A Hong Kong conservation group said Tuesday increasing high-speed ferry traffic is contributing to a dramatic decline in Chinese white dolphin numbers in the city's waters.

Research by the Hong Kong Dolphin Conservation Society showed that catamaran and jetfoil ferry traffic is displacing dolphins from their habitats in the mouth of the Pearl River Delta.

"Dolphins depend on sound to survive in their environment, they are acoustic creatures," society chairman Samuel Hung said.

"There is a lot of noise generated from high-speed ferries... this noise creates a lot of stress for the dolphins and can change their habits."

High-speed ferry traffic between Hong Kong, mainland China and Macau increased 76 percent from 1999 to 2011, according to the Hong Kong Marine Department.

"In particular, since the opening of six ferry services at the Sky Pier of Hong Kong International Airport in late 2003, the dolphin number has dropped dramatically in North Lantau waters," the society said in a statement.

Dolphins will not hear a high-speed ferry until it is as close as 100 metres (330 feet) away, giving them only around 10 seconds to react, Hung said.

As a result, dolphins were leaving their habitats and preferred feeding grounds in search of safer waters.

Numbers of Chinese white dolphins in Hong Kong waters have dropped from 158 in 2003 to 78 in 2011, with about 2,500 in the Pearl River Delta, the body of water between Macau and Hong Kong.

Earlier this year a Hong Kong conservation group set up a DNA bank for the rare sub-species, also known as the pink dolphin, in a bid to save the mammals.

Related to Indo-Pacific humpback dolphins, they are known for their pink or white skin. They are listed as "near threatened" by the International Union for Conservation of Nature.

The mammal was the official mascot at the handover ceremony when the former British colony of Hong Kong returned to Chinese rule in 1997, while dolphin watching is a favourite tourist attraction in the city of seven million people.

Hung said plans to build a third runway on reclaimed land at the Hong Kong international airport would increase ferry traffic and place further strain on the dolphin population.