While there is a sharp rise in the overall Hilsa population and production in the country, the number of Padma Hilsa is diminishing due to an insufficient period of ban on fishing aimed at preserving the stock, experts say.
There is a 22-day fishing ban imposed during spawning season. But the Padma Hilsa take 10 more days for spawning because they take a longer path. This makes them vulnerable to the fishing nets before releasing their eggs.
Two surveys conducted by Worldfish – an international organisation that works to preserve fish stocks – in 2018 and 2019 clearly show that the 22-day ban period does not ensure safe spawning for Padma Hilsa, said Hilsa expert Dr Abdul Wahab.
Dr Md Jalilur Rahman, a scientist (fish population biology) at Worldfish, said, "Data from 2018 and 2019 show that nearly 18-20 percent of the Padma Hilsa released eggs just after the spawning period ended, as opposed to 48 percent of the Meghna Hilsa. This means 82 percent of the Padma Hilsa did not manage to spawn during the period the ban was in effect."
Most of the Padma Hilsa caught after the ban ended were found mature, meaning they would have released eggs if allowed a few more days.
Dr Wahab said if this continues, the total stock of the Padma Hilsa will deplete eventually.
"This is obviously something we have found happening already when we randomly explored fish markets around the country."
According to estimates by the Department of Fisheries, total production of Padma Hilsa in the 2017-18 fiscal year was 3,809 tonnes, compared to more than 46 times the number – 176,399 tonnes – in the Meghna River during the same period.
Meanwhile, Hilsa production has doubled in a decade. According to government data, production of the silver fish stood at 517,000 tonnes in the 2017-18 fiscal year, up from 279,189 tonnes in the 2006-07 fiscal year.
"We need to make some corrections in the 22-day blanket ban period that is currently in force. Either we extend the total ban period for 10 more days, apply an additional ban period for the Padma River and its tributaries, or we start a delayed ban for that particular area," explained Dr Wahab.
The agency responsible for proposing an effective ban period is the Bangladesh Fisheries Research Institute.
Anisur Rahman, chief scientific officer of the institute's Riverine Station in Chandpur, said the 22-day ban was calculated after years of research.
"A decade of observation has revealed that the ban is mostly effective, given the sharp rise in overall production of Hilsa. We have to strike a balance between the sustenance of the fishermen and the Hilsa population," he said.
He also said any increase in the ban period will prove too taxing for the fishermen as this will shorten the precious catching season.
Masud Ara Momi, assistant director of the Department of Fisheries and project director of Enhanced Coastal Fisheries in Bangladesh, said the government has not received any new proposal regarding change in the ban period yet.
"If new research findings warrant it, the authorities are open to consider that," she added.
A unique taste
Not all Hilsa fish taste the same.
"I always look for the Padma Hilsa because it is the tastiest one," said Anowara Parveen, a school teacher who came to buy fish at a kitchen market in Mohammadpur – a mid-income neighbourhood in Dhaka.
It was the off-season for Hilsa in mid-winter, but the fish was still available at the market.
Anowara said anyone who is a Hilsa aficionado can tell the difference in taste.
Jasim Uddin, a fish seller in that market, said most of the buyers ask for the Padma Hilsa, which is pretty rare.
"The fish sellers try to dupe customers by claiming every Hilsa to be Padma Hilsa," he said.
Asked whether he can tell the difference, the trader said, "I cannot, but some claim they can."
Dr Wahab said the taste varies because of the different eco-systems the fish traverses.
One fish, three variants
There are three different varieties of Hilsa. A recent study has traced the differences to the genetic level.
The findings of the study were published in Scientific Reports Nature, an associate publication of the renowned scientific journal Nature.
Published in the first week of November last year, the study found that a distinctly different river route is taken by the Padma Hilsa – one of the three variants of the Hilsa stock – for spawning during the breeding season.
In the language of experts, it is called the turbid freshwater (Western Riverine) variant. The two other variations are clear freshwater (Eastern Riverine) and brackish-saline (Southern Estuarine-Marine).
Explaining the findings of the study, Dr Abdul Wahab, the co-author, said, "Millions of Hilsa shad, scientifically named Tenualosa ilisha, start their journey upstream from the sea in mid-October. To be precise, they begin at the full moon during the Bengali month of Ashwin. That is their main breeding season although they breed all year round."
From outward appearance, all members of the upstream Hilsa stock may look the same. But there are two distinct variants among them, and their routes diverge when they reach the confluence of the Meghna and the Padma rivers.
One variant takes the eastward route, and enters the Meghna and its tributaries – the Surma and the Kushiara. The other goes westward and enters the Padma and its tributaries.
However, there is a third variant of the stock that does not enter the river system at all. Members of this variant stay close to the estuary and release their eggs in the water that becomes semi-saline due to close proximity to the sea.
"The three variants thus embrace three different riverine ecosystems," said Dr Wahab, a renowned Hilsa expert.
To him, Hilsa fish variants are almost analogous to racial differences of human species.
According to the co-authors of the study, it was the first-ever high quality, elaborate genetic study on Hilsa carried out the world over. They said it clearly explained the different variations of this anadromous fish stock at the genetic level.
To analyse the data of the study – jointly carried out by a group of Bangladeshi and Malaysian scientists – SNP, a modern genetic technology, was used in the Apical laboratory in the US.