The limit was 0.50 ppm in the 1970's, during which 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."