Hilsha is not exclusive to the Meghna basin only. They enter Irrawaddi River in Myanmar and Bhagirathi-Hooghli River in West Bengal
When someone buys a hilsha from a regular city market, chances are pretty high that the fish will be a female one with eggs inside. Male hilshas are rare. All the fish markets in every corner of the country boast of carrying female hilshas.
Experts say during breeding season in October, both male and female hilsha wade up the stream from the estuary almost in equal numbers. So, where do the male members all go? How do they manage to vanish in thin air?
In mid-September when I visited the fish landing station in Chandpur, the biggest hub of hilsha trade in the country, I was intrigued by the fact that all the fishes laid in heaps on the floor were female ones.
I enquired the traders repeating the same question I set out my journey with: Where have all the male hilshas gone?
No one in the market could provide me with any plausible answer to the mystery.
"We do not bother about the male fishes. We are just happy to see that all the fishes are female, bulging with eggs, because that's what the customers want. It sells," said Meraj Ahmed, a fish trader, who owns a trading house named Hazi Siraj Enterprise.
"Studies carried out by Ecofish project have shown that during the breeding season, in a hoard of hilsha, the average female-male ratio is 55:45," said Dr Abdul Wahab, a hilsha expert from WorldFish, an international organisation that works for preserving fish stocks.
So why do we see only female fishes everywhere? Several years back, Abdul Qaium, a journalist and writer of popular science, began looking for a clue.
He said, "After enquiring a number of fishermen and experts I found that male hilshas are not as rare as we take them to be. Fishermen do catch male fishes in their nets, but they cannot tell a female from the male one, as the male do not have any genital organs."
"Males release their sperm in the water the way the females release their eggs. So, they look almost alike," said Dr Wahab, "with the exception that male ones do not have eggs inside, but so do the female ones who have exhaustively released all the eggs."
Only an expert eye can determine which stocks are male, as does Kinckar Saha, a research associate of Ecofish, based in Chandpur.
Kinckar said, "Several years back, on a research errand, I was on a fishing boat in Char Bhairab, a village 10 kilometres south of Chandpur. After pulling the net from the water, the fishermen became sad at the yield of the catch. The fishes in the net were small in size and they looked comparatively weak, thin and unimpressive. On closer inspection, I found out that almost 62 percent of the fishes were male. It was a predominantly male hoard, which is pretty rare, because usually males populate 40-45 percent in a hoard."
"The male fishes lack the lustre the female hilshas have. They look sort of unattractive, if not ugly," said Dr Anisur Rahman of the Fisheries Research Institute, Chandpur.
These fishes do not end up in the regular fish markets. The fishermen separate them and send them to be sprinkled with salt and dried up, a local technique of preservation popularly known as "nona Ilish".
Hilsha, explained Dr Wahab, is an anadromous fish, which means like salmon, smelt, hickory shad, lamprey and gulf sturgeon they follow a special pattern of life cycle. Hilsha fish are born in freshwater, for eight months they migrate to the ocean as juveniles where they grow into adults, before migrating back into freshwater to spawn.
Hilsha do not perform sexual intercourse. The male fish releases the sperm in the water like foams and the female fish releases the eggs on that foam-like broth, fertilising them.
During the breeding season, which researches have finally pinned down to the full moon during the Bengali month of Ashwin, the fish start moving upstream along the River Meghna and its numerous tributaries.
"The strong river currents send shivers down the spines of the fish – both male and female," Dr Abdul Wahab elaborated. "It stimulates them sexually and they swim upstream, faster and faster."
Hilsha is not exclusive to the Meghna basin only. They enter Irrawaddi River in Myanmar and Bhagirathi-Hooghli River in West Bengal. Hilsha has its presence even further. Starting from the Mekong delta in the east, the fish can be seen as far west as the Persian Gulf. Even in the Euphrates River, one can find a Hilsha.
"But nowhere on Earth do these fish rush in such large numbers as it does in the rivers of Meghna basin," said Dr Wahab, who heads the EcoFish project of WorldFish.
"Not all the female fishes have egg sacks filled either," said Dr Anisur Rahman. "We can find eggless female fishes in the market. After emptying the egg sacks on a return journey these female fishes also look exhausted. They appear as unattractive as the male ones, making it hard for a regular fisherman to tell one from the other. Fishermen call them 'paik machh' in local language."
Manik Dewan, a fisherman on the Meghna, said, "We know, we have heard that there are male hilshas in our catch, but we do not bother identifying them. All we bother about is the size and look of the fish.
For us, there is good fish and 'paik machh'."
Wandering in the Meghna River, near Haimchar, an area famous for hunting hilsha, I came upon a rumour that on a return journey, after releasing sperm in the water and impregnating the eggs externally, the male hilsha undergoes a gender metamorphosis and turn into female ones.
"We do not have any proof as such," said Dr Wahab. "No research has revealed even a hint of such a metamorphosis."