According to Health Canada canned tuna has lower levels of mecury than fresh or frozen.
Mercury levels in freshwater fish varies, but in general bass, pike, muskellunge and walleye have high levels and should be eaten in moderation.
The consumption of mussels, pollock, salmon, scallops, shrimp and sole ? the majority of aquatic species consumed in Canada ? are not of concern.
More info & references at Canadian Medical Association web page here: www.cmaj.ca/cgi/content/full/167/8/897
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