A century ago, thousands of river dolphins were reported swimming along huge stretches of major rivers in the north and east including the holy Ganges and the mighty Brahmaputra.

"They are as endangered as the better known tigers, if not more endangered," Subrata Pal Chowdhury, West Bengal state wildlife officer, told Reuters.

Dozens of rare river dolphins get killed each year after getting snagged in fishing nets or slashed by boat propellers.

Others die due to sewage, toxic effluents and pesticides that flow into India's rivers from cities, factories and fields.

The first pilot count of dolphins is under way in the Sunderbans, where salty creeks and tributaries wind their way around slushy islands of mangrove forests into the Bay of Bengal.

Around 50 wildlife officers will scour the muddy waters in two dozen boats for a week to note the behaviour of dolphins.

"With this pilot census, we will have a benchmark to carry out future counts of freshwater river dolphins upstream," Atanu Raha, chief conservator of forests of West Bengal, told Reuters.

The dolphins in the Sunderbans are called Irrawaddy dolphins - Orcella brevirostris - and are round-headed, snub-nosed and do not have dorsal fins.

Upstream from the Sunderbans, fresh water dolphins - Gangetic dolphins or Platanista gangetica - are blind and they swim in turbid water and locate prey by sonar.

River dolphins are also found in China, Brazil, Pakistan, Nepal, Myanmar and Bangladesh.