Each fish is as large as an adult male’s arm or thigh, with an average weight of 6-10kg. The potential devastation if these species were left to dominate our river basins is unimaginable. What’s more alarming is that both species have a lifespan of over a decade, and reproduce rapidly in large numbers.

“This is no longer a minor issue but a national concern,” said Azahar Khalid, a member of SFM who took part in the operation earlier this month.. “I’ve been an avid angler for the past 20 years and, over that time, we’ve noticed that native fish like the toman (giant snakehead) and haruan (striped snakehead) are becoming harder to find, when they were once abundant.”

During a previous operation, the group managed to capture over 200kg of armoured catfish, excluding other invasive species found in the same river.

Founded in 2020, SFM has 100 members and uses slingshot or catapult techniques with darts to catch invasive fish, particularly the armoured catfish native to South America.

“We use this harpoon-like method because the slingshot allows the dart to reach far distances, and the specially made metal darts can penetrate the thick scales of the armoured catfish. After that, we reel the dart in along with the fish,” Azahar explained.

Invasive fish that are caught are either sold to traders, disposed of, or turned into fertiliser.

Highly adaptable

Describing the dominance of invasive fish species as a “ticking time bomb” threatening native river fish populations, fisheries department director-general Adnan Hussain said the issue is difficult to address as many people fail to understand the impact of these species on the country’s river ecosystems.

“Invasive fish refers to species not native to the area but are introduced by humans for various purposes. For instance, tilapia and African catfish are brought in as protein sources, while koi and arapaima are kept as ornamental fish. The gambusia is used to control mosquito populations.

“Some invasive fish enter our waters unintentionally, such as through ship hulls, or escape from aquaculture enclosures,” he explained.

Invasive fish, he said, fall into two categories: “naturalised species” do not cause harm, and in fact have been economically beneficial to farmers, such as the Javanese carp and Siamese gourami.

“On the other hand, invasive species that disrupt ecosystems pose risks that ultimately threaten ecological balance, as seen with the freshwater redclaw crayfish, which has proliferated since its introduction over 15 years ago.”

Based on a study, the dominance of alien fish species like tilapia, African catfish, redtail catfish, and armoured catfish primarily originates from commercial farming and ornamental rearing.

Research by the department’s Glami Lemi Fisheries Research Institute reveals that the armoured catfish, redclaw crayfish, and peacock bass are highly dominant in the waterways of peninsular Malaysia.

Adnan noted that while official data on the impact of invasive species on local river fish landings is still being compiled, the department has received numerous complaints from fishermen about declining catches of high-value fish, as well as damage to fishing equipment caused by invasive species.

“More concerning is the fact that invasive fish adapt easily to their environment, even in polluted rivers,” he added.

Looming danger

Adnan emphasised that the presence of invasive fish in river ecosystems creates intense competition with native species for food, space, and breeding grounds.

“Invasive fish are more aggressive and reproduce rapidly, which can drastically reduce the population of native species. Predator species, in particular, prey on native fish, disrupting population balance.

“One such predator is the redtail catfish, which poses a threat to local species such as the temoleh, kelah, sepat, and freshwater prawns. This fish grows quickly, further compounding its threat to local species.”

Adnan also noted that some invasive fish burrow into riverbanks to create habitats and spawning grounds, altering habitat structures, affecting water quality, and indirectly damaging aquatic ecosystems.

Moreover, invasive fish can carry diseases or new parasites that can harm native species, he cautioned. Among diseases commonly associated with invasive fish are Edwardsiellosis, which can damage the liver and pancreas of fish; Columnaris, a disease that causes tissue decay in fish skin; and white spot syndrome, which threatens native species.

He warned that if left unchecked, invasive species could have a negative impact on the fisheries, aquaculture, and tourism industries, ultimately leading to economic losses for the country.

To tackle the issue, Adnan said the fisheries department has implemented various programmes, including regular studies and monitoring to assess the current status of waters affected by invasive fish species.

“We have also actively implemented mitigation programmes, such as alien fish-hunting initiatives. Since 2022, 15 such programmes have been organised, involving state governments, local communities, NGOs, associations, private companies and universities.”

Adnan highlighted the positive results of awareness campaigns conducted among various groups.

“More citizens are now helping to reduce the population of invasive fish. For example, a community project in Sungai Kuyoh, Selangor, has shown impressive results, with invasive-fish catches dropping significantly from 600kg at one time to 159kg more recently.”

Adnan said the department is taking measures to ensure the sustainability of native species like the patin buah, kelah and freshwater prawns through breeding and reintroduction into their natural habitats.

A total of 13 native fish sanctuaries have been established to protect the habitats of these species. Under the 12th Malaysia Plan, 23.9 million fish fry have been released into their original waters, supported by an allocation of RM1.5 million.

Additionally, Adnan shared that the department collaborates with universities to gain insights from researchers studying alien or invasive fish species, in addition to partnering with NGOs and local communities to restore habitats, raise awareness, and educate future generations.

“Through these efforts, the department hopes to rehabilitate river ecosystems, transforming them into thriving habitats rich with life. This will provide a sustainable source of livelihood and priceless natural beauty for future generations to cherish,” he concluded.