Image copyright Kabir Uddin/Getty Images Image caption Fish are disappearing in the Bay of Bengal

Overfishing off the coast of Bangladesh is creating a "fishless" zone in one of the world's largest marine ecosystems, scientists are warning.

Most fish species are in decline, with some nearing extinction, a report on fish stocks in the Bay of Bengal says.

"Some seas in the world, like the Gulf of Thailand, have run out of fish," one of the authors of the report, Sayedur Rahman Chowdhury, told BBC Bengali.

"We don't want our Bay of Bengal to end up like that."

Hundreds of large vessels are overfishing at an unsustainable rate, monitors suggest. Local fishermen say the government is turning a blind eye as the trawlers target key fish species they rely on.

A resource running dry

Bangladesh is one of the most densely-populated countries on Earth, with its population crammed into a delta of rivers that empty into the Bay of Bengal.

At least 1.5m people in the country are dependent on fishing for their livelihoods and fish remains the most important source of animal protein for the population overall.

But a three-year report commissioned by the government shows the largest and most valuable species, like tiger prawns and Indian salmon, are almost completely gone.

Image caption Millions rely on fishing for their livelihoods in Bangladesh

Jasim is a fisherman who's worked out of the port of Chittagong for 35 years. He says until a few years ago he only had to sail for a couple of hours to catch fish but now he and his colleagues travel for up to 20 hours before they find anything.

"There are many species of fish we used to catch before but we can't find nowadays," Jasim says.

He and the other artisanal, or small-scale, fishermen blame the scarcity on the presence of large trawlers in the bay.

Trawlers out of control

There are about 270 trawlers off the coast of Bangladesh, the biggest of which can catch up to 400 tonnes of fish each trip, 20 times the amount of the largest artisanal vessel.

"We are truly worried that if the fishing effort is not substantially reduced, we may lose this resource for generations to come," Mr Chowdhury says.

The fees the government receives from trawler licences are a small fraction of the profit made by the handful of companies which own the industrial fleet.

Image caption Industrial trawlers often come into conflict with local fishermen

These companies sell and trade fishing licences that were issued by the government years ago, making it difficult for the department of fisheries to control the amount of vessels operating.

New legislation that will give the department the power to cancel old licences is currently making its way through parliament.

But enforcement officers from the department are often taken to court by operators over their attempts to enforce existing regulation and risk being held personally liable if they lose.

A senior official at the Naval Trade Department, Captain Mohammad Giasuddin Ahmed, said no new licenses should be issued to trawlers until there is concrete information about stocks.

"If this carries on then our fishing ground will become fishless,'' he says.

With fish increasingly scarce, trawlers have begun targeting hilsa, a species crucial to the livelihoods of artisanal fishermen and the food security of Bangladesh.

A success story in jeopardy

The hilsa, a kind of herring and Bangladesh's national fish, is the only species showing some signs of recovery.

Image copyright oqba/Getty Images Image caption The market for hilsa is worth £2bn

For several years now, the government has implemented a 22-day annual ban on hilsa fishing that takes effect every October.

The seasonal ban allows the hilsa time to migrate from the bay into river estuaries to spawn.

As compensation, the government gives artisanal fishermen a subsidy of around 44 pounds of rice per household not to fish.

But many say they struggle to feed their families and face financial ruin during this time.

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When the government announced in May last year that it was adding a further 65-day ban for all fishing activity without the benefit of subsidies, hundreds of fishermen took to the streets to protest.

The bans have helped hilsa stocks recover but artisanal fishermen aren't the ones reaping the rewards, according to Mr Chowdhury.

"Although hilsa conservation affects millions of poor fishers, a large proportion of the benefit is going towards the industrial trawl operators who are catching thousands of tonnes of hilsa without providing much social benefit or revenue to the government," he says.

The hilsa recovery has also begun attracting "'super-trawlers" from abroad that are fitted with equipment to track and target the hilsa schools.

A new threat arrives

Super trawlers have double the capacity of existing industrial vessels.

Their size and engine power makes them quick enough to catch the fast-moving hilsa and they are fitted with sonar equipment to help them locate the shoals.

Four of these huge vessels arrived in Chittagong from abroad last year.

Bangladeshi operators bought the four super trawlers moored in the port and claim they have rights to use them to fish.

Image caption Sea View and Sea Wind were seen docked in Chittagong by the BBC

Two of the four vessels, Sea View and Sea Wind, originally of Thai origin, are subject to an Interpol notification for illegally fishing in Somalia.

International monitoring organisations OceanMind and the International Justice Mission (IJM) have been following the super trawlers since 2018 and confirmed, using satellite imagery, the presence of both vessels in Chittagong port.

Under international law Bangladesh must notify the government of Somalia about the presence of Sea View and Sea Wind in its waters.

When asked about the blacklisted vessels, Capt Giasuddin Ahmed said: "We are not aware of the presence of Sea View and Sea Wind vessels.

"They first entered Bangladesh with the excuse of repair work, but they were later expelled."

Image copyright Satellite image ©2020 Maxar Technologies Image caption Sea View and Sea Wind, seen here circled in red, were traced using satellite imagery

The BBC approached the Bangladeshi government for comment and was told the fisheries department was closed because of the coronavirus outbreak and the minister was unable to respond.

But Mr Chowdhury is worried about the impact the super trawlers could have on fish stocks and the future of fisheries management in Bangladesh.

"These super trawlers are a threat to marine resources,'' he says.

"If such illegal vessels can enter Bangladeshi waters and get registered without any obstacles, then it could be suggested that Bangladesh has become a safe haven for blacklisted vessels."