Nearly all seafood also contains traces of mercury, which is not a health risk for most people, the FDA and EPA state. However, the higher levels of this metal in some species of fish can harm the nervous system of unborn babies or young children, the agencies say. So in 2004 they issued a few guidelines for young children, nursing mothers and women who are or might be pregnant:

• Do not eat swordfish, shark, king mackerel or tilefish. These large species have high levels of mercury.
  
  
• Eat up to 12 ounces (about two servings) of a variety of low-mercury seafood per week. Examples include salmon, shrimp, clams, tilapia, catfish, pollock and canned light tuna. Or eat up to only 6 ounces a week of albacore ("white") tuna or tuna steak, which aren't as low in mercury as canned light tuna.
  
  
• If no advisories warn against eating locally caught fish, it's OK to eat up to 6 ounces if you don't eat any other fish that week.
  
  
• Follow these guidelines with young children but offer smaller portions.
  
  

The recommendations from the FDA and EPA have sparked a lot of follow-up. For example, Consumer Reports found that some canned light tuna had more mercury than albacore did. Environmental Defense Fund says that bluefin, yellowfin (ahi) and bigeye tuna should be eaten infrequently if ever because of their high mercury content.