What’s the deal with mercury in fish and shellfish? Mercury is a naturally occurring substance. Generally it falls from the air and will accumulate in rivers and oceans. Once in the water, mercury turns into methylmercury. Methylmercury is what we need to limit our intake of. Fish absorb this substance when they feed and then it builds up in their bodies. Some fish absorb more than others, which is why we recommend certain fish over others.
AVOID these fish: Marlin, Orange roughy, tilefish, swordfish, shark, mackerel (king), tuna (bigeye, ahi).
LIMIT intake: Sea Bass (Chilean), bluefish, grouper, mackeral (Spanish, gulf), tuna (canned, white albacore & yellowfin).
For a complete list visit: http://longevity.about.com/od/lifelongnutrition/a/fish_mercury.htm