There's an impressive amount of mercury bioaccumulated in ahi large enough to make a 2" thick filet.
"Children under six, as well as women who are pregnant or planning to become pregnant, are the most vulnerable to mercury's harmful effects. They should restrict or eliminate certain fish from their diet, including ahi or bigeye tuna, tilefish, swordfish, shark, king mackerel, marlin, orange roughy and fish caught in any waters that are subject to a mercury advisory. Women with elevated mercury levels should ideally begin avoiding or restricting their consumption of mercury-laden fish as much as a year before they become pregnant."
http://www.nrdc.org/health/effects/mercury/protect.asp
http://www.newsweek.com/mercury-leve...ch-year-304062