I keep a can opener in my office drawer because lunch for me is opening a can of tuna and eating it right from the can at my desk....at least three times a week! According to CR, the heavy metal accumulates in tuna and other fish in an especially toxic form, methyl-mercury, which comes from mercury released by coal-fired power plants and other industrial or natural sources like volcanoes. So, no more simple canned tuna at my desk for lunch anymore.
CR goes on to report that it's especially important for women who are pregnant or might become pregnant, nursing mothers, and young children, to choose lower-mercury fish (salmon and shrimp) that are also rich in healthful omega-3 essential fatty acids benefits, because fetuses and youngsters seem to face the most risk from methyl-mercury's neurotoxic effects. As a precaution, pregnant women should avoid tuna entirely.
Besides salmon, shrimp and other low-mercury fish, there are foods labeled as fortified with the omega-3 fatty acids EPA and DHA. Omega3 supplements provide the fish oil you need and can be safe if taken in appropriate doses per day. But always consult your doctor first.
Source: Consumer Reports January 2011