It's true that certain types of fish and seafood* are high in mercury. Salmon, mackerel, herring, rainbow trout, Pollock (Boston Bluefish), sole, hake and canned light tuna are low in mercury and high in DHA. You can also look into DHA-enriched eggs.

*Health Canada provides advice for limiting exposure to mercury from certain types of fish.

Related Link
Limiting Exposure to Mercury