Midnapore, Sept. 1: The discovery of four Irrawaddy dolphins since May upstream in Kalighai river near Sabang, 150 km from Calcutta, in East Midnapore has set forest officials thinking.

The Irrawaddy dolphins (Orcaella brevirostris), as their name suggests, are natives of the Irrawaddy and Mekong rivers in Myanmar. This species can survive in both saline and sweet water, but prefers to keep to the coasts.

Officials said these dolphins are often seen in the Sundarbans. “In the coastal regions of Bengal and Orissa, there are about 300 such dolphins according to our estimate,” said one.

Subrata Bandopadhyay, the forest officer of Hijli Social Forest Division, said the original migratory zone of the dolphin is in the rivers of Myanmar. It has also been sighted in the state’s coastal regions and in the Hooghly river.

“But why are they shifting from the depths of saline and sweet water areas of their migratory zones to shallow rivers like Kalighai? Why are they doing this? Are they doing it to chase food or is it because of some other biological need? We have drawn up an elaborate plan to find these things out,” said Bandopadhyay.

The state forest department has embarked on a year-long project to research the reasons behind the seeming shift in habitat of the mammals.

The Irrawaddy dolphins are slow swimmers. They surface in a rolling fashion and lift their tail flukes clear of the water only when going for a deep dive. When they spring out, they release streams of water from their mouth.

These mammals are being hunted mercilessly for their oil. They are also caught as they make good performers in water parks.

Bandopadhyay suspects an increase in fishing activities in the coastal areas of the Bay of Bengal may have pushed them up the smaller rivers, sometimes as far as 30 to 35 km from Haldia on the coast.

“It is really surprising why these dolphins are migrating to Kalighai. There is a possibility that they have drifted in to avoid pollution along the stretches of Hooghly where industrial units are located,” said a forest department official.

Bidhan Biswas, the forest ranger in Panskura, said the first dolphin, a six-and-half-foot male weighing 100 kg, was caught in a fishing net at Kantakhali on May 24. The second, seven-and-half-foot long and weighing 200 kg, was also caught by fishermen, at Shiulibar.

On July 23, two dolphins — a male and a female — were caught in the net of fishermen in Uttar Nimki-Mohar.

“The female dolphin was probably pregnant and that is why it was abnormally heavy. We carried these dolphins in trucks with the help of villagers to the Roopnarayan river and released them there,” said an official.