PETALING JAYA: Terengganu’s Tasik Kenyir, Malaysia’s largest man-made lake, was once teeming with native fish species like tengas, lampam and kelah.
Not anymore. The native species have been almost wiped out. And it’s not the only lake affected.
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Several other water bodies across the country, including Tasik Timah Tasoh, Tasik Telabak and rivers in the Klang Valley, are also now in a critical state as invasive species continue to dominate.
Invasive ornamental fish, often released into lakes by breeders or pet owners, are killing off the local fish.
One predatory species, the peacock bass (ikan raja), is now literally the king of many lakes, having eaten up most of the native fish.
“They lack natural predators, are bred to be hardy and can tolerate environmental stress. Many are also fast breeders,” said marine scientist Alvin Chelliah.
Once these species establish themselves, they begin to disrupt the natural balance of the ecosystem, a pattern already observed in lakes such as Tasik Telabak and Tasik Timah Tasoh in the Klang Valley.
“Invasive fish feed on native fish, including their eggs and larvae. They also consume insects, algae and aquatic plants, all key food sources for local species,” he said.
He added that invasive species also compete for food, shelter and breeding grounds, often outcompeting native fish. Some also cause physical damage, particularly in rivers like Sungai Klang and Sungai Langat.
“Invasive fish like the suckermouth catfish (ikan bandaraya) burrow into riverbanks, causing erosion, making rivers shallower and the water murky, disrupting the entire ecosystem.”
Once these species take hold, controlling them becomes a major challenge.
“It is very difficult to remove invasive fish without harming native species,” said Chelliah.
Marine life expert Dr Syafiq Musa described the issue as a classic example of biological invasion, where non-native species establish themselves and outcompete local fauna.
“They are highly tolerant of poor water quality, low oxygen and pollution, which allows them to thrive, especially in degraded or urban waterways where native biodiversity is already weakened,” he said.
He added that the absence of natural predators – known as “enemy release”– allows populations to grow unchecked.
Dr Syafiq said the impact extends beyond predation and competition to the structure of ecosystems.
“These species can trigger what we call trophic disruption, where the entire food web is destabilised.
“Some even consume fish eggs, leading to ‘recruitment failure’, where there are no young fish to replace ageing populations,” he said.
Eradication is very difficult.
“Rivers and lakes are interconnected systems, allowing invasive species to spread and recolonise quickly. Management then becomes long-term control rather than complete removal,” he added.
Malaysian marine scientist Prof Datuk Dr Aileen Tan Shau Hwai said the problem often begins with human behaviour.
“It often starts with a simple act, releasing an unwanted ornamental fish into a nearby water body. It may seem harmless, but it can trigger a chain reaction that disrupts the entire ecosystem,” she said.
She noted that Malaysia’s tropical climate allows these species to adapt and reproduce quickly.
“Without natural predators or environmental limits, their populations can grow rapidly and spread across connected waterways,” she said.
Dr Aileen said Malaysia was already seeing long-term impacts in several locations, including rivers in the Klang Valley and lakes such as Tasik Timah Tasoh and Tasik Telabak.
“In many cases, ecosystems do not return to their original state even after intervention,” she said.
