Six pieces of sushi from most of the restaurants and stores would contain more than 49 micrograms of mercury. That is the amount the Environmental Protection Agency deems acceptable for weekly consumption over a period of several months by an adult of average weight, which the agency defines as 154 pounds. People weighing less are advised to consume even less mercury. The weight of the fish in the tuna pieces sampled by The Times were 0.18 ounces to 1.26 ounces.