The dolphin, which appears pink because of blood vessels beneath its skin, was the official mascot of the former British colony's handover to China in 1997.

But large projects, including recent work on the $US19 billion bridge across the Pearl River estuary, threaten the animals, whose number has slumped nearly 80 per cent over the past decade to just 47.

"The construction of this bridge poses quite a significant negative impact to the dolphins and caused their really great decline," said Samantha Lee, an official of conservation group WWF in Hong Kong.

Also known as the Indo-Pacific humpback dolphin, the species has become much harder to spot in nearby waters, Lee added.

A spike in sea traffic around Hong Kong, pollutants and overfishing have contributed to the loss of their habitat.

Greater underwater noise from construction makes it hard for the animals to feed and communicate, said Lindsay Porter, a scientist at marine mammal research consultancy SMRU Hong Kong.

"When you increase that underwater noise, you not only stop dolphins from being able to forage ... we also stop them communicating with each other," Porter said, adding that such interruptions threatened their survival.

More than half the dolphins spotted showed signs of distress, such as skin lesions and infections due to stress and high noise levels, Porter added.

In an email response to Reuters, Hong Kong's conservation department said the decline in dolphin numbers was a complicated issue that needed further data analysis and close monitoring.

"To further protect Chinese white dolphins in Hong Kong, the government plans to designate more marine parks expeditiously," it added.

Government efforts to compensate for infrastructure projects with conservation sites, such as the Brothers Marine Park (BMP), have not brought back the dolphins, however, one environmentalist said.

Australian Associated Press