Most fish and shellfish can be part of a healthy diet. They're packed full of protein and are good sources of fats (they're low in saturated fat and full of omega-3 fatty acids). Most also contain mercury, at least some amount, and some fish contain more than others. Swordfish, for example, is one of those mercury-dense varieties (also included on this fishy list are king mackerel, marlin, orange roughy, shark, and tilefish). Albacore tuna, too, contains more mercury than canned light tuna, so choose wisely.
High levels of mercury in the fish we eat may harm the developing brain and nervous system of a fetus or young child because mercury is a neurotoxin, and because of this the FDA and EPA recommend against eating these fish, especially if you're pregnant, may become pregnant or are a nursing mother -- and kids need to avoid it, too.