They say severe drought that has been moving down the Amazon basin from the upper reaches of the river in Peru have caused fish populations to plummet.
This has left the Amazon river dolphins, which can grow to more than 9 feet in length, struggling to find enough food. Surveys conducted in the Peruvian Amazon have revealed a 47 per cent drop in numbers.
Dr Richard Bodmer, an ecologist from the University of Kent and the Wildlife Conservation Society who has been working with environmental charity Earthwatch to monitor changes in the area, said extremely low water levels in tributaries to the Amazon River had dramatically impacted on dolphin numbers.
He said that as the drought has moved further downstream, similar effects are being seen elsewhere in the Amazon basin.
He said: "This year there has been quite a difference in the numbers of pink river dolphins we are seeing.
"They are an important river species that tell us a lot about the health of the aquatic habitat in the river.
"The drought in the Amazon has been extremely harsh and water levels have been very low. We saw large numbers of dead fish when we were moving around on the river.
"This has a knock-on effect for a lot of species and especially the river dolphins that are being forced to seek refuge in some of the larger channels of the Amazon."
Pink river dolphins, which are only found in the Amazon, are the largest species of river dolphin and are the only one able to turn their neck, a trait that has evolved to help the aquatic mammals navigate around underwater tree roots.
Whereas most dolphin species have a distinctive dorsal fin, the pink river dolphin has a hump on its back. It has fully adapted to the freshwater environment and unlike other river dolphins is unable to live in salt water. Its skin colour ranges from off-white to bright pink.
In Peru and throughout the Amazon, the pink river dolphin has an almost-mythical status among indigenous populations. It is considered bad luck to kill one. There are many legends that describe these elusive creatures as shape-shifters.
Dr Bodmer said: "The drought that was seen in Peru has now moved down into Brazil, and we will almost certainly be seeing similar affects on the wildlife there."
A state of emergency has been declared in many parts of the Amazon after the area has experienced its worst drought for many years.
The dry spell reached its peak in the upper reaches of the South American river in late August and now the impact of the low water levels are moving downstream, leaving many smaller channels and tributaries almost completely dry. Brazil's Rio Negro has fallen to its lowest level in a century.
Dr Bodmer studied the impacts on wildlife in the Samiria River, which is a major tributary to the Amazon in Peru.
He found that there were half as many pink river dolphins in the river as there were last year. He also found that numbers of a smaller, related dolphin, the grey river dolphin, had fallen by 49 per cent.
Other animals affected by the low water levels include the spectacled caiman, a smaller relative of the crocodile. Birds have also suffered, including the chestnut-fronted macaw.
Dr Bodmer said the drought in the Amazon this year gave a stark warning of what could happen if global temperatures continue to rise under climate change.
He added: "If the extreme climatic events continue, both the wildlife and the local people will be severely impacted."
Conservation experts have also warned the dolphins are under threat from Amazonian fishermen who are increasingly slaughtering the creatures to use as bait.
Wendy Elliott, the WWF's species manager for whales and dolphins, said: "River dolphins are really fascinating creatures that much of the world has overlooked. They manage to live in rivers where others couldn't; they are very special.
"When we see river dolphin populations declining it is the first sign that the river system isn't healthy."
By Richard Gray, Science Correspondent
http://www.telegraph.co.uk/earth/wildlife/8114754/Pink-river-dolphins-at-risk-from-drought.html
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