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In a startling discovery last year, an Ichthyological survey documented the presence of Pterygoplichthys disjunctivus—a species of Sailfin Catfish native to the Amazon—in the Lohandra river of Eastern Nepal. Known for its hardiness and invasive potential, the species has already disrupted ecosystems in India and Bangladesh. Due to the significant ecological impact of Loricariids species, Bangladesh has imposed a ban on any species of the suckermouth catfish group. 

As a non-edible species, its presence in natural waters points towards potential source—pet trade. The record of this species has raised serious concerns among conservationists. “The numbers suggest they have already adapted to natural water bodies and monitoring is necessary to determine whether they have become fully invasive or not,” said Jash Hang Limbu, lead researcher of the study.

The aquarium trade link

The Loricariids species, including Pterygoplichthys and Hypostomus, are popular aquarium fish for their ability to clean aquaria by feeding on algae. Sold as “plecos” or “sucker fish”, they are easily available in aquarium stores across the country. When small, they remain mostly at the bottom of aquariums, feeding on algae and detritus. However, these species can escape or are released into natural waters, once they outgrow aquariums. They have the capacity to adapt, which can result in disruption of native aquatic biodiversity. 

While the precise route of introduction into the Lohandra River remains unclear, researchers suspect a combination of accidental releases from aquariums and migration from Indian rivers as a potential cause.

Back in 2018, a specimen was recorded at a commercial fish farm in Dhanushadham municipality, and was released into Dhanush pond, a natural pond within Dhanushadham protected forest. “We failed to identify the species. However, it had an eye-catching body structure, so we rescued and released it in the pond,” said Dev Narayan Mandal of Mithila Wildlife Trust. 

Recently, two additional sightings were reported, both in commercial fish farms from the same locality. Despite the potential risk of invasion, the fish was released into nearby rivers. “To my information one farmer killed the fish but the other released it in a nearby river,” said Mandal. 

The presence of these fish in remote villages far from aquarium shops hints at the possibility of alternative introduction pathways, such as migration from India or accidental release via fish stock imports. In 2024, a specimen was also captured from the Narayani irrigation canal in Bara district. 

According to Limbu sailfin catfish have been recorded in irrigation canals and other water bodies in Morang and Jhapa districts in recent surveys as well. He suspects the presence of two species of sailfin catfish (Pterygoplichthys disjunctivus and Pterygoplichthys pardali) in eastern Nepal, although taxonomical identification is difficult.

In both India and Bangladesh—where the fish has invaded ecosystems, the primary source of introduction is the aquarium trade. Limbu suspects that the fish might have migrated from India through inter-connected river systems. As Nepal’s rivers are linked to those in India, the possibility can’t be dismissed. 

However, experts like Kripal Datt Joshi, retired principal scientist at Indian Council of Agricultural Research, are sceptical of the migration theory. “Nepal’s rivers are less polluted and fast flowing - conditions that are not congenial for this species,” he said. 

Likewise, Loricariids (both Pterygoplichthys and Hypostomus) are known for their ability to breathe air and even show terrestrial locomotion. Experts say considering this adaptive behaviour, the fish may have found alternative dispersal methods.

Growing threat of invasion in Nepal

The pet fish trade is not the only pathway for exotic invasive species in Nepal. Despite the tremendous native fish diversity - 236 indigenous fish species, introduction of exotic fish species started during the 1950s. A 2015 study by Prem Budha, Professor of Zoology, Tribhuvan University, documented 19 exotic fish species in Nepal. However, the list doesn’t include fish species reared in aquariums. 

Many were introduced for aquaculture without proper risk assessments. “If such exotic species escape or are released to natural ecosystems they might adapt to the environment, breed, and can become invasive,” said Budha. Species like Tilapia, has already outcompeted native fish in lakes of Pokhara. Recently, rainbow trout—a species listed under “100 of the World's Worst Invasive Alien Species”—has escaped and are recorded in natural waters, said Bharat Babu Shrestha, professor at Tribhuvan University. “It’s time to regulate exotic species, including Tilapia,” said Professor Shrestha.

Ornamental fish industry in Nepal

Nepal’s ornamental fish industry has grown rapidly, with aquarium shops flourishing in Kathmandu, Pokhara and other major cities. “The number of people buying fish has increased in recent years,” said the owner of Hobby Aquarium, one of Nepal’s oldest shops. 

