The limit was 0.50 ppm in the 1970's. Around that time canned tuna was found to surpass that amount. Subsequently, 12 million cans of tuna were recalled - and the limit was quickly raised. Canada's limit is still 0.50 ppm.
Here is the FDA/EPA's current table of mercury levels in fish. As you can see, the data is not recent. The FDA has promised to sample 29 species of fish this year to update their numbers.
Mercury Levels in Commercial Fish and Shellfish
Click chart for full table.
The FDA also said it would look into the concern over mercury levels in canned tuna - a concern raised by the Chicago Tribune's investigation:
"Responding to a Tribune series this month on mercury in fish, the FDA said it will review the possibility that there are elevated mercury levels in some cans of "light tuna," one of America's best-selling seafoods and a product the agency has recommended repeatedly as a low-mercury choice."
Back to the present (April 2009): I can't find a chart from the FDA that is more recent than the one above. As you can see, the data is old, in some places almost 30 years old, and there are a limited number of samples.
The Chicago Tribune published the results of their investigation into mercury in fish back in December 2005. <-- Worth a look. Some quotes:
- Reporters working for the Chicago Tribune conducted an 8-month investigation of mercury in fish.
- They tested 144 fish samples from the Chicago area - "one of the nation's most comprehensive studies of mercury in commercial fish".
- "The testing showed that mercury is more pervasive in fish than what the government has told the public."
- "Regulators do not inspect seafood for mercury - not in ports, processing plants or supermarkets." In fact, "no federal testing program exists for mercury."
- Even when found, "The government does not seize high-mercury fish that violate U.S. limits."
If I find anything else I'll post it. I thought this was a good article:
Mercury Pollution Threatens Health Worldwide, Scientists Say, Science Daily, August 2006