Food safety authorities in Hong Kong are investigating whether freshwater fish and shrimp contaminated with male hormone stimulants and antibiotics might have been imported from the mainland. The Food and Environmental Hygiene Department said it was concerned by reports that tests in Guangdong found traces of methyltestosterone and chloramphenicol in samples from fish farms, mostly among tilapia (pictured) and shrimps. 'The department's food monitoring scheme has continuously been taking food samples for tests of veterinary drug residues and additives,' a department spokeswoman said. 'Regarding this report, the Food and Environmental Hygiene Department will contact the Guangdong authorities to inquire about the situation.' The department said freshwater fish imported from the mainland must carry hygiene certificates to prove they do not contain banned substances such as the cancer-causing chemical malachite green. Local fish breeders said most imported tilapia came from Nanhai , Zhongshan and Shunde , whereas most contaminated samples were from Maoming . 'We predict the fish farms in question also breed other aquatic products and put this hormone in the ponds, where the tilapia are fed as well,' said Tommy Hui Hon-man, of the Hong Kong, Kowloon and New Territories Freshwater Fish Wholesale Association. According to a report by the Guangzhou-based Information Times on Friday, the Guangdong Oceanic and Marine Bureau tested aquatic products in the province from May to July and found more than 10 per cent of 288 samples tested to be contaminated with various pesticides and chemicals. The report said quality inspectors in Guangdong had begun a second round of testing. Medical experts warned that excessive intake of hormones could increase the risk of premature births and affect sperm counts.