However, the customs import data depicts a different story, showing a sharp decline in ornamental fish imports—an ironic contrast to the booming domestic market. 

Currently, imports are processed through the Nepal National Single Window (NNSW), where importers must list species name and quantities. “Based on that online application, import permissions are granted,” said Sumitra Laudari, Senior Fisheries Development Officer at Central Fisheries Promotion and Conservation Center (CFPCC).

Aquarium owners reported that importers import fish based on market demand and international availability. Most imports come from India and Thailand, with occasional imports from China, Indonesia, and other east-Asia-pacific countries.

Nepal lacks comprehensive research and a centralized database on aquarium fish imports. Limited studies exist, mostly in Kathmandu and Pokhara. A 2024 study published in International Journal of Biological Innovations journal reported 53 exotic and four native ornamental fish species sold in Kathmandu aquarium shops. Arowana was the most expensive, while Guppy the cheapest.  

Professor Budha noted that international trade is a major pathway for invasive alien species globally. “With growing demand, more non-native species are imported, increasing risk of introduction into ecosystems,” he added.

Another concern is “fish liberation”—religious practice where believers release captive live fish into water bodies.  “With exotic fish easily available, there’s a high chance non-native species are being released,” added Professor Shrestha.

Weak regulations, invasion risk and conservation concerns

The Nepal Fish Fry Standard 2061 (amended in 2077) lists 144 ornamental fish species allowed for import. However, many unlisted and high-risk exotic species such as Arapaima, Alligator Gar, Redtail Catfish, Pacu, Iridescent Sharks, Giant Snakehead, Red Devil and Peacock Bass are openly sold in the market. Known for their predatory behaviour and rapid growth, these species have caused severe ecological damage in other countries after escaping captivity. 

The aquarium trade also threatens conservation efforts. Some IUCN Red List endangered species like Reticulated Hillstream Loach and Bala Shark (Vulnerable) and Red-Tailed Black Shark (Critically Endangered) are easily available in the market, though they are not listed under the Nepal Fish Fry Standard 2061. 

Some of these are smuggled from India, often hidden among frozen fish shipments. “The mortality rate is high with such fish, so we avoid purchasing from those sources,” said a Kathmandu-based shop owner. Nepal’s open border and weak import inspection further increases the possibility of such illegal imports. 

Officials acknowledge enforcement challenges, citing a lack of technical officers at quarantine checkpoints. “Due to the lack of fishery experts at quarantine centres/posts, these fish species are often not identified correctly,” said Laudari. Ashok Adhikari from the Ministry of Agriculture and Livestock echoed a similar challenge, “Veterinary officers manage quarantine posts, but they often lack the expertise to identify fish species”.

A potential solution: Breeding native ornamental fish

Nepal has significant potential to develop a local ornamental fish industry. The CFPCC and Pokhara’s Fishery Research Station have begun breeding programs.

Though 15 native species with ornamental potential have been identified, only one is currently being bred. “Identifying and developing technologies for breeding native fish species for aquariums could be an important step in reducing import dependency” said Md Akbal Husen, Senior Scientist at Pokhara’s Fishery Research Station. For instance, River stone carp (Psilorhynchus sucatio)—a native algae eater known as “sucatio”, is being imported despite its local presence. Such species, if bred domestically, could serve as an alternative to invasive plecos or sucker fish. 

While the domestic production has increased slightly over the past five years, it still doesn’t meet the market demand, said Husen. According to Laudari, efforts are underway to expand breeding programs at CFPCC. 

Officials also reported even private breeders are starting to breed exotic species, but their operations remain unregulated.

The way forward

As the spread of invasive exotic fish species accelerates, Nepal must take proactive measures. Nepal has also adopted Kunming Montreal Global Biodiversity Framework (GBF), Target 6 of which aims to reduce the rate of introduction and establishment of invasive alien species by 50 percent, and to reduce/mitigate the impacts of invasive alien species on biodiversity and in ecosystem services. Nepal is currently drafting the National Biodiversity Strategy Action Plan (NBSAP)—a document that translates the global targets into national actions.

The approval of Invasive Alien Species Management Strategy and Implementation Plan 2081 has raised new hope. “This now provides us with a regulatory foundation,” said Professor Shrestha who was involved in drafting the strategy. “Conducting thorough risk assessment—both pre-border and post-border, is crucial in mitigating the risk of invasion” added Professor Shrestha.  

