winds bring heavy rain.

Malaysia’s central bank said in 2024 that floods are the cause of 85 percent of the country’s natural disasters, with their frequency increasing since 2020.­

Though far from his favourite fish to catch, Haziq discovered that suckerfish roe could be used as bait for other bigger fish, and he earned some money selling their eggs to other fishing enthusiasts. He also gained a following by putting his exploits on social media. Further research then led him to learn about the threats posed by invasive species.

Harziq started to attract like-minded anglers, and, in 2022, they decided to form a group for hunting suckermouth, meeting nearly every week in a river to carry out a cull.

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Their public profile and popularity are growing. The group’s membership has now grown to more than 1,000, and it has a strong fan following on social media.

“People kept asking how to join our group, because we were looking at the ecosystem,” Haziq said.

Focusing first on Malaysia’s Selangor state and rivers in the capital Kuala Lumpur, the fish hunter squad netted nearly 31 tonnes of suckermouths alone in 2024. They have also visited rivers in other states in Malaysia as their campaign expands.

Dispose, use for research, or cook and eat?

During a hunt in the Klang River earlier this year, Haziq and his comrades deployed to the river’s banks on a mission to see how many suckermouth they could catch during a single outing.

But hunting for invasive fish can be tricky. Without boats, the hunters have to wade into the fast-moving polluted waters from muddy banks, while navigating underwater debris such as rubbish on the riverbed.

Almost all the fish they caught were of the invasive kind, but once in a while, they do net a local.

“Haruan (snakehead)!” shouted ex-navy diver Syuhaily Hasibullah, 46, as he showed off a small fish half the size of his arm, taken from a net containing several suckermouths.

“This one is rare! There used to be a lot of them in the river,” he told Al Jazeera.

Haziq said if the hunters found many invasive species in their nets, they would organise another outing to the same location, bringing along more people to take part.

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The day they set out to calculate how many invasive fish they could catch in a single outing turned out to yield half a tonne of suckermouth in just three hours – so many they had to stuff them into sacks.

Previously, the hunters buried their hauls in deep holes away from the river. Now, they have found more creative ways to dispose of what is, generally, an unwanted fish.

At the event earlier this year, sacks of suckermouths were handed over to a local entrepreneur looking to experiment with turning the fish into a form of charcoal known as biochar.

Some local universities have also started researching the possible use of the suckermouth. One university research article explored the potential of suckermouth collagen for pharmaceutical use, while another considered its use as fertiliser or even as a type of leather.

On some occasions, the hunters even eat the fish they catch, though that depends on which river they have been taken from.

While redtail or African catfish are considered delicacies by some, the suckermouth, also known in India as “devil fish”, is a less attractive snacking option – but not out of the question when it comes to a quick riverside grill.

“If the fish is from the Klang River, we don’t eat it,” Mohd Zulkifli Mokhtar told Al Jazeera, before dozens of hunters broke their fast during the holy Muslim month of Ramadan.

“But if it’s from the Langat River, it’s still OK,” Zulkifli said, as dozens of suckermouth caught in the less polluted Langat River, located in Bangi some 25km (16 miles) south of Kuala Lumpur, were gutted, marinated in satay and grilled on skewers.

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Studies from Bangladesh and Indonesia have found varieties of catfish with high levels of heavy metals and contaminants. A 2024 article by Malaysia’s Universiti Teknologi Mara cited a study that showed the level of contaminants in the suckermouth was “heavily influenced by the level of pollution in the river”.

‘If we don’t act now, it would be worse’

While Malaysia’s fisheries department said there were no records of local species becoming endangered because of invasive ones, native fish nevertheless face threats.

Local fish either faced becoming prey or have had to fight to survive, with the department finding in a survey that 90 percent of the fish in six rivers in the Selangor and Kuala Lumpur region were now foreign arrivals.

The department’s Director-General Adnan Hussain said various measures had been put in place, including the release of some 33.6 million native fish and prawns into rivers nationwide from 2021 to 2025 to “balance the impact” of invasive fish.

Late last year, the state government of Selangor also came up with a scheme to pay anglers one Malaysian ringgit ($0.23) for every kilogramme (2.2lb) of the suckermouth fish removed from two rivers. The captured fish were to be turned into animal feed and organic fertiliser, an official said.

Restrictions on the import of certain foreign aquatic species – including entire species and groups – into Malaysia were also imposed last year, and he added that programmes and collaboration with the fish hunters had also helped to deal with the problem.

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In one river in Selangor state, Adnan said the amount of invasive fish caught following one eradication programme had dropped from 600kg (1,300lb) in a May 2024 event to just more than 150kg (330lb) four or five months later.

However, Universiti Malaysia Terengganu fish researcher Professor Amirrudin Ahmad said it was “almost impossible” to fully exterminate the country’s invasive fish.

“So many species live in (native water bodies) and getting rid of invasive species by the means of poisoning the water is not feasible at all,” he said, adding there were close to 80 recorded fish species introduced in Malaysia so far.

He further warned that rising temperatures caused by climate change may even allow species like the predatory Mekong redtail catfish to proliferate in cooler upstream waters in Malaysia.

“They are here to stay,” Amirrudin said.

“It is simply,” he said, “that the environment is mostly similar to their native country, or these species are highly adaptable.”

That this is an ecological war that can never truly be won is a point that Haziq and his fellow fish hunters are fully aware of. Nearly every river they visited in recent times had almost nothing but invasive fish, he said.

But their mission will carry on, he added, along with the hunting and public awareness that has spurred thousands to follow his social media videos on the subject.

“Yes, this fish won’t be completely gone from our rivers,” he told Al Jazeera.

“But if we don’t act now, it would be worse,” he said.

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“It’s better to take action than to just leave it alone,” he added.

“At least we can reduce the population, than allow it to completely take over our local fish.”