Conservation activists say the number of rare Chinese white dolphins in Hong Kong waters has significantly dropped due to the construction of the world's longest sea-crossing bridge linking the territory to mainland China.

Key points: The number of Chinese white dolphins in Hong Kong waters dropped from 80 to 47

The number of Chinese white dolphins in Hong Kong waters dropped from 80 to 47 Conservation activists say the measures put in place to protect dolphins failed

Conservation activists say the measures put in place to protect dolphins failed Hong Kong-Zhuhai-Macao Bridge Authority says $68m was allocated to protect the dolphins

It comes amid reports that the 55-kilometre-long bridge — dubbed the "bridge of death" by some local media — has also claimed the lives of 20 workers and injured more than 500 people during its construction.

The $20-billion bridge, which took almost a decade to build, was officially declared open by Chinese President Xi Jinping on Tuesday.

Taison Chang Ka-tai, chairman of the Hong Kong Dolphin Conservation Society, told the ABC the dolphin population in Hong Kong waters had dropped by more than 40 per cent — from an average of 80 sightings in 2012 to 47 in 2017.

Mr Chang said the impact of construction on the white dolphin population was evident in the distribution of the dolphins in the area.

"During the construction of the bridge, we can see the dolphins in north Lantau [Island] almost all disappeared from that area, which is the closest area to the construction," he said.

"So we can see a very clear relationship between the construction and the distribution of the dolphins."

Dolphins spotted 'congratulating' bridge opening: Xinhua

Sorry, this video has expired China opens world's longest sea crossing (Photo: AP)

Despite the drop in dolphin numbers, a report from Chinese state media CCTV said high priority had been given to protect the white dolphins, also nicknamed "China's marine panda".

A statement on Hong Kong-Zhuhai-Macao Bridge Authority's website states $68 million was allocated to protect the dolphins.

Mr Chang said that while mitigation measures were put in place by Hong Kong Government's Environmental Protection Department — such as pausing work for 30 minutes when a dolphin was spotted — the measures proved ineffective.

A Chinese white dolphin swims off Lantau Island in Hong Kong. ( Reuters: Bobby Yip, file photo )

"After they saw the measures were not effective, they didn't do anything to try to make the dolphin number increase again or stop the construction for a while to see if they can improve the situation of the marine environment," he said.

"I can imagine the situation in Chinese waters is even worse than Hong Kong."

State media Xinhua commented on its Weibo account that white dolphins were spotted "dancing around the bridge" on the opening day, as if they were "congratulating it on its birthday".

Mr Chang called the comments "ridiculous" because it was natural for dolphins to poke their heads up for air.

'No other country is capable of building this bridge'

The world's longest sea bridge lit up in Hong Kong. ( AP: Kin Cheung )

Chan Kam-hong, chief executive of the Association for the Rights of Industrial Accident Victims, told Radio Television Hong Kong (RTHK) on Tuesday morning that the project had also resulted in a "very critical" number of casualties.

"What we know so far is that there were 11 workers who died on the construction site in Hong Kong part [of the bridge], and nine workers who died on the mainland part," he said.

"It's very difficult for us to compile the exact number of casualties because the Government didn't give us any means to obtain it.

"This is a huge project, but our view is that while it is a huge and challenging project, it is not an acceptable reason for causing a greater number of casualties."

Despite the hefty casualties, Chinese state media and social media posts about the Hong Kong-Zhuhai-Macau Bridge have been largely positive.

One user with the nickname yuanshuizhiyuan commentated on Weibo that "no other country in the whole world is capable [of building this bridge]".

The bridge officially opened to public traffic at 9:00am (local time) on Wednesday after major delays and cost overruns.

Construction was suspended after 66-year-old Chu Yee-wah applied for a judicial review of the project's environmental assessment report in Hong Kong's High Court in April 2011.

It resulted in an 18-month delay of the project, which was originally scheduled to be completed in 2016.