A suckermouth catfish kept in an aquarium. Image by Maxmann via Pixabay. Used under a Pixabay License

A highly invasive vermiculated sailfin catfish, originally from the Madeira River Basin in South America, is poised to be a major threat to native fish in the rivers of Eastern Nepal. Around four years ago, in July 2020, a pair of suckermouth catfish was found in an irrigation canal in Belbari, Morang District of Eastern Nepal. The news quickly went viral in news outlets and social media. Similar reports emerged from the Lohandra River in October 2021, with anecdotal evidence of the fish being found as early as May 2018 in a pond in Dhanusha District, approximately 200 kilometres west of Morang District.

Vaskar Nepal, Assistant Professor of Biology at Western Illinois University, tweeted:

Non-native suckermouth catfish in Nepal. Second such capture in the country. Unfortunately, it was released back. Experts suspect that the fish was released from aquarium trade. https://t.co/cWBHzsPyj2

— Vaskar Nepal, PhD (@NepalVaskar) October 31, 2021

A team led by Jash Hang Limbu, a researcher with the College of Fisheries and Life Science at Shanghai Ocean University, studied the prevalence of this fish in river systems of Eastern Nepal. According to a research paper published in the journal BioInvasions Records, the team collected 43 individuals of Pterigoplichthys from the Lohandra River. The fish were identified as Pterigoplichthys disjunctivus, commonly known as vermiculated sailfin catfish or sailfin suckermouth catfish.

How did a fish from South America land in Nepali waters?

Speaking online with Global Voices, Limbu said, “Although the fish was sighted for the first time in Belbari, it has spread across places like Belbari, Pathari and Ratuwa River. It has an established population in Lohandra River.” He added that a different species of fish of the same genus was recently found in a canal in Damak, a town in the Jhapa District of Eastern Nepal.

“The fish might have escaped from an aquarium or might have travelled northwards from Indian rivers,” Limbu said with uncertainty.

Researchers in neighbouring India also found the same species in the waters of the Ganges River, West Bengal during their surveys in 2018. Since the rivers from Nepal flow into the Ganges and other rivers, some speculate the fish swam northwards to infiltrate Nepal's waterways.

Likewise, the vermiculated sailfin catfish has been recorded in 17 rivers across Bangladesh, with well-established breeding population in four localities.

A highly invasive ornamental fish

Commercially traded and often kept in aquariums, these fish can survive in all sorts of environmental conditions — ranging from cool, fast-flowing, highland streams to slow-flowing, warm lowland rivers and even in swamps, floodplain lakes and stagnant pools with polluted water and low oxygen level. Named after its suctorial mouth which can attach to surfaces, the suckermouth catfish eats detritus and algae and has an enlarged stomach which appears to function as an accessory respiratory organ.

Although their effect on native fish in the river is not comprehensible, researchers opine that they might threaten the native inhabitant species by disrupting the aquatic food chain and competing with native fish for food. A food competition experiment was carried out in Bangladesh by a team led by Md. Taksin Parvez demonstrated that the growth and survival rate of native fish can be adversely impacted in the presence of sailfin catfish.

As they are voracious feeders and have no natural predators, their number might increase rapidly enough to displace native fish species.

How to stop the spread of suckermouth catfish?

With its ability to adapt to any kind of environment, the vermiculated sailfin catfish, if not stopped spreading in Nepal’s waters on time, might pose a high risk to the native species and livelihoods of traditional fishermen, suggest Limbu and the team.

Conducting mass awareness campaigns is a must to inform the public, policymakers, researchers and traders about threats posed by the introduction of such invasive species in natural water systems.

Moving forward, scientists say it is crucial to monitor the spread of this fish and implement strict measures to prevent the intentional or unintentional release of non-native fish in natural water systems.