A joint study by several organizations, including India's Central Fisheries Research Center, found that many hilsas that enter freshwater rivers to lay eggs from the sea never return to the sea.

Scientists involved in the study told the BBC that this was the reason why hilsa was now available all year round, about two hundred kilometers upstream of the Ganges estuary - and they were very good in taste and aroma.

They are speculating that the swarms of hilsa are staying in the fresh water for fear of the leafy fishing nets in the estuary. But why and how is the Bay of Bengal becoming a permanent resident of freshwater?

Although hilsa is a fish of the sea, they flock to the river to lay their eggs - this is what hilsa-loving Bengalis have known since time immemorial.

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But a recent study funded by the Indian Institute of Sciences in Bangalore found that many hilsas enter the Ganges to lay eggs but never return to the Bay of Bengal.

Professor Asim Kumar Nath, one of the leading scientists of the research team, said that they have found evidence of this by looking at the differences in the amount of different chemicals in the 'autolith' of Hilsa.



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There is a lot of demand for hilsa in the market

He said, "An organ of the autolith fish, which is in the head in the case of hilsa, is also present in the pharyngeal region of some fish. Analysis of this autolith gives an idea of ​​the migratory route of a fish.

"We can now see that the autolith of hilsa has been cut and the ratio of different chemicals is such that it is clear that many hilsas are not returning to the seaThey have also weighed a lot in fresh water - five hundred or five hundred and fifty grams - while small size hilsas weighing five to ten grams are also found. "

BK Mohapatra, chief scientist at the Central Institute of Fisheries Education in India, thinks that not returning to the sea is a kind of survival or 'natural selection' of these hilsas.

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"Lobsters can also be seen in Haridwar, from where they move to the mouth of the Sundarbans to lay their eggs. This type of fish is called catadromas. But hilsa is anadromas, they go from the sea to lay their eggs in the river."

"But why don't they want to go back now?Not wanting to, because there are more than fourteen thousand nets spread across the Ganges estuary - so they stay in fresh water to survive. It is seen as evolution or natural selection, "said BK Mohapatra.

It was seen in Gujarat many years ago that swarms of hilsa entered the Ukai reservoir along the Tapti river and started living there, laying eggs and giving birth.

Now a lot of similar things can be seen in the Ganges - says Professor Asim Kumar Nath.

"If you go to Balagarh more than two hundred kilometers above where the freshwater starts at Nishchindapur below Kakdwip in the Ganges, we can see January-February-March-Pre-Monsoon or Post-Monsoon by calendarHilsa is found all over the area throughout the year. "