[heleny]

a quarter of the orders were near or above the limit where the agency says fish should not be old(sic)

Most sushi restaurant patrons will eat sushi made from a variety of different fish, not just tuna, and Japanese food typically has smaller portion sizes, so a person will probably only consume an ounce or so of tuna in a meal -- much less than if he ate a tuna steak somewhere.

Is there a difference in mercury levels of different varieties of tuna or in different parts of the fish?

[1rudeboy]

Generally speaking (and I am not an expert), the fattier the fish, the higher the level of mercury.