Bluefin tuna contaminated with mercury. High Mercury Levels Are Found in Tuna 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.

Granted, that's a lot of sushi. Over time, however, the cumulative effect of these miniscule doses is what can do us harm.