As a general rule, fish that are larger and caught when they are older have the most mercury. Salmon, which are usually caught around the age of three, have some of the lowest levels (plus the highest levels of omega-3s), making them a great choice. Other small, young fish include herrings and sardines.
Among the fish with the highest levels are tuna (especially albacore), tilefish, swordfish, king mackerel, and shark. The Food and Drug Administration recommends that people eat albacore tuna once a week at most and chunk tuna no more than twice a week. Pregnant women or those trying to conceive should avoid high-mercury fish as much as possible.