Conservationists Tuesday warned that the number of rare Chinese white dolphins in Hong Kong waters has fallen to its lowest level in a decade of monitoring, and urged the government to immediately create more protected areas.

The number of the marine mammals, also known as pink dolphins for their unique colour, has fallen from an estimated 158 in 2003 to just 78 in 2011 and 61 in 2012, the Hong Kong Dolphin Conservation Society said.

"They will slowly disappear from Hong Kong if we don't do anything now," society chairman Samuel Hung told AFP.

"We are very worried that the dolphin numbers will keep dropping and will never recover."

The group blamed the dramatic decline in numbers on construction and land reclamation work for a 50-kilometre (30-mile) bridge linking Hong Kong to Zhuhai on mainland China and Macau.

The number of dolphins near the site, on the east of Hong Kong's Lantau island, fell to just four from 11 a year earlier, Hung said.

Other construction projects, including proposals for a third runway at Hong Kong International Airport on Lantau island, mean the dolphins "are at a crossroads where they will face an uphill battle to continue to survive", the group said in a statement.

Hung urged the government to immediately establish more marine protection areas.

"What the government can do is to create more capacity for the dolphins to survive in Hong Kong so that may be able to compensate for the habitat loss," he said.

Fewer than 2,500 of the mammals survive in the Pearl River Delta, the body of water between Macau and Hong Kong, with the majority found in Chinese waters and the rest in Hong Kong.

Experts say dolphin numbers have also dropped in the past few years because of overfishing, an increase in marine traffic and water pollution.

In April, a tour guide from Hong Kong Dolphinwatch spotted a group of pink dolphins helping a grieving mother support the body of her dead calf above the water in an attempt to revive it.

The scene, captured on video and widely shared on Facebook, raised fresh concerns about the dwindling population.

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.

Related to Indo-Pacific humpback dolphins, they are listed as "near threatened" by the International Union for Conservation of Nature.