Mercury is a natural contaminant of the marine system and since methyl mercury moves up the food chain easily (in some circumstances actually biomagnifying) some large predatory fish such as tuna and swordfish have naturally high levels which may approach or even exceed acceptable limits for human consumption in large, older specimens. There are normally no apparent adverse health effects in the fish themselves, the risk is to the next level of consumer in this case humans. (However, wildlife such as birds can be adversely affected directly)
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