Walker acknowledged that small cheap tours run out of Tai O were a possible nuisance to the dolphins, but said those trips were only a minor part of the threat.

"I just think they are trying to pass the buck. These boats are such a teeny weeny per cent of the problem compared to a damn great big bridge and airports in their habitat," she said.

"They are being disingenuous blaming dolphin-watching activities. I don't accept that."

A study by the Dolphin Conservation Society released on Tuesday said the number of pink dolphins, also known as Chinese white dolphins, in Hong Kong waters dropped to 61 last year from 78 in 2011, and 159 in 2003.

Society chairman Dr Samuel Hung Ka-yiu said the drop had been particularly acute in the past two years and blamed noise and marine traffic involved in the construction of the bridge.

The department said it had taken measures to minimise the impact on the dolphins' habitat and marine ecology and that other factors, including dolphin-watching tours, could be behind the decline.

Walker defended the Hong Kong Dolphinwatch tours, saying they were aimed at building awareness about the dolphin and were sensitive to the welfare of marine life.

She said their four-hour trips stringently followed all the guidelines set out by the Agriculture, Fisheries and Conservation Department (AFCD) in its voluntary Code of Conduct for Dolphin Watching Activities.

But Walker said that did not appear to be the case for many of the small boats operating out of Tai O.

"On some days there will be five or six of these boats whizzing around after two dolphins. We have even seen them getting in between a mother and her calf," she said.

"These boats have 10 or 15 seats and they speed out of Tai O, chase over to any dolphins they can find, let everyone take pictures, then they speed back in. They are probably not a great thing for the dolphins because they have no regard for the AFCD's code of conduct.

"The conduct is a very good set of guidelines and we do abide by them, but we wish everyone else would do the same."

Walker said she had complained about the boats' activities to both the agriculture department and the Marine Department but nothing had been done.

"If the Highways Department thinks it is an issue, then they should do something about it."

A spokeswoman for the agriculture department said operators of dolphin watching activities had to observe the Wild Animals Protection Ordinance, which made it an offence to cause wilful disturbance to dolphins.

It also insisted they comply with marine control legislation regarding the safety and manning of vessels carrying passengers as monitored by the Marine Department.

"The AFCD has also organised educational and publicity activities for operators of dolphin-watching activities and members of the public through promulgating a Code of Conduct for Dolphin Watching, conducting seminars and making available educational and publicity pamphlets to teach people what they should do and what they should not do when participating in dolphin-watching activities," she said.

"The Marine Department and AFCD will continue their efforts in enforcing the law to enhance marine safety and protect the Chinese white dolphins."

The Highways Department refused to comment on the issue of dolphin-watching tours but said that according to its own site-monitoring records, the water quality in the vicinity of the bridge project was "more or less the same" as before the construction began.