KUALA LUMPUR - With a slingshot in one hand and a fishing reel in the other, freshwater fisherman Muhamad Nasrullah Maludin waited on a bridge over a river in Sentul, Kuala Lumpur.
"See that black shape breaking the surface? That's ikan bandaraya," Mr Nasrullah told The Straits Times, using the local moniker for the pleco fish that compares it to a city council's refuse collector, due to its habit of cleaning up algae and leftover food in aquariums.
Snapping into action, he released his slingshot and snagged one of the armoured invaders, then happily flung it into a gunny sack for later disposal.
Invasive species like pleco and African catfish have spread to 39 locations, including major rivers, dams and reservoirs, across Peninsular Malaysia and Labuan in Sabah, prompting warnings from fisheries officials that native fish populations face extinction.
Now, avid anglers like Mr Nasrullah are taking matters into their own hands, turning recreational fishing into an environmental rescue mission to save the country's rivers from ecological distress.
Malaysia's Department of Fisheries, which has been tracking the spread of fast-breeding invasive species through nationwide surveys since 2021, has warned that native fish such as lampam (tinfoil barb) and tengas (copper mahseer) risk disappearing.
Meanwhile, conservationists warn that invasive species like pleco, also known as bottom-dwelling suckermouth catfish, are driving native fish towards extinction and raising flood risks by destabilising riverbanks. Other unwelcome species include peacock bass, alligator gar and redtail catfish.
Dr Nur Azalina Suzianti Feisal, who studies invasive species at the Management & Science University in Selangor, called plecos "riverbank destroyers".
"The constant digging makes our river water permanently muddy," she said. "Just like plants on land, underwater plants need sunlight to survive. The cloudy water blocks the sun, causing these plants to die."
"As these big fish scrape the river bottom for food, they act like bulldozers, tearing up the plants that provide clean water, oxygen, and shelter for other aquatic life."
The pleco acts like a living vacuum cleaner, outcompeting native fish by stripping away algae - the main food source of many species, especially juveniles - leaving them to starve. They also monopolise hiding spots, forcing smaller native fish into the open where they become vulnerable, accelerating their decline.
Plecos burrow to build nests by digging deep tunnels into riverbanks. With thousands of holes weakening the banks like a sponge, heavy rain can cause them to collapse, sending mud and soil into rivers, said Dr Nur Azalina.
This clogs waterways, making them shallower and narrower, slowing the flow of rainwater and raising the risk of floods in nearby neighbourhoods.
Dr Intan Faraha A. Ghani, senior lecturer at the Faculty of Engineering and Life Sciences of University Selangor, said invasive fish thrive because they grow fast, reproduce quickly and adapt easily.
"They compete for food and habitat, prey on native eggs and even degrade the river environment itself," she told ST.
Introduced as pets to clean aquariums, invasive species like pleco have multiplied since the Covid-19 lockdown, as irresponsible owners released them when they outgrew their aquariums.
For Mr Nasrullah, 30, the "invasion" has brought financial hardship. Back in 2020, a day at the rivers would earn him RM200 (S$60). Today, he struggles to make a quarter of that, if lucky.
Rivers once alive with native species such as lampam, baung (mystus catfish) and patin buah (wild pangasius) are now crowded with pleco, tilapia and African catfish, he noted.
"Nine times out of 10, I catch pleco," he added. "I've had to find delivery work just to survive."
The scale of the problem is staggering. At the Denai River in Selangor, a community sweep pulled out 1.1 tonnes of pleco - compared with a mere 1.9kg catch of native snakehead fish, called haruan by locals, and others such as giant freshwater prawn and keli (freshwater catfish).
"It shows how far the balance has shifted," said fisherman Ahmad Nawawi Husin. "If this continues, our local species may vanish."
Mr Nasrullah and his peers in Slingshot Fishing Malaysia (SFM) spend their weekends eradicating the fishy "invaders". Formed in 2021 with about 100 members, the group now has 15,000 Facebook followers.
Slingshot fishing combines a handheld slingshot and a reel. The dart or arrow is tied to a fishing line, which is shot into the water when a fish is spotted.
While slower than using a net, in polluted rivers like those visited by ST, it is best to fish from dry land. The technique also provides the thrill of the hunt as it requires skill to operate the gear.
Department of Fisheries director-general Adnan Hussain has described the dominance of invasive fish species as a "ticking time bomb" threatening native river fish populations. "Invasive species that disrupt ecosystems pose risks that ultimately threaten ecological balance," he said.
Meanwhile, the department has tightened biosecurity rules and organised river sweeps, while also urging the public not to release aquarium fish into natural waterways, in efforts to protect biodiversity and native species. It is also finalising nationwide legislation against fish dumping, a measure that so far only the northern state of Perlis has put in place.
In Selangor, the state government, with support from the fisheries department, has gone a step further by offering RM1 per kg of invasive fish caught to encourage their removal. The initiative has been met with enthusiasm by grassroots fishing communities and clubs, in person and online.
On Facebook, besides SFM, there is Skuad Pemburu Ikan Asing (SPIA) or Invasive Fish Hunting Squad, with 27,000 followers.
Local anglers' efforts are paying off. The first series of the Selangor initiative in September 2024 netted 5 tonnes of invasive fish while the second series in December saw 11 tonnes. A third round is currently being planned.
"We need to hunt constantly. It doesn't wipe them out, but at least it gives our local fish a chance to breed and survive," Mr Nasrullah noted.
Businesses have also joined the effort, turning invasive fish hunts into part of their corporate social responsibility programmes, a shift from the usual tree-planting initiatives.
Invasive fish that are caught are either sold to traders, disposed of, or turned into fertiliser. Some even end up on the dinner table.
On a recent fishing trip, a group led by Mr Azahar Khalid netted 45kg of pleco and African catfish. The catch is collected and placed in a communal composting bin for breaking down into fertiliser.
Fishing communities like Mr Nasrullah's have also begun finding creative ways to make use of the aquatic invaders caught in cleaner waters. Once dismissed as inedible due to its tough exterior, pleco can be grilled as satay, turned into fried snacks or made into curries.
His group has experimented with 10 recipes, including buttermilk-battered fish and chips and pleco-patty burgers.
"Cooked right, it (pleco) tastes almost like chicken," Mr Nasrullah said. "But that depends on the water. If the river is clean and controlled, pleco and species like African catfish are safe to eat. If it's fished out of a polluted river, you don't want it on your plate due to its toxicity."
Other community groups have joined the fight, such as SPIA, which sells pleco roe to anglers and fish-pond owners as bait, and works with local researchers to turn invasive fish meat into feed pellets. River Monster Hunter's anglers target certain invasive species such as redtail catfish.
Experts warn that the risks may extend beyond Malaysia's borders. While Singapore's food supply may not be immediately disrupted, Dr Nur Azalina cautioned that its reservoirs and waterways are equally vulnerable.
"A single unwanted pet (fish) in a reservoir can alter the ecosystem. Invasive fish outcompete local species, reduce biodiversity and upset the balance that keeps freshwater systems healthy," she told ST.
Agreeing, Dr Intan said: "Such ecological shifts can be long-lasting and costly to manage. Once invasive fish become widespread, eradication is almost impossible."