What we do know is that Breast Feeding improves the health of both mothers and babies which far outweighs any toxic effects of mercury-- in most circumstances. IF you are worried and are in an environment where there is know to be high Mercury levels see a clinician. Know that  if Mercury is present ( and there is some everywhere)  Levels go down within in 2-3 months after birth -- and again there is more mercury in cows milk based formula's than in human breast milk.  The damage of not breast feeding an infant is far more lethal in terms of disease  than mercury.  However-- an environment where a mother is exposed to high levels of mercury during pregnancy -- can be dangerous and cause Minamata disease --but that mercury is passed across the placenta into the blood stream of the baby.  But Mercury is poorly transmitted to and through breast milk. If you have the luxury of avoiding mercury ingestion by all means do. But if you have to choose between breast feeding and not -- mercury contamination is not your biggest problem.  You might see the film Breast Milk and follow journalist, writer health communicator Kimberly Seals Allers (author of the Big LetDown).