A joint study by several institutions, including India's central fisheries research centre, shows that many hilsa that enter freshwater rivers from the sea to spawn never return to the sea.

Scientists involved in the study told the BBC this is exactly why hilsa are now found all year round nearly 200 kilometres upstream of the Ganges estuary, and they taste and smell excellent.

They believe the shoals are staying in fresh water out of fear of the fishing nets set in the estuary. But why and how are Bay of Bengal hilsa becoming permanent residents of fresh water?

Hilsa is a sea fish, but it enters rivers in shoals to spawn. That is what hilsa-loving Bengalis have always known. A recent study funded by the Indian Institute of Sciences in Bangalore, however, shows that many hilsa enter the Ganges to spawn and never return to the Bay of Bengal.

Professor Asim Kumar Nath, one of the lead scientists in the research team, said they found the evidence in variations in the levels of different chemicals in the hilsa's otolith.

"The otolith is an organ of fish, which in hilsa is in the head, and in some fish in the pharyngeal region. Analysing the otolith gives an idea of a fish's migratory route," he said.

"Cutting hilsa otoliths, we now see chemical ratios that make it clear many hilsa are not going back to the sea. In fresh water they have grown quite heavy, 500 or 550 grams, while tiny hilsa of five to ten grams are also being found."

B K Mahapatra, principal scientist at India's Central Institute of Fisheries Education, believes that not returning to the sea is a kind of survival effort, or natural selection, by these hilsa.

"Giant freshwater prawns are also seen in Haridwar, and they come down to the Sundarbans estuary to spawn. Such fish are called catadromous. But hilsa is an anadromous fish: it goes from the sea into rivers to spawn."

Source: BBC Bangla. First published in Bengali on The Agro News.