Beijing -- Shanghai's food safety watchdog issued a warning over a popular fish dish after small amounts of a cancer-causing antibiotic were found in samples of turbot.

In the wake of the warning, one seafood market said it would tell vendors not to replenish stocks of the fish, while city restaurants said they would continue to serve turbot to customers unless there was an official ban.

Shanghai Food and Drug Administration officials said tests on 30 turbot samples taken from wholesale markets, supermarkets and restaurants showed low levels of AOZ, a carcinogenic antibiotic, in all of them.

Some samples contained other antibiotic residue, the officials said, adding that some fish farms use antibiotics to boost the low immune systems of turbot.

"All the antibiotics are forbidden fishery medicines," said Gu Zhenhua, director of the Shanghai Food and Drug Supervision Agency. "Though they won't cause immediate harm to people's health, there is a hidden danger if people eat small amounts of antibiotics over long periods."

The highest amount of AOZ residue found in a sample was one milligram per kilogram.

Administration officials said the levels were not high enough to cause cancer, and were only about 0.3 percent of the recommended dosage when AOZ is used to treat diarrhea.

National standards stipulate no antibiotics should be found.

About 40,000 tons of turbot is farmed in China every year.

Gu said the administration would decide in a few days how to punish farms that sell turbot with traces of antibiotics.

After hearing the news, Huxi Aquatic Products Wholesale Market told vendors to stop restocking turbot until the administration issued further instructions.

"We believe the measure will help our vendors reduce their losses," said Zhao Yuan, a manager of the market. "If some vendors insist on selling turbot, we will ask them to provide qualified test reports."

In the nearby Tongchuan Road Aquatic Products Wholesale Market, the city's largest seafood market, turbot was still for sale.

"So far we haven't received any notice from the administration to ban turbot," said Yang Jihua, manager of the marketing department of the market.

One fish vendor surnamed Zhang at Huxi market said the different reactions of market managers were confusing.

"But I don't dare to buy a lot of turbot now in case the demand decreases after the news is released," Zhang said.

Turbot was still available at city hotels and restaurants.

"The fish is still on our menu," said an unidentified member of staff at the Jinjiang Hotel. "But we will discuss with our chief chef whether to continue to serve the dish."

The Equatorial Hotel and Mei Lin Ge Restaurant both said they would serve the fish unless there was an official ban.

Turbot is a popular dish in China, partly because its Chinese name translates as "many treasures."

Most stocks come from Shandong Province.