More on mercury in fish

High mercury levels are found in tuna sushi - International Herald Tribune

Hey, whenever I decide there's nothing to blog about, I find a few things in quick succession.

Further to my recent post about mercury in fish, here's an article indicating that it is a real problem even for the relatively small amounts of tuna eaten as sushi:

Recent laboratory tests found so much mercury in tuna sushi from 20 Manhattan stores and restaurants that at most of them, a regular diet of six pieces a week would exceed the levels considered acceptable by the Environmental Protection Agency.

Sushi from 5 of the 20 places had mercury levels so high that the Food and Drug Administration could take legal action to remove the fish from the market. The sushi was bought by The New York Times in October.

"No one should eat a meal of tuna with mercury levels like those found in the restaurant samples more than about once every three weeks," said Dr. Michael Gochfeld, professor of environmental and occupational medicine at the Robert Wood Johnson Medical School in Piscataway, New Jersey

Canned tuna has been recognized as a potential problem for some years too, but these sushi tests were even worse:

In 2004 the Food and Drug Administration joined with the Environmental Protection Agency to warn women who might become pregnant and children to limit their consumption of certain varieties of canned tuna because the mercury it contained might damage the developing nervous system. Fresh tuna was not included in the advisory. Most of the tuna sushi in the Times samples contained far more mercury than is typically found in canned tuna.

The levels of mercury in large fish everywhere seems to be a phenomena that just doesn't attract the degree of public concern that it deserves. (In past posts, I have noted the problem with mercury in dolphins eaten in part of Japan, and in whale meat eaten somewhere else.)

What I don't understand is where the mercury is coming from, and is it too late to do anything about it? And if they have so much mercury in their flesh that we shouldn't eat them, how come the dolphins, whales and fish don't get sick themselves?