The survey would be simultaneously conducted in Bihar, Jharkhand, Uttar Pradesh and West Bengal from October this year and results are likely to be available by February next year, sources at the National Mission for Clean Ganga (NMCG), functioning under the Union Ministry of Water Resources, River Development and Ganga rejuvenation said.

According to NMCG sources, it would be the country's first national-level survey for the head count the Ganga dolphin (PlatanistaGangetica) in the river using the direct count method of the International Union for Conservation of Nature (IUCN).

"Though some States have independently surveyed Ganga dolphins in the past, they followed different methodologies for their enumeration. Henceforth, we have decided to conduct a collective survey using a standard method to know the population and distribution range of the species," an NMCG official said.

Rs two crore has been earmarked for the purpose, the official said, adding about 450 surveyors, including stakeholders such as local villagers, academicians and NGOs would be engaged for the survey. The survey would be carried out by the forest departments of the respective state governments, the official said.

The survey is being conducted as part of the NDA government's flagship 'NamamiGange' programme, which integrates the efforts to clean and protect the Ganga river.

The presence of Ganga dolphins is a clear indicator that the river is in good health for human consumption, while its decline warns about the poor status of the river.

As per an estimate, the number of dolphins in the Ganga was around 3,500 in 2014, a drastic drop from over 5,000 in the early 1980s due to a variety of reasons, the sources claimed.

Industrial pollution, pesticides, unregulated use of fishing nets, arbitrary sand mining, poaching and accidental deaths are cited as major threats to the survival of these dolphins.

Officials claim anywhere between 10 and 15 per cent of Ganga river dolphin are killed annually due to the above reasons. "In fact, Ganga river dolphins are like the tigers of forests. These dolphins are in the top position of the food chain in the eco-system of the river. So, their growth is important for the good health of river Ganga," an official observed.

As part of the survey, there are also plans to carry out the head count of other indicator species such as turtles, gharials, otters and aquatic birds.

The ministry of environment and forests notified the Ganga dolphin as the National Aquatic Animal in 2010. They are locally known as 'susu' due to the noise they make while breathing. This species inhabits parts of the Ganga, Meghna and Brahmaputra rivers in India, Nepal, Bhutan and Bangladesh. It is also found in Karnaphuliriver in Bangladesh.

The Ganga dolphin is a critically endangered species in India and has been included in the Schedule I of the Wildlife (Protection) Act, 1972.