“Strengthening technical capacity at quarantine checkpoints is also essential to prevent the entry of invasive exotic species”, emphasized Professor Budha.

In recent centuries, Earth has seen rapid growth in population of a single species, humans. Humans have explored every nook and corner, deep into the Earth to the depths of Oceans, and even beyond the Earth to space. And this rapid growth has resulted in an increase in trade throughout the globe. During this trade and expansion, humans have been introducing species to new habitats, intentionally or unintentionally. According to the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPEBS), over 37,000 alien species have been introduced through human activities worldwide, with 37 percent of these reported since 1970—attributed to the increased global trade and human movement.

Not all these introduced species become invasive, but many are capable of or forcing their adaptation to the new ecosystem/habitat, making them a potential invasive species. Once the exotic becomes invasive, they grow rapidly and become dominant changing the overall ecosystem dynamics. In most cases, their growth and invasion is supported by the absence of a natural enemy. For the same reason, they are termed biological pollution or green cancer. More than 3,500 species, including 6 percent of alien plants; 22 percent of alien invertebrates; 14 percent of alien vertebrates; and 11 percent of alien microbes, have become invasive, threatening the native biota, ecosystem functioning, human health, and the economy. The IPBES Global Assessment Report (2019) highlights invasive alien species as one of the five most important direct drivers of biodiversity apocalypse, contributing to 60 percent of global extinctions and being the sole cause of 16 percent of recorded animal and plant extinctions. Besides the ecological loss, indigenous communities who are directly dependent on ecosystem services are directly affected by invasive alien species.

Exotic fauna in Nepal

It is plants which are among the major focus when it comes to alien species in Nepal. In Nepal, there are over 182 naturalized alien floral species reported, 27 of which have become invasive. Four of them are among the world’s 100 most invasive species. In contrast, there is limited data on exotic faunal species in Nepal, mostly due to sparse research. A research conducted by Prem Bahadur Budha in 2015 documented 64 alien faunal species in Nepal. The list is mostly dominated by animals introduced intentionally for commercial farming including livestock breeds, fisheries, aquaculture, and for pets and ornamental purposes. However, several alien animal species have invaded different ecosystems and threatened native animal species but are understudied. Some of the reported alien faunal species are among the world’s 100 most invasive species such as African giant land snail, Western Mosquito fish (Gambusia affinis), Mozambique Tilapia, Rainbow trout (Onchorhynchus mykiss), Brown trout (Salmo trutta) and Terrestrial flatworm (Platydemus manokwari), etc.

Contributing factors

In Nepal, many exotic fish species are introduced for augmenting fisheries for their high economic value and rapid breeding, and some for ornamental trade. Some are introduced illegally by farmers into fishery ponds. Currently, Nepal has at least 16 exotic fish species (www.fishorg.com), with some studies reporting up to 19.  These species often escape into natural water systems due to floods or unintentional dumping by aquarists, posing a threat to native species.

A great irony is that some of these species have been reared in natural ecosystems too. For instance, Tilapia fish have been released in large natural lakes such as Phewa, Begnas, Rupa Lake of Pokhara valley, Indrasarobar and other parts. Tilapia is listed among the “100 of the World’s Worst Invasive Alien Species”. An investigation from Begnas Lake found the introduction of alien species led to a 42 percent reduction in native fish. Another study reported native fish catch in Phewa Lake decreased to 13.7 percent of the total, with exotic species making up over 86 percent of the catch. Additionally, a study in Kailali district revealed that 80 out of 101 natural lakes were being used for exotic fish farming, mostly carp.

Despite these ecological risks, there is an increasing trend of importation of exotic species. However, these introductions are questionable. It is not clear whether any scientific assessment has been conducted before introducing these species.

Ornamental fishery

Ornamental fish trade also contributes to the introduction of invasive species. Aquariums housing ornamental fishes and other aquatic animals have become increasingly common, with annual import of ornamental fishes in Nepal rising from 318.1 metric ton (Rs 13.3m) in 2009 to 1233 metric ton (Rs 153.4m) in 2017. A recent study recorded 57 exotic fish species in Kathmandu’s aquarium shops, a notable increase from  27 exotic fish species reported back in 2019.

One concerning example is the presence of Vermiculated Sailfin Catfish (Pterygoplichthys disjunctivus) from Lohandra River, Morang likely introduced through ornamental pet trade. The species is known for rapid growth, high reproduction rates, and adaptability to various habitats, which facilitates its establishment in natural ecosystems. A total of 43 individuals were collected from two sites along the Lohandra, indicating a likelihood of successful reproduction in the wild. A social media video last month showed a local fisherman capturing this species in the nearby Rate Khola, Morang, suggesting its potential spread.

