By Alastair Lawson
BBC News, in the Sunderbans of Bangladesh
Seeing the river dolphins of Bangladesh is not something that is easily forgotten. They rise arc-like and majestic out of the water only inches from boats that ply the rivers of the country's south.
In a country where the wildlife population has been denuded because of over-crowding and pollution, dolphins provide visitors with a beautiful and memorable surprise.
But conservationists say they are increasingly concerned over the future of the country's river dolphin population, some of which they warn may even be at risk of extinction.
They say that it is rapidly declining because of over-fishing, a shortage of prey, pollution and declining freshwater supplies.
Experts are particularly concerned over the fate of two species - the Ganges river dolphin and the Irrawaddy dolphin whose numbers they say have significantly reduced over the last decade.
"This is probably because of intense human activities - such as farming and fishing - that takes place in their river and near shore water habit," said dolphin expert Elisabeth Fahrni Mansur.
Over-fishing is being blamed for the dolphins'' decline
"But they are also at risk because of the clumped nature of their overall distribution, which results in a patchwork of relatively small groups demographically isolated from each other."
While Bangladesh currently supports relatively large populations of Ganges river dolphins and Irrawaddy dolphins, conservationists argue that it's crucial to address the threats they face now, while the potential for long-term survival of both species is still relatively high in comparison to other areas in Asia.
While other species such as Indo-Pacific humpback dolphins and Indo-Pacific bottlenose dolphins are not currently threatened, their future can by no means be taken for granted. But it's the Ganges river dolphin and the Irrawaddy dolphin which give the most cause for concern.
"The most dire threat to them comes in the form of accidental deaths caused fishing nets," said Ms Mansur.
"Fishermen don't target the animals, but when they often become entangled in nets they easily drown because they are breathing mammals.
"A more long term threat comes from declining freshwater supplies - primarily due to water extraction upstream in India - and sea-level rises which have led to profound changes to the ecology of their habitat."
The bulk of the country's freshwater dolphin population live in the south-west of the country, especially in the rivers and waterways of the Sunderbans mangrove forest.
Rising salinity is killing the animals
Experts point out that these rivers are particularly affected by toxic and industrial waste which is dumped in the water further upstream.
"Rising salinity through both climate change and declining freshwater supplies is also a real and a long-term challenge to the ecology of the Sunderbans," said Ms Mansur.
Dolphins in the forest tend to partition themselves according to the level of salinity - Ganges river dolphins for example are found in mangrove channels with high freshwater inputs, while Irrawaddy dolphins live in more salty mangrove channels further downstream.
Experts say that the level of salinity in these areas is crucial to the survival of the animals and to the livelihoods of over 30,000 fishermen in the Sunderbans. Already at least 11 species of fresh water fish are extinct.
The animals can surprise with their arc-like appearances
In what many environmentalists see as an ominous development, the finless porpoise - primarily a coastal species - has recently been discovered in the Sunderbans which provides another indication of rising salinity.
Steps are now being taken to combat the problem.
The Bangladesh Cetacean Diversity Project (BCDP) and the New York-based Wildlife Conservation Society (WCS) are proposing a protected area for dolphins, which would consist of three priority sites in the Sunderbans.
In addition to monitoring salinity levels, accidental killings of dolphins by fishermen would also be surveyed.
"The idea is that fishermen will be provided with relatively inexpensive global positioning systems and depth sounders, in addition to being trained how to use them so that they can navigate safely to shore during storms," said Ms Mansur.
"In return the fishermen would safely release live animals found entangled in their nets, and collect samples and basic information on animals found already dead.
"But the battle to save these animals is not going to be easy. Salinity and over-fishing are in many respects facts beyond our control. We are the local end of a global battle."