Recent sightings of the endangered Chinese white dolphin in Lantau waters near construction of the Hong Kong-Zhuhai-Macau bridge have dwindled by as much as 60 per cent, the project website shows.
A conservation group says the species is facing unprecedented challenges to its survival because of the planned and ongoing marine works in north Lantau.
Works officers of the Highways Department downplayed the significance of the reduction, saying it was not "directly related" to the construction of the border crossing.
But the alarming fall in numbers emerged as eight white dolphins have so far been found dead across Hong Kong this year - one more than the total of seven last year. Only 61 dolphins now lived in the city's waters, compared with 158 a decade ago, WWF Hong Kong said.
The group wants more transparency in dolphin monitoring and better measures to protect the species.
WWF marine conservation officer Samantha Lee Mei-wah urged the department to release more details of the monitoring and of any contingency measures employed to stop the population from shrinking further.
"We have no idea what or whether contingency measures have been introduced to mitigate the impact, and whether the measures have been effective," she said.
The border-crossing construction plan calls for creating more than 100 hectares of land east of the airport.
According to data released on the project website, the "encounter rate" of the dolphin in the monitoring area between March and May fell below the department's predefined limit.
Highways Department officers calculate the encounter rate by dividing the number of sightings by the boat distance travelled. They conduct observations twice a month along 23 lines in waters around the worksite.
Conservationists see the rate as an indicator of whether the dolphin is under stress from the land reclamation works.
The department does not say what its limit is. But in a letter submitted in May to contractor China State Construction Engineering, its environmental team said exceeding the limit meant at least 60 per cent below the baseline recorded before construction began in 2011.
The limit takes into account natural fluctuation in population numbers.
Translated into the encounter rate, a 60 per cent drop means that, for every 100 kilometres surveyed, there are fewer than 2.6 dolphin sightings east of the worksite, and 3.9 to the west.
Lee said the contractor was obliged to find out the cause of the reduction and take mitigation measures, as promised in an environmental-impact assessment report. In the worst case, the works might need to be suspended, she said.
The department said the decline could be related to "general environmental factors" and "seasonal fluctuation". It said the encounter rate should not be a yardstick to determine whether dolphin numbers had dropped.
It also said the number of dolphins had recently risen to "close to the norm", but did not elaborate or offer proof.
"There are many factors affecting the possibility of the dolphins being observed," a spokesman said. "Distribution of the Chinese white dolphin exhibits strong seasonality, and the encounter rate would fluctuate."
The project also missed water-quality targets several times in May and June, the contractor reported, but it did not attribute this to the project either.