In addition, invasive alien species like red-eared slider (Trachemys scripta elegans) have been found in Nepal’s water bodies, possibly through accidental escape or intentional release from aquariums. Unfortunately, this species is listed among “100 of the World’s Worst Invasive Alien Species” and poses a significant threat of becoming invasive if not regulated.

Existing legal instruments

The Kunming-Montreal Global Biodiversity Framework aims to reduce the rate of introduction and establishment of invasive alien species by 50 percent by 2030. Nepal’s National Biodiversity Strategy and Action Plan (NBSAP) (2014-2020) recognized invasive species as a major threat to biodiversity and native species but focused primarily on plants. The NBSAP prioritizes formulation and implementation of a plan to regulate the introduction and expansion of invasive aquatic fauna by 2018 (WB-B8), but progress on this priority action remains unclear. The Invasive Alien Species Management Strategy has been drafted in 2016 (2074) but hasn’t been approved yet.

Preventing the introduction of non-native species to the natural ecosystem is the most effective measure to avoid invasions. Besides, preparedness, early detection and rapid response are effective at reducing invasion rate by alien species. In addition, eradication can be successful and cost-effective when the invasive populations are small and slowly spreading. There is need for strict measures to prevent intentional or unintentional release of non-native faunal species and to enhance awareness among public and merchants of ornamental fish about risks of exotic invasive species. Strengthening the institutional capacity, capacity of customs and quarantine departments is also crucial for controlling invasion and spread of alien species. Studies have reported that about 15 native fish species of Nepal are ornamental or have the potential to become ornamental fishes. Prioritizing these native fish species for ornamental market trade can help reduce the import of potentially invasive exotic species.

Nepal’s aquatic ecosystem is already jeopardized by infrastructure constructions in the name of beautification, blockage and damming projects, pollution and climate change. Extensive trade and introduction of exotic species has further augmented the threats to aquatic ecosystems. Urgent action is needed to prevent the introduction of alien faunal species and protect Nepal’s biodiversity. 

Every winter, the skies of Nepal welcome migratory birds, embarking on epic journeys from Siberian realms covering thousands of kilometers. Their graceful flights wow many hearts and minds. On a parallel realm, beneath the shimmering surface of the river, a similar marvel unfolds with the onset of monsoon (rainy season)—the migration of majestic Mahseer. Migration, a vital journey taken by animals regularly for a particular reason, usually at a particular time of the year, is an essential adaptation strategy for survival. Unlike bird migration, fish migration is intricately tied to the availability of free-flowing waters. This crucial journey is being jeopardized by a rapid development of hydropower projects and river modifications. These engineering feats, while harnessing energy and water resources, are disrupting the natural flow of the river, obstructing the Mahseer’s migration cycle and driving them toward extinction.

As the monsoon rains recede, a mesmerizing natural event unfolds in the rivers of Nepal. Freshwater fish embark on epic journeys, traversing hundreds of kilometers to reach their spawning grounds. This epic phenomenon is a critical part of many fish species’ life cycles, including the iconic Mahseer. Fish migration is the periodic seasonal movement between breeding and foraging grounds. This is an important cycle of life for most migratory fish species meant to prevent the extinction of species.

Mahseer, often referred to as ‘the tiger of the river’ (first used by conservationist Jim Corbett), symbolize Himalayan rivers. Mahi means ‘fish’ and sher, meaning ‘tiger’, and the word reflects their cultural, economic and recreational significance. These are apex species, which indicate the overall health of the ecosystem, just like apex terrestrial species such as tigers. They are important food fish and a delicacy for many indigenous communities. They are important game fish among anglers, for their strength, stamina and splendor. Mahseer undertakes awe-inspiring journeys, navigating rapids in search of critical spawning habitats. Golden Mahseer are listed as endangered on the IUCN Red List of Threatened Species.

Nepal rivers flow through varied ecological realms, hosting many indigenous, rare and endangered fish species. Unfortunately, the migration behavior of these species remains understudied and misunderstood. This knowledge gap has contributed to overlooking the impact of hydropower and irrigation projects. Recent reports from the Asian Development Bank (ADB) reveal that dam locations are selected without considering the cumulative impact on fish migration routes.

