Chinese white dolphins will face a new threat if the construction of the third runway of Hong Kong International Airport gets approved, environmentalists say.

The environmental impact assessment report on the runway has been criticized by groups including the World Wide Fund for Nature and the Hong Kong Dolphin Conservation Society.

Chinese white dolphins mainly inhabit the Pearl River and the waters off Taiwan, and are a national first-class protected animal.

According to the environmental groups, the number of Chinese white dolphins in Hong Kong waters has declined from 154 to 62 over the past decade, mainly due to land reclamation, construction and pollution.

The report says the third runway will be built on 672 hectares of reclaimed land in open water. The runway is being built to raise the airport's passenger capacity to 102 million tons and freight capacity to 8.9 million tons per year by 2030.

The report admits that there would be an "insignificant to medium-level impact" on sea animals, and promises to exclude underwater blasting that is likely to cause big damage. A sea park of 2,400 hectares to lure dolphins back is also being planned.

Environmentalists say the L-shaped runway would steal more water from sea animals than the reported land reclamation area.

According to Lam Chiu-ying, a professor at the Chinese University of Hong Kong, squeezed water space, noise and possible incidents would take a toll on sea animals. He also said it's unknown whether the sea park would lure the dolphins back. He suggests raising the efficiency of the two existing runways instead.