- Researchers discovered specimens of Pterygoplichthys disjunctivus, an Amazonian catfish, in a river in eastern Nepal, marking the first record of this species in the country’s rivers.
- How the fish ended up here remains uncertain, with possible explanations including religious release or upstream migration from India.
- Experts warn the presence of the species could severely disrupt Nepal’s freshwater ecosystems, due to the lack of natural predators and their potential to outcompete native species.
KATHMANDU — The discovery in a Nepali river of an armored catfish native to the Amazon has alarmed researchers and conservationists, who say it’s still not clear how the species found its way from the South American rainforest to the Himalayan nation.
Forty-three specimens of Pterygoplichthys disjunctivus, known as the vermiculated sailfin catfish, were discovered in the Lohandra River during surveys of various rivers in eastern Nepal from May 2019 to July 2023. The findings were documented in a newly published study in the journal BioInvasions Records.
“Our study for the first time shows the presence of genus Pterygoplichthys fish in Nepal’s river system,” study co-author Asmit Subba, a zoologist at Tribhuvan University in Kathmandu, told Mongabay. “However, we can’t conclusively say where the fish came from.”
Fish in the genus Pterygoplichthys are commonly called “janitor fish,” as they’re used in aquariums to eat algae and keep the tanks clean. From the Amazon Basin via the aquarium fish trade, they’ve since found their way into river systems around the world, including in South Asia, East Asia and the U.S.
P. disjunctivus is adapted to living in the murky waters of the Amazon, where it swallows air and absorbs the oxygen through its intestinal linings rather than extracting oxygen from water the way most other fishes do. This ability allows it to survive even when it’s out of water for hours. “Because of its vermiculated scales, it’s very difficult to kill and not even palatable to humans,” Subba said.
Hari Sharma, an associate professor of zoology at Tribhuvan University, who wasn’t involved in the study, said the species had previously been recorded “in various ponds in southern Nepal,” but that what the new paper documents are probably the first findings in a river in the country. He said the discovery needs to be taken seriously as the fish could have a huge impact on freshwater ecosystems across Nepal.
In neighboring India, various Pterygoplichthys species have been recorded in Bihar, Uttar Pradesh and West Bengal — three states that border Nepal — as well as farther south in Andhra Pradesh, Telangana, Maharashtra, Kerala and Tamil Nadu states. In areas where they’ve spread, fishers have reported declining catches of native fish, raising the alarm that the invasive species are displacing or even killing off native species.
For their study in eastern Nepal, Subba and colleagues used various fishing tools to scour a network of eight rivers, including the Lohandra, which flows south into India. When they encountered fish they suspected weren’t native, they photographed them in their natural habitat to document their appearance accurately. They then preserved the captured fish in a solution for later analysis.
When it came to P. disjunctivus, they confirmed the specimens’ identity through close examination of their physical characteristics, such as size and shape, and comparing them to known species.
The findings have raised several questions that remain unanswered for now, chief of which is: Where did the janitor fish come from?
One possible answer is that they swam upstream from India. However, Sharma said he’s not convinced that was the case. “If it had come from India, why was it that the individuals were found in a certain stretch of the river” rather than more widely, he said.
He added that because janitor fish can survive in harsh conditions, they’re a sought-after species for releasing into rivers as part of religious rituals. “Native fish can’t survive in plastic bags or containers. Traders wouldn’t want the fish to die before they are released into the river,” Sharma said. This, he added, suggests the specimens discovered could have been locally released into the river.
But that hypothesis has a key flaw: Most of the pet trade in fish in Nepal is concentrated in big cities such as Kathmandu and Pokhara, and not so much in the area where the janitor fish were found, Subba said.
Another question raised is whether the species has spread to other rivers in the country. There’s no definitive answer, Subba said: “Although we didn’t find the fish in any other rivers we surveyed, we can’t say for certain that they aren’t there.” Given that the river network in the region is interconnected, there’s a good chance this species has moved upstream from India and into other river systems in western and central Nepal, he added.
Regardless of how the fish found their way into a Nepali river, their presence here could have severe impacts on freshwater ecology across the country, adding to the litany of other stresses being put on Nepal’s rivers, researchers say. Genuine Prajapati, an aquatic biologist at Tribhuvan University, who wasn’t involved in the study, said the lack of predators for P. disjunctivus in the Himalayan river system is the major concern. “As we don’t have predators who can kill these vermiculated fish, they might proliferate very quickly and displace native species of fishes,” she said.
There’s already precedent for this. A 2020 report commissioned by USAID recorded 16 species of exotic fish in Nepal, ranging from the Nile tilapia (Oreochromis niloticus) to the rainbow trout (Oncorhynchus mykiss), both of which are farmed for food. “Similarly, we are also seeing a massive invasion of glass fish such as Chanda nama,” Prajapati said.
The report states that Nepal lacks procedures and standards for aquarium traders and farms to release waste and effluent into natural water bodies, and this could be the major source for invasive species proliferating in rivers. Similarly, laws are needed to regulate the import of aquatic species into the country, it adds.
Climate change, a cross-cutting issue, could also be playing a role in the proliferation of invasive species. “As Nepal is a geographically diverse country, we could see species from the lowlands move upward to the hills and mountains and vice versa,” Prajapati said, adding that invasive fish aren’t necessarily just those that come from outside the country.
While some measures can be adopted over the long term to address the issue of invasive species, others need to be taken immediately, researchers say. In the case of the janitor fish, the government needs to increase awareness, especially among fisherfolk, Subba said. “The only way we can control its spread is by killing it whenever and wherever we find it before it gets too late,” he said.
Banner image: A giant otter with an Amazonian catfish catch. Image by Bernard Dupont via Flickr (CC BY-SA 2.0).
Abhaya Raj Joshi is a staff writer for Nepal at Mongabay. Find him on 𝕏 @arj272.
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Citation:
Limbu, J., Rajbanshi, D., Subba, A., Khanal, L., Yang, J. Q., & Li, C. (2024). First record of the non-native vermiculated sailfin catfish Pterygoplichthys disjunctivus (Weber, 1991) from Lohandra River, Eastern Nepal. BioInvasions Records, 13(2), 557-564. doi:10.3391/bir.2024.13.2.20
