Mercury Content in fish and how to choose the right kind.

All fish contain trace amounts of mercury. For most people, the small amounts in fish do not pose a health problem. Some fish, however, contain high amounts of mercury -- enough to damage a fetus or newborn. That is why pregnant and nursing mothers must be very careful about the amounts and types of fish they eat. Young children should also avoid eating fish high in mercury. According to the FDA, pregnant women and small children (under 6) should not eat more than 2 servings of fish each week -- and should only eat those fish with low mercury content (see below).
Mercury levels can build in adults too -- eventually becoming harmful to health. High mercury levels can cause permanent damage to the kidneys and brain.
Which Fish Have the Most Mercury?
LOWEST MERCURY
Eat 2-3 servings a week (pregnant women and small children should not eat more than 12 ounces (2 servings):
• Anchovies
• Catfish
• Clam
• Crab
• Crawfish
• Flounder
• Haddock
• Herring
• Mackerel
• Mullet
• Oyster
• Perch
• Pollock
• Salmon
• Sardine
• Scallop
• Shrimp
• Sole
• Squid
• Tilapia
• Trout
• Whitefish
MODERATE MERCURY
Eat six servings or fewer per month (pregnant women and small children should avoid these):
• Bass
• Carp
• Cod
• Halibut
• Lobster
• Mahi Mahi
• Monkfish
• Perch
• Snapper
• Tuna (Canned Chunk light)
HIGH MERCURY
Eat three servings or less per month (pregnant women and small children should avoid these):
• Bluefish
• Grouper
• Sea Bass
• Tuna (Canned Albacore, Yellowfin)
HIGHEST MERCURY
Avoid eating (everyone):
• Marlin
• Orange Roughy
• Shark
• Swordfish
• Tilefish
• Tuna (Ahi)
Sources:
Food and Drug Administration (FDA). Mercury Levels in Commercial Fish and Shellfish