Hilsa follows water, not policy: Can Bangladesh's national fish survive climate change?

Hilsa does not know when October begins. It does not know the date of a government ban. It moves when the water tells it to move.

It migrates when rivers speak through temperature, salinity, depth, oxygen, rainfall and tides. 

When those signals shift, Hilsa may also change its routes, timing and breeding grounds.

That subtle dependence is now at the heart of a growing question, and that is – can Hilsa survive in a warming Bangladesh, where climate change is intensifying pressure on already stressed rivers and estuaries?

For decades, Bangladesh has treated Hilsa as a conservation success story. The fish contributes an estimated 12% of national fish production, according to the Department of Fisheries. 

Official figures from the fisheries department show production at 5.29 lakh tonnes in FY2023-24, worth over Tk20,000 crore. Authorities also claim Bangladesh supplies more than 80% of the world's Hilsa.

More than 80% of the world's Hilsa comes from Bangladesh and the fish contributes around 1% to GDP and supports lakhs of people directly and indirectly, said Fisheries and Livestock Minister Amin Ur Rashid recently.

But beneath the success narrative lies growing strain. 

Production has steadily declined in recent years, according to fisheries department data cited in media reports. Output fell from 5.71 lakh tonnes in FY2022-23 to 5.29 lakh tonnes in FY2023-24, with further estimates indicating a drop to 5.12 lakh tonnes in FY2024-25.

While Bangladesh remains the dominant producer globally, experts say the warning is not in collapse, but in transformation.

A fish whose geography is changing

The biggest shift is not just in numbers – it is in geography.

Research over past decades, including "The Shifting Habitat of Hilsa: River to Sea" published in SEAFDEC's Fish for the People, shows a clear shift in Hilsa distribution from inland rivers toward marine and estuarine waters. 

In the 1950s and 1960s, riverine catches accounted for about 94% of total production. By 2012-13, the pattern had reversed, with roughly 72% of catches coming from marine waters and 28% from rivers.

Experts link this shift to a combination of upstream water control structures, disrupted migration routes, siltation, habitat loss and heavy fishing pressure in river and estuary zones.

Dr Md Manzoorul Kibria, professor and coordinator of the Halda River Research Laboratory at the Department of Zoology, University of Chittagong, does not see climate change as the only or biggest immediate threat.

"In my view, more than climate change, our man-made impacts are the biggest," he said, pointing to pollution, river filling, reduced navigability and overfishing as more immediate threats.

His central point is that climate change is arriving on top of already degraded systems. And in such conditions, even small environmental changes can have outsized effects on a migratory species like Hilsa.

"Hilsa does not migrate by date," he said. "It migrates when water quality and climatic conditions become favourable."

Reading the river: How Hilsa makes decisions

Hilsa migration is governed by environmental cues rather than fixed time cycles. 

A 2014 study, "Discovering spawning ground of Hilsa shad (Tenualosa ilisha) in the coastal waters of Bangladesh" published in Ecological Modelling, found that temperature, dissolved oxygen, salinity, pH, turbidity, river current and rainfall all influence where the fish travels and breeds.

A GIS-based ecological model identified only a small fraction of Bangladesh's aquatic ecosystem as highly suitable spawning habitat, suggesting that optimal conditions are geographically limited and sensitive to change.

More recent research, such as "Assessment of spawning and nursery habitats of Hilsa in the Tetulia and Meghna River estuaries" highlights the importance of salinity levels in particular. 

In estuarine zones such as the Tetulia and Meghna, Hilsa spawning and nursery habitats depend on very low salinity conditions. Even small changes in freshwater flow can alter suitability.

The Tetulia River estuary, for example, has been identified as a consistently favourable breeding and nursery ground due to its near-freshwater conditions throughout the year. Scientists recommend strict year-round protection of juvenile and brood Hilsa in such areas.

This makes the species highly vulnerable to any disruption in freshwater flow, whether from climate variability, upstream withdrawal, or erratic rainfall patterns.

Climate change meets pollution

The concern is not climate change alone, but how it interacts with pollution, siltation and overfishing.

Rising temperatures can reduce oxygen levels in water. Reduced river flow allows saltwater to penetrate deeper into estuaries. Industrial discharge and agricultural runoff alter nutrient balance and phytoplankton composition – the very base of Hilsa's food chain, study highlights.

