Sushi from 5 of the 20 places had mercury levels so high that the Food and Drug Administration could take legal action to remove the fish from the market. The sushi was bought by The New York Times in October.
Unfortunately, the NY Times doesn’t really go into how that mercury got into the tuna in the first place.
To understand how mercury contaminates fish, consider the mercury cycle. It begins with mercury being emitted to the atmosphere by sources such as coal-burning power plants. The mercury washes out of the air with precipitation and comes down on land and water.