All fish have some levels of mercury in them. Some fish have more mercury than others. Fish that eat other fish, or fish that are long-lived tend to have more mercury than smaller, less predatory fish.
The following poster identifies the typical (average) amounts of mercury in commonly consumed commercial and sport-caught fish. Using this chart allows you to make lower mercury fish a larger part of your diet. Pregnant and nursing women should be especially careful about eating fish that have higher mercury levels.