Community models could help conserve the disappearing migratory freshwater fish [Commentary]

Migratory freshwater fish have declined by 81% globally since the 1970s, with the Ganges-Brahmaputra-Meghna basin as a critical migration landscape where river giants like the golden mahseer are quietly disappearing.
As riverbed habitats are disappearing with the impacts of climate change, impassable hydropower dams like Tehri and Srinagar are trapping fish into small, inbred, genetic islands.
Decentralised, local stewardship can successfully restore wild fish populations while sustaining river livelihoods.
The views in the commentary are that of the author.

When the United Nations Convention on the Conservation of Migratory Species of Wild Animals (CMS) published its Global Assessment of Migratory Freshwater Fishes report earlier this year, the numbers confirmed what riverine communities had suspected for decades. Migratory freshwater fish populations have dropped 81% globally since the 1970s. Nearly all (97%) of the 58 fish species listed under the convention now face extinction. The report, drafted in collaboration with the UN Environment Programme-World Conservation Monitoring Centre (UNEP-WCMC) names the Ganges-Brahmaputra-Meghna (GBM) river system as a global priority region.

Zeb Hogan, a fish biologist and one of the report’s lead authors, describes the GBM basin as among South Asia’s greatest migration landscapes. It is a single connected pathway from Himalayan headwaters down to the Bay of Bengal. Yet, despite supporting an immense biomass of migratory species, the basin is collapsing under severe habitat fragmentation driven by aggressive hydropower expansion and altered flow regimes.

Public debate about the GBM usually stops at the lower reaches, majorly at barrages like Farakka. The damage is worse far above the plains. In the high-altitude headwaters of the Indian Himalayan Region, the lifelines that flow-dependent species depend on are being cut apart piece by piece.

Cascading hazards and the dislocation of aquatic life

In late August 2026, a massive ice and rock avalanche struck the Nepal-Tibet border. The debris tore into the Lende Khola before surging down the Bhote Koshi and Trishuli rivers. Monitoring stations run by the International Centre for Integrated Mountain Development (ICIMOD) at Galchchi recorded the water rising nine metres in just 30 minutes. That is how fast it happened.

The flood claimed hundreds of lives, wiped out 281 megawatts of installed hydropower capacity, and among it the 42-year-old Devighat plant. The riverbed itself came out unrecognisable. And then came the stranger thing. As the muddy, debris-choked water rolled into India’s Gandak (Narayani) river system and flooded districts like West Champaran and Gopalganj in Bihar, fish from the high altitudes came down with it. Species the floodplain villages had never seen before were suddenly in their nets. The Bihar government had to put out public health alerts telling people not to eat or sell the unfamiliar catch.

Community models could help conserve the disappearing migratory freshwater fish [Commentary] — Heavy flooding and debris flow along the Alaknanda river. Such disasters may bury deep pools and gravel beds necessary for fish spawning and survival. Image by Anand Kumar.

C.P. Rajendran, a geo-scientist at the National Institute of Advanced Studies (NIAS), views the disaster as a symptom of a larger condition. He warns that the Himalayan ecological framework is near collapse due to an intense infrastructure expansion with little regard for environmental limits. Slopes are unstable. Rivers are losing their efficiency. Mountains and rivers do not simply reset after a disaster of this scale. As glaciers retreat, they leave behind vast, unstable deposits of rock and silt that geologists call ‘sediment bombs.’ A slightest trigger such as a shift in rainfall pattern or a sudden glacial lake outburst flood (GLOF) can set them off. When they go, the resulting cascades scour out river channels, and the aquatic landscape is permanently disfigured for years.

In October 2023 a GLOF was triggered on the Teesta in Sikkim. The Teesta River basin is a stronghold for the snow trout (Schizothorax richardsonii). It is a primitive, rheophilic species adapted to cold, fast-flowing Himalayan currents. According to an assessment study from 2025, conducted by ICAR-National Bureau of Fish Genetic Resources (NBFGR), the flood expanded the Teesta’s floodplain by 138% in the upper reaches and widened the channel so significantly that the river’s complex ecology was physically ‘simplified’.

Millions of tonnes of mobilised sediment buried the deep pools and gravel beds that snow trout and other minor carps use for winter refuge and spawning. The critical hydrologic connectivity between the main channel and its side-channels was severed. The fisherfolk downstream. were left catching smaller, less valuable fish, because the big species had either died or moved into less damaged tributaries.

An unusual catch of mixed riverine fish following catastrophic floods in the Ganges, including catfish of the genus Bagarius (left), locally called "the Goonch". In the aftermath of disasters occurring along Himalayan rivers, aquatic species are often displaced into downstream floodplains. Images by Upendra Singh. — An unusual catch of mixed riverine fish following catastrophic floods in the Ganges, including catfish of the genus Bagarius (left), locally called “the Goonch”. In the aftermath of disasters occurring along Himalayan rivers, aquatic species are often displaced into downstream floodplains. Images by Upendra Singh.

