Katherine's points are well taken. The question of hazard due to chemical contaminants in fish and other seafood is complex. For example, the 1 ppm level cited earlier for mercury applies only to methyl mercury, which is not the same thing chemically as elemental mercury, and does not necessarily apply at all to any other metals, much less to organic contaminants.
In fact, much is known about risk levels for the more common contaminants, though much work has yet to be done. The current US Food and Drug Administration guidelines ("action levels") for common contaminants are available here:
http://vm.cfsan.fda.gov though that is only half the picture as it does not tell you the levels present in food you are about to buy or eat.
In general, many contaminants introduced into the marine or aquatic environments both bioconcentrate (concentration in organisms greater than that in the environment) and biomagnify (concentration increases up the food chain). Biomagnification is why predatory (someone earlier said aggressive, which is sort of similar) fish generally have higher concentrations of contaminants than fish that feed lower on the food chain. In general, it is true that older fish will have higher levels of contaminants than younger fish.
Another factor is fattiness, more properly called lipid content. Many of the organic contaminants - dioxins and PCBs, for example - partition preferentially into fatty tissue (are lipophilic), so fatty fish such as bluefish or mackerel are more likely to contain contamination than fish such as flounder, all other things being equal. As a practical matter, reducing the amount of fatty fish consumed in favor of less fatty fish is a worthwhile thing to do. Another very simple thing to do is not eat the small area of darker flesh along the body midline that most fish have - that's where most of the contamination will be found. In particular, never, ever eat the tomalley (green material) in a lobster, which can be loaded with contaminants.
If you've followed me this far, you may have guessed that I make my living working with this, and you would be correct. The few simple guidelines above are the ones I practice to reduce my overall exposure to contamination from seafood.
[This message has been edited by Dick (edited September 07, 2000).]
[This message has been edited by Dick (edited September 07, 2000).]