Three types of saltwater fish reared in Pearl River Delta farms have been found to contain cancer-causing organochlorine pesticides such as DDT.
Mainland scientists heading a study advised the public not to eat orbfish , red drum and mulloway from the region.
Hong Kong Chamber of Seafood Merchants chairman Lee Choi-wah confirmed that these species were on sale in Hong Kong and that some stocks were imported from fish farms in the Pearl River Delta.
Zeng Yongping, director of the Chinese Academy of Sciences' State Key Laboratory of Organic Geochemistry, who headed the study, said even if consumers ate orbfish just once a month, the likelihood of getting cancer was more than one in 10,000, or 100 times higher than the maximum set by the World Health Organisation.
'My advice is that consumers stop eating fish, especially the three named species produced by Pearl River Delta sea farms, until we can single out the exact sources of the pollution,' Professor Zeng said.
The scientists also found poisonous pollutants spread much faster through the fish than previously thought. It was believed that most harmful materials were concentrated in internal organs such as the liver, but the latest study shows the contaminants were evenly distributed throughout the body.
'Fish muscles seem to be able to absorb more pollutants than we previously thought. Some people believed that it was safe to eat just the meat, but this does not hold up,' Professor Zeng said.
The research team investigated fish farms in Yangjiang that revealed some astonishing results. They found hundreds of fish farms with poor sanitary conditions and few anti-pollution measures. They listed possible pollution sources, from fish feed to neighbouring factories to banned DDT-tainted paints.
Most freshwater fish farms, however, were in better condition because they depended on spring water and rain water, and many farmers knew to isolate them from rice fields.
Detailed results of the study will be published soon in the international academic journal Environmental Pollution.
University of Hong Kong professor or biological sciences Steven Chen Feng said DDT was banned in the United States and fish might be contaminated if their feed was sprayed with the pesticide. The pesticide would cause cancer and damage to the human nervous system.
A Centre for Food Safety spokeswoman said it did not have import records for fish.
The spokeswoman said the centre had conducted tests on 1,300 samples of freshwater and seawater fish from January to last month. The tests covered levels of heavy metals, preservatives and DDT in the samples. Forty-two samples were found to be unsatisfactory.
The Centre for Food Safety has found a trace amount of the cancer-causing antifungal malachite green in a sample of frozen bream filled. The level of the banned agent in the sample, from a supermarket in Tsim Sha Tsui, was too low to cause adverse health effects, it said. More samples have been taken for testing.