A 2015 study, "Climatic and anthropogenic factors changing spawning pattern and production zone of Hilsa fishery in the Bay of Bengal," highlights Hilsa migration routes and spawning grounds have been disturbed, displaced or destroyed by human activities, climate change, siltation and river-basin changes.

The study found inland Hilsa production declined by about 20% over two decades, while marine yield rose about three times. It also found major spawning areas shifting towards lower estuarine regions such as Hatia, Sandwip and Bhola.

In some stretches, water depth has fallen far below what is considered necessary for free fish movement, creating what experts describe as "biological barriers" rather than just navigational hazards.

This same zone – Meghna-Hatia-Sandwip-Monpura-Bhola belt – is under growing pressure from fishing intensity, river fragmentation and sedimentation.

The fisheries department identified multiple obstruction points across key rivers where migration is partially blocked. Sandbars, reduced depth and narrow channels are limiting the fish's ability to move between sea and spawning grounds.

The result is a narrowing ecological corridor – where Hilsa survival depends on a few increasingly fragile zones.

The nursery question: Jatka and future production

The health of Hilsa populations depends heavily on nursery areas where juvenile fish, known as jatka, grow before returning to the sea.

Chandpur, the lower Padma and parts of the Tetulia estuary remain critical nursery zones. But scientists warn that if these habitats shrink or become concentrated in fewer areas, pressure on remaining zones will intensify.

Dr Kibria warned that harmful gears, including pocket nets, may catch large brood Hilsa before they can migrate, spawn and return to the sea. 

According to him, the gear-specific claim needs more official research, but the broader concern is well established: indiscriminate catching of juveniles and brood fish, small-mesh nets and rising fishing pressure have weakened riverine Hilsa.

Do bans still match biology?

Bangladesh's management still depends heavily on fixed bans: the 22-day mother Hilsa ban, the eight-month jatka ban, sanctuary restrictions and the marine fishing ban. 

These measures helped Bangladesh rebuild the Hilsa fishery. But if rainfall, salinity, flow and temperature are shifting, fixed dates may not always match the fish's biological calendar.

Dr Kibria said the ban period should be reviewed every year before being declared, using expert advice, weather forecasts, rainfall, temperature, salinity, water quality, biological indicators and field data.

The logic is simple – humans can fix a date, but Hilsa responds to water.

He also called for regional coordination, saying that, as it is migratory, if Bangladesh imposes bans while neighbouring waters remain open, Bangladeshi fishers may bear the cost while fish are harvested elsewhere. 

A livelihood system at stake

Behind the ecological debate lies a major livelihood issue. Millions depend directly or indirectly on Hilsa fishing and trade.

Any change in migration patterns or catch volume affects income, food security and local economies along riverine and coastal Bangladesh.

A 2023 Frontiers in Marine Science study on climate adaptation strategies for small-scale Hilsa fishers in coastal Bangladesh, recommended alternative livelihoods, access to credit, use of fishers' local ecological knowledge, stronger sanctuaries and better protected-area networks.

Without such measures, stricter conservation rules risk deepening rural economic vulnerability.

A system under stress, not collapse

Despite the pressures, Bangladesh's Hilsa fishery has not collapsed. It remains one of the world's largest and most productive Hilsa ecosystems.

It has shown Hilsa can recover when jatka protection, brood fish bans, sanctuaries and enforcement work together. But the next challenge is harder. It is not only about stopping fishing for a few weeks. It is about keeping the whole river-estuary-sea system suitable for migration, spawning, nursery survival and return.

River pollution control is now Hilsa policy. Dredging and navigability are Hilsa policy. Industrial effluent control, freshwater flow, climate forecasting and fisher compensation are now Hilsa policy.

Hilsa will not argue. If the river becomes too polluted, too shallow, too warm, too saline or too oxygen-poor, it will simply stop coming back the way it once did. 

The future of Hilsa in a warming Bangladesh will depend not only on how many fishers are stopped during a ban, but on whether Bangladesh can keep the waters Hilsa depends on suitable for return.

Because Hilsa will not adapt to the policy, it will only follow water.