Larger fish such as shark, king mackerel, swordfish, and tilefish tend to be highest in mercury because they eat smaller fish and consume the mercury in them as well.

CNN: Seafood benefits outweigh risks, government says

The agency warns, however, that pregnant and breast-feeding women shouldn't eat swordfish, tilefish, shark, or king mackerel and should limit consumption of white, albacore tuna, a commonly canned variety.

WSJ: Guidelines for Eating Sustainable Seafood