Mercury levels in three alfonsino fish samples ranged from 609 to 1,370 micrograms per kilogram, exceeding the legal 500 microgram limit, a Centre for Food Safety study has found. The samples were also found to contain a higher level of methylmercury, ranging from 509 micrograms per kilogram to 1,010.

Mercury is a metallic element widely present in the environment from natural sources and human activities. It accumulates mainly in the organic form of methylmercury in the food chain, particularly in fish. Concerns about methylmercury in food are related to its possible effects on the nervous system, particularly in developing foetuses.

The centre's Community Medicine Consultant Dr Ho Yuk-yin said the study covered 280 fish samples, including 266 whole fish and 14 canned fish of 89 species found in the local market.

Study findings

Mercury levels in the samples ranged from three to 469 micrograms per kilogram. The levels of methylmercury detected ranged from three to 430.

The study also found comparatively higher average levels of methylmercury in yellowback seabream, yellowtail barracuda, and canned albacore tuna, ranging from 205 to 253 micrograms per kilogram.

The average level of methylmercury for different tuna fish species, including canned albacore, canned yellowfin, skipjack (canned and whole fish), also varied, ranging from 85 to 205 micrograms per kilogram. The highest average level was found in canned albacore. More study findings are available here.

The centre estimated secondary school students' dietary exposure to methylmercury was 31% to 41% of the provisional tolerable weekly intake adopted by the Joint Food & Agriculture Organisation - World Health Organisation Expert Committee on Food Additives. The estimated dietary exposure for high consumers may be 94% to 106% of the provisional tolerable weekly intake.

Dr Ho said: "The current study shows most of the fish available on the Hong Kong market have relatively low levels of total mercury and methylmercury, while a small proportion contain higher levels. The amounts of methylmercury relative to total mercury in different fish species vary considerably.

"For the high consumers among secondary school students, their estimated dietary exposure to methylmercury may exceed the provisional tolerable weekly intake, therefore possible health risk cannot be ruled out."

Health tips

Consumers should maintain a balanced diet. Moderate consumption of a variety of fish is recommended as fish contain many essential nutrients, such as omega-3 fatty acids and high-quality proteins.

Pregnant women, women planning pregnancy and young children should avoid eating large predatory fish and those which may contain high levels of mercury, such as shark, swordfish, marlin, alfonsino and tuna, especially the bigeye and bluefin species.