The risk is very minimal. Look at the numbers for mercury content of sardines or salmon. The amounts are very low, around 1/20th of the levels in tuna. I say don't eat any tuna, swordfish, shark, marlin, bass, halibut, or other high mercury fish, but moderate amounts of low mercury fish such as sardines or salmon(8 ounces or less up to twice a week)won't have any significant effect. Keep in mind the 20 to 1 or so ratio of mercury content for high mercury fish vs low mercury fish.

http://www.gotmercury.org/article.php?list=type&type=75