Invasive species: The biological pollution or green cancer
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.
Saving a lifeline and a majestic fish
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.
