While the government's dream project Padma bridge is widely expected to boost the country's economy, there is also fear that it may take a heavy toll on the famous hilsa fish of the river.

According to the environment impact assessment (EIA) report for the bridge project, migration of juvenile hilsa will be seriously hampered during the construction.

The Padma is an important migratory route for the national fish and the construction activities may result in temporary changes in the migration pattern, it reads. "Because hilsa is very sensitive to noise and vibration."

The report, however, does not say anything about the long-term effect on the fish.

Researchers who have been studying migration pattern and biological habits of hilsa say the bridge might have lasting effect on the life-cycle of the fish.

"After the construction, silt deposit will increase under the bridge and pattern of Padma's current would change. It will have an impact on hilsa," said Anisur Rahman, director of Bangladesh Fisheries Research Institute.

The fish may not migrate beyond the Padma bridge if the pattern of current changes or too much siltation occurs in the river, he told The Daily Star.

"Following our conservation activities, shoals of hilsa again started moving up to Rajshahi, even to Maida in India, during the spawning season. But the construction work would hamper this migration process,* he said.

Traditional fishermen of the Padma report decreasing catches in the area near the Padma bridge construction site.

"We used to net more fish before the beginning of the construction of the bridge. Nowadays we get less," said Mohammad Ismail near the Mawa ferry ghat last Sunday.

An increased traffic of heavily loaded vessels of the construction companies are also having on adverse effect on the fish, he added.

Hasan Matobbar, a fish wholesaler at Mawa Ghat, said fish prices have gone up due to less fish coming every day.

An engineer, who is with the project, said all large constructions negatively affect birds, animals and fishes. "Quite naturally, number of fish will decrease in the bridge area," he said preferring not to be named.

According to the EIA report, different types of work for the bridge, especially the pile driving which generates loud noise and vibration, would leave a serious negative impact also on dolphin vocalisation and migratory bird habitat.

Experts say the hilsa migrates from the sea to the estuaries and rivers for breeding and feeding during its spawning seasons, mainly in September-October.

Generally, the hilsa prefer to swim against strong currents in around 25-feet-deep water but they have stopped migrating to many rivers in the country for loss of navigability.

Even in the 80s, hilsa was available in more than 50 rivers, including all the big ones. But now they are found in only 10 rivers --Padma, Meghna, Jamuna, Paira, Tentulia, Bishkhali, Shibsa, Baleshwar, Kirttonkhola and Arial Kha, experts say.

There is a two-week ban on hilsa harvesting in October to increase the production.

Hilsa production in the country slumped to 1.99 lakh tonnes in 2002-03 season, creating a crisis. But the government initiatives helped increase hilsa catch to 3.85 lakh metric tonnes in 2013-14.

Hilsa makes up to 45 percent of the total catch in the Padma every year.