Mercury is a highly toxic heavy metal that can damage the central nervous symptom and has been linked to high blood pressure and increased fish of heart attacks.. It is even more of a health concern for pregnant women or women who may become pregnant, nursing mothers and young children. Fish and shellfish absorb mercury due to water pollution and it then becomes concentrated in their bodies. Species that are higher on the food chain and therefore live longer such as marlin, tuna, shark, swordfish and king mackerel contain higher concentrations of mercury. Studies have shown that eating a lot of larger species of fish is linked to higher levels of mercury in humans. The health benefits of fish outweigh the risks of mercury exposure as long as you eat it in moderation and stick to the lower level species.