Their weight had also dropped to under 2 kg (4.4 lb) from 5 kg (11 lb) in the 1980s, the chairman of the Commission for Mekong Dolphins Conservation said.

"A group of 10 full-grown dolphins living in the upper Mekong River had no babies at all this year," he said, blaming a shortage of fish and rising water temperatures which might have affected their reproductive systems.

There are about 150 dolphins living in the upper Mekong River, up from only 90 before a 2006 ban on net fishing in the eastern provinces of Kratie and Steung Treng.

Conservationists had hoped for a surge in newborn dolphins after the ban was imposed.

"Global warming may be a possible indirect threat to the dolphin population, particularly if their fitness is reduced," said Teak Seng of the World Wildlife Fund.

"Dolphins are very sensitive to changes in their environment such as water temperature and quality. Other factors may be more influential such as diseases and water pollution," he said.