If you want a good look at the horrifying problems with mercury, I urge you to review this interesting study of store-bought fish and funded partly by the environmental group Oceana (free text link below).
University of North Carolina scientists collected samples of tuna and swordfish from more than 20 states and a variety of national supermarket chains, including Safeway, Albertsons and Whole Foods.
Half of the swordfish samples tested by researchers exceeded the FDA's action limit of mercury 1 parts per million (ppm). Even worse, swordfish bought in Maine and Rhode Island contained more than twice as much mercury than the FDA allows. Overall, tuna samples averaged 0.33 ppm of mercury and those analyzed from Maine, Washington, D.C., Maryland and Montana exceeded .50 ppm.
Sadly, fish would be a near-perfect food, if virtually all of them weren't contaminated with mercury. And, because this toxic substance rapidly bioaccumulates in the food chain, it is commonly found in higher concentrations of larger, older fish and predatory species like shark, tuna and swordfish.
If you eat a lot of fish, you'll want to know how much mercury you may be consuming by using a free online mercury calculator.
A safe option at your disposal to get the nutritional benefits of fish without the mercury: Taking a high quality fish or cod liver oil daily.
Oceana Free Full Text Report
Sacramento Bee September 16, 2005