Most of Nepal’s river systems are already obstructed by dams, restricting seasonal fish migration to the upper reaches for spawning and feeding and returning. The consequences extend beyond the blockage of migratory paths, affecting water discharge downstream, breeding grounds, limnological parameters, nutrient flows and the overall ecological balance.

It is widely observed that the barrier effect is responsible for a sharp decline in fish populations in rivers with dams. Case studies from the Kaligandaki have revealed that populations of migratory species like Goonch, Raj Baam and Sahar have declined after the construction of the Kaligandaki A project (144MW). Similar is the case in other rivers. Government policies waiving environmental impact assessments (EIA) for projects up to 50 MW capacity have exacerbated the issue, impacting fish species without accountability.

The implementation of mitigation measures such as fish passages, environmental flow, restriction on fishing and sand mining from rivers is crucial for the conservation of fish species. Unfortunately, their effectiveness has been compromised by poor implementation and lack of monitoring. The correct choice of fish passage design is a critical factor, and needs to be tested for known fish species by fishery specialists. Engineers and environmental experts must find the best modifications that facilitate migration. Overlooking the migratory pathways is a mistake which is hampering the economy and ecological balance of aquatic systems.

Saving a lifeline

The Karnali river stands as a testament to the untamed beauty of Nepal's waterways. As the last free-flowing river in the nation, it holds a unique ecological heritage, providing sanctuary for a diverse range of species, including the revered Mahseer. The river supports habitat for 50 migratory species, including the Golden Mahseer and 24 species currently on the IUCN Red List for conservation.

Access to quality spawning habitats is vital for survival and productivity of fish populations. The uninterrupted natural course of the Karnali river provides pathways for migratory fish like Mahseer to spawn and thrive. However, the rapid development of hydropower projects and river modifications has cast a shadow over this pristine watercourse, threatening the very existence of its iconic inhabitants. If such happens, Mahseer and other migratory fish face insurmountable obstacles, jeopardizing their populations and causing a ripple effect of ecological disruption.

The Karnali, as the last free-flowing artery in Nepal, is not only a sanctuary for fish; it is the lifeblood of communities lining its banks. The annual migrations of fish hold profound cultural, historical and economic significance. They fuel local economies through activities such as angling, ecotourism and fisheries. Moreover, the presence of healthy fish populations sustains countless individuals, who rely on the river for their livelihoods.

Despite their ecological and economic importance, the Mahseer populations have witnessed dramatic declines. Overexploitation, habitat fragmentation and loss, and pollution pose significant threats. Destructive fishing practices and unregulated sand and boulder mining have further deteriorated the potential breeding grounds of the Mahseer.

Hydroelectric dams, touted as symbols of progress, inadvertently disrupt the natural flow of rivers, obstructing migratory routes. Reduced migration means limited access to crucial breeding grounds, leading to a decline in genetic diversity and overall Mahseer population. With each passing year, their numbers dwindle, pushing this iconic species closer to the brink of extinction.

The Mahseer are migratory species and rely on free-flowing rivers. Free-flowing rivers are the lifeblood of migratory fish species. The unobstructed waterways provide the essential pathways that allow fish to reach their spawning grounds, contributing to the sustainability and resilience of their populations. However, the rapid pace of hydropower development has threatened their survival.

The cost of decreased Mahseer and other migratory fish populations is not just ecological, it creates a domino effect of ecological disruptions, affecting the health of ecosystems, local communities, and cultural traditions of various indigenous riverine communities. 

Their annual migrations have become interwoven with local traditions, providing livelihoods, and enhancing the cultural fabric to the communities.  Mahseer are an important game species, thus communities can also make earnings through ecotourism and angling. Additionally, healthy fish populations contribute to the livelihoods of countless individuals dependent on the rivers. Their loss could lead to food security issues among fishing communities. Beyond their intrinsic value, conservation of Mahseer yields a multitude of ecosystem services, from balancing the aquatic food chain to supporting healthy aquatic and terrestrial environments. Once lost, restoring and conserving such obstructed habitats will be a costly process.

In conclusion, the preservation of natural river flows is not just an environmental endeavor; it is a pledge to safeguard the legacy of Nepal’s rivers for generations to come. Collaborative efforts between engineers and conservationists, along with prioritizing research, are essential for ensuring that future generations witness the grandeur of the Mahseer in Nepal’s rivers.