The health ministry has issued a warning to pregnant women not to eat broad-bill swordfish and certain types of sea bream more than twice a week because mercury in the fish can be harmful to fetuses.

The Health, Labor and Welfare Ministry is also advising pregnant women to limit consumption of shark and sperm whale meat to no more than once a week and bottlenose dolphin to no more than once every two months.

It is the first time the ministry has issued such warnings on consumption of specific types of marine products.

The ministry said consumption of any kind of fish does not pose health problems for those who are not pregnant.

Swordfish, tuna and whales, which are at the top of the food chain and feed on smaller fish, are known to have accumulations of poisonous methylmercury, which can occur naturally in the environment or get into water by way of industrial pollution.

Through the consumption of fish and other marine products, the intake of methylmercury — the cause of Minamata mercury poisoning — can affect the development of a fetus’s nervous system. This can take place with small mercury levels that would not affect an adult.

The ministry held discussions over the need for intake limits for fish that Japanese often consume, based on results of mercury content studies conducted by the Fisheries Agency and other organizations, before coming up with the warnings.

The Fisheries Agency said a study found no health problems stemming from methylmercury levels in tuna, swordfish and other fish species eaten domestically, as they are within international safety standards.

The study results were presented Tuesday to a health ministry panel that is considering safety measures taking into account recent research showing that exposure to methylmercury can cause health problems for pregnant women and their fetuses, and can also impair mental development in fetuses and small children.

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