Formaldehyde has been found in noodlefish - a small variety commonly used in Chinese cuisine and sushi - but safety chiefs say the quantity of the cancer-causing chemical present is too small to threaten consumers' health.
The chemical was found at levels of between 160 and 620 milligrams per kilogram in nine of 15 samples taken from vendors city-wide.
It was the first time that the centre detected formaldehyde in seafood.
The centre's principal medical officer, Tina Mok, was concerned that the traders might have abused the use of formaldehyde after the chemical was found in nine out of 15 noodlefish samples collected recently. The centre refused to provide the exact time frame. The tainted samples were taken from the Jusco supermarket in Tseung Kwan O and wet market stalls in North Point, Central, Aberdeen, Fanling, Kowloon Bay, Sha Tin and Western District.
Dr Mok said formaldehyde occurred naturally in fish at levels of up to several hundred milligrams per kilogram, but the chemical detected in the nine samples was believed to have been added after the fish were caught and while they were being transported or in storage. She said scientists suspected the formaldehyde had been added to the fish because another chemical, dimethylamine, which was usually associated with naturally occurring formaldehyde, was not found in any of the tainted samples. Formaldehyde is used in cosmetics, to coat textiles and in the manufacture of plastics.
The centre has asked Jusco and the fish stalls concerned to stop selling noodlefish for the time being.