Hilsa swim upriver into freshwater to spawn, then return to the sea once they have bred. Any loss of navigability or any build-up of shoals along that long migratory route disrupts their passage.

After dredging on the Rabnabad channel serving Payra port, silt has been accumulating and shoals forming in several rivers upstream, researchers say, raising fears that the hilsa’s path could narrow, and that spawning and fry-rearing grounds could also be affected.

Figures from the fisheries department point to a recent decline in hilsa production. The 2022–23 fiscal year saw a record 571,342 tonnes. Output has fallen in each of the two years since: down 7.33 percent to 529,487 tonnes in FY 2023–24, then down a further 12.49 percent to 500,288 tonnes the following year. Full figures for FY 2025–26 have not yet been published, but a source at the department believes the catch fell below 500,000 tonnes.

Researchers attribute the decline in part to changes in river navigability and migratory routes. Dr Anisur Rahman, a hilsa specialist and former director of the Bangladesh Fisheries Research Institute’s Chandpur river centre, told Bonik Barta that hilsa catches had never fallen this far before, and never for three years in a row.

“They had been rising steadily, and we could see that reflected in the markets. Now there’s very little large hilsa in the market. Medium and small fish make up most of what’s available. I’ve spent 35 years researching hilsa and have never been this worried,” he said.

Explaining the impact of shoal formation, Rahman said: “When water flow drops, siltation increases and shoals form. Shoals mean the migratory route is ruined. On the way from the mouth of the sea to the main rivers, hilsa faces huge barriers at those points. This has been happening for several years, to varying degrees. And it’s getting worse.”

The hilsa expert believes the change in water flow caused by dredging the Rabnabad channel for Payra port is a major factor. “Rabnabad, Arial Kha, Hijla and the Meghna, along with the other rivers of the south, reach the sea mainly through the Meghna,” he said.

“These rivers, their tributaries and distributaries have long served as the hilsa’s migratory route. When water quality and temperature change because of various structures and alterations to waterways, that can affect the hilsa’s food availability, movement and breeding.”

Research has also flagged the rapid siltation that followed dredging between 2022 and 2024 to keep the Rabnabad channel navigable. A 2026 study published in the Bangladesh Maritime Journal titled “Payra Port: Challenges of Sedimentation and the Way Forward” analysed sediment deposition in the channel adjoining Rabnabad after dredging and the long-term difficulty of maintaining navigability there.

Deepening one stretch of a river alters more than that stretch’s bed alone, the study found: water flow, the pace of the tides and the natural path of sediment carried in the water can all change as well.

A 2025 study of the Yangtze estuary in China likewise found that deepening a channel strengthens the flow of water along the riverbed towards the land. More sediment can enter the river from the sea as a result, the study noted, and the altered shape of the channel can make it more prone to trapping that sediment.

There is, however, not enough evidence to confirm that the Yangtze findings apply directly to the Rabnabad channel.

Molla Emdadullah, a former project director of the Hilsa Resource Development and Management Project, also attributes the loss of navigability and the formation of shoals to dredging in the Rabnabad channel. “There’s no water in the rivers. How will the fish come?” he told Bonik Barta.

“More than 250 shoals have formed in the Meghna-Tentulia alone. Barriers have appeared on the route from the sea into the rivers and on the way back. So the hilsa can’t enter the rivers in the spawning season.”

He warned that the hilsa’s migratory route, breeding grounds and nursery grounds were all being destroyed, adding that pollution had also risen. Shoals are preventing juvenile fish from reaching the sea, and without that journey the hilsa’s growth suffers.

Researchers believe industrialisation and pollution in the south are adding to the pressure, apart from the navigability crisis. The hilsa is a highly sensitive species and changes in water temperature and quality can affect its breeding. Silt accumulating on the riverbed and narrowing the passage is one constraint; pollution and changes in water quality are another. Both can hinder spawning and movement.

In the researchers’ view, this points to a need to monitor regularly how dredging to keep the Rabnabad channel navigable affects rivers both upstream and downstream, rather than simply maintaining the depth of a single waterway. The entire migratory route must be treated as one system, they argue, taking into account the river flow, sediment deposition, water quality and breeding grounds.

Dr Md Yamin Hossain, a professor in Rajshahi University’s fisheries department who has spent years researching hilsa, told Bonik Barta: “With navigability reduced, there’s no current at all in the rivers during the breeding season. That creates barriers on the route between the sea and the rivers.”

Hilsa researchers say the Rabnabad, Andharmanik and Tiakhali rivers in Patuakhali connect the sea to inland waters for a range of migratory fish. The environmental assessment for the Payra power plant likewise identified Rabnabad and Tiakhali as hilsa routes between sea and river.

Fisheries and Livestock Secretary Md Delowar Hossain could not be reached for comment as his phone number was switched off.