Impassable concrete and genetic islands

Where the rivers are not being physically reshaped by sediment, they are being structurally walled in. The recent Nepal disaster showed how exposed, surface-built dams are in a seismically active region. Yet the push to build more has not slowed. Every new barrier directly undermines the CMS mandate to maintain longitudinal connectivity along essential river corridors.

Take the golden mahseer (Tor putitora) for example. A megafish which is known for its striking golden hues. It can grow to over 50 kilograms. Mahseer once swam freely up the Ganges tributaries to spawn ahead of the monsoon. Its life cycle demands a continuous, uninterrupted network where deep pools, shallow riffles, and lateral side-channels are perfectly interconnected.

In Uttarakhand, that journey is now heavily blocked by dams. A radio telemetry research conducted by the Wildlife Institute of India (WII) demonstrated how rapid hydropower development has broken golden mahseer habitat into fragments, changing the species’ ecology and spawning biology along the way. The Tehri dam complex on the Bhagirathi river and the Srinagar dam on the Alaknanda river are both impassable. And the damage is not limited to the blocked movement. The species’ genetics indicate that the Srinagar dam has cut off the upstream Alaknanda stocks entirely. The small, inbred clusters are left trapped above the concrete walls, marooned on these genetic islands.

The ICAR-Directorate of Coldwater Fisheries Research (DCFR) in Bhimtal has dedicated years trying to offset mahseer wild population losses through hatchery management and breeding programmes. Yet, it remains a desperate, uphill fight. The fish’s own biology works against it. Because mahseer matures late and produces relatively few eggs, captive breeding cannot keep pace with habitat loss on this scale.

Engineering fixes in the wild have also failed. Pool-type passes and fish ladders (engineered structures that help migratory fish pass through) are rarely built into dams in India. And if they exist, surveys find they do not function for large, heavy-bodied migrants like mahseer struggling against heavily altered, hydropeaking flows.

The Srinagar hydroelectric dam on the Alaknanda river. Infrastructure like this acts as an impassable barrier for migratory megafish. Image by Anand Kumar.

The community alternative

If the engineering fixes and the top-down management keep failing, the working answer may lie in community conservation models that need absolutely no concrete.

Meghalaya’s Garo Hills offer one such success story. The Simsang river runs clear here. It is spared from the pollution, fragmented habitat and destructive dynamiting that ruin other waters in the region. The reason is the Wachi Wari Community Fish Sanctuary. A wari is a deep river pool where fish gather when the water drops. The villagers guard them, and they have done it without the support of a government infrastructure or formal regulatory paperwork. These protected pools host hundreds of rare chocolate mahseer (Neolissochilus hexagonolepis). Strict village bans on dynamite and poison fishing keep the populations thriving; creating a natural overflow into open, unprotected waters that eventually feeds the community and fuels the local economy.

Across the border in Bhutan, indigenous communities protect snow trout the same way, simply by not fishing during spawning seasons. A concrete dam never adapts. A biological refuge like these moves with the river.

Further east, along Nagaland’s Tepuiki river, the residents of Poilwa village recognised and responded to the rapid depletion of their local fisheries. No external state enforcement, no outside agency. The community mapped eight spawning grounds along a seven-kilometre stretch and protected them. Fishing and extraction were barred in the breeding zones, and the native fish recovered within a few seasons. The river also recovered. Household incomes followed it up.

A protector of the Wachi Wari Fish Sanctuary, a community reserve in West Garo hills. Image by Parineeta Dandekar.

Bridging policy and practice

The recent floods in Nepal and Sikkim, and the structural fragmentation of rivers in Uttarakhand, confirm that our current approach to managing Himalayan rivers is fundamentally broken.

The CMS global assessment report leaves us with a clear mandate. Freshwater fish conservation must be brought out of the margins and into the mainstream through shared tracking, protected migration pathways, and restored connectivity. For the Indian Himalayan Region (IHR) and the wider GBM basin, that means accepting something uncomfortable. Transboundary cooperation must extend beyond water quotas and hydroelectricity trade. It must be about protecting a single, living, continuous biological flow, stretching from the glaciers to the delta.

Policymakers will have to move past the hydropower-first strategy. The remaining free-flowing, undammed stretches in the Himalayas must be strictly protected as fish conservation zones. For existing dams, adequate and seasonally appropriate environmental flows must be legally mandated and strictly enforced to preserve remaining aquatic biodiversity.

A mahseer population is quietly inbreeding above a dam. High-altitude fish are washing into the flooded plains of Bihar. These are warning signs. We must heed the global frameworks, certainly, but the deeper, more urgent and important lesson is already being lived out in Meghalaya, Bhutan, and Nagaland by grassroots communities who never needed a UN report to tell them how to keep a river alive. A river without fish is just geography.

Banner image: A golden mahseer in a free-flowing, unobstructed stretch of the Ganga before the Srinagar dam. Maintaining these clear, uninterrupted waterways is critical for the survival, seasonal migration, and genetic variability of Himalayan megafish. Image by Saurabh Dewan.

Saurabh Dewan is a freshwater biologist, conservationist, and the founder of the Himalayan Conservancy Foundation, a non-profit dedicated to conserving Himalayan ecosystems through science, community participation, and collaborative action.