Toxic chemicals including DDT are being washed into the Pearl River Delta and poisoning dolphins.
The $4.6 million study found high levels of 'environmental contaminants' in dolphins' bodies.
'What effects these are having on the animals' longevity and reproduction are unknown,' the report concluded. 'But a high incidence of neonate [newborn] strandings may be related to organochlorine poisoning.' Dr Jefferson said the chemicals were probably harmful to the dolphins.
He said the source of the chemicals was not known - but they could come from paint used on ship's hulls.
'And there's possible illegal use of DDT in China . . . we must co-operate with authorities there,' he said.
The report recommended a 'major effort' to clean up local waters.