The blind river dolphin is an endangered species (Photo: WDCS)

The census, the first of its kind in the Sunderbans delta region in West Bengal state, will also cover the neighbouring states of Bihar and Assam.

Just 2-3,000 river dolphins remain in India, down from thousands a century ago, wildlife officials believe.

Plummeting numbers have been blamed on pollution, hunting and fishing nets, in which dolphins become entangled.

Poachers kill river dolphins mainly for their oil, which is then used by fishermen as bait, wildlife officials say.

Most river dolphins are blind - the only species in the world to have eyes without lenses.

They navigate through the silt and mud using a sophisticated sonar system.

Scanners

India holds censuses of river dolphins "once, or even twice a year" in the northern states of Bihar and Uttar Pradesh, Aniruddha Mookerjee, a Wildlife Trust of India (WTI) official, told BBC News Online.

But this census is being seen as significant because it is the first of its kind in the Sunderbans delta, home to the world-famous Royal Bengal Tigers.

Since the river dolphin is at the head of the aquatic food chain, its presence in adequate numbers symbolises greater bio-diversity in the river system Colonel Shakti Banerjee World Wildlife Fund (WWF)

"They are as endangered as the better-known tigers, if not more endangered," Subrata Pal Chowdhury, a senior West Bengal wildlife official told Reuters news agency.

He said the census would reveal the actual number of dolphins left in the rivers of eastern India.

Wildlife officials say they are scanning the Ganges and its tributary, the Bhagirathi river, for dolphins using sophisticated photo imagery equipment on boats fitted with searchlights.

World Wildlife Fund (WWF) official Colonel Shakti Banerjee told the BBC that the presence of dolphins in a river points to a healthy ecosystem.

"Since the river dolphin is at the head of the aquatic food chain, its presence in adequate numbers symbolises greater bio-diversity in the river system," he said.