Many larger fish are widely cited as having the highest concentrations of mercury, particularly king mackerel, swordfish, shark, tilefish, and some tunas. After growing up on the coast, and working on fisheries issues for over a decade, I’ve never been served either a tilefish or a king mackerel, so those shouldn’t be tough to give up. And there are plenty of reasons beyond mercury not to eat shark—many species are overfished primarily as a result of the inflated value of their fins for shark fin soup, but also because sharks are very slow to reproduce.