Chris asked me about mercury levels, how dangerous they are, and how mercury can be abated (not because I knew anything a priori, but because he knows I sometimes like to look into these things).

On the one hand, 41 states have issued fish advisories for mercury (ref). But only 13 states have issued statewide advisories (including fish from all bodies of water), and it's unclear how serious an advisory is. The EPA says most consumers don't need to worry. Generally only pregnant women eating unusual amounts of fish daily or from particularly contaminated sources would need to worry. The FDA is currently reviewing its advice so this may change.

Note that mercury abatement processes exist -- there are small companies (like Acute Services or Enviro-Vac) that can be paid to clean up mercury spills. There's even been a small amount of backlash against proposals to even more severely lower the recommended maximum mercury intake. Even at current levels, the US recommendations are already several times stricter than the Australian recommendations (four servings of high-mercury fish per week upper limit for pregnant women only).

Overall, it doesn't seem like a serious thing Americans need to worry about. Even if you travelled to some highly mercury-polluted place and ate nothing but fish for a month, it would still be balanced by a normal American diet over the period of a couple years.