New research which linked DNA and mercury contents shows that levels of the metal are elevated in certain species of tuna. They were higher in blue fin , sushi from lean dark red tuna, and big eye tuna samples. Yellowfin , the type typically sold by supermarkets tested lower.
Researchers at Rutgers University say the likely reason is that mercury accumulates differently in certain tissue types. The leanest fish tend to have the highest concentrations.
Also, yellowfin are typically harvested at a younger age, so the mercury does not have a lot of time to build up.