Chris Kresser spent quite a bit of time in his Real Food Summit presentation discussing the "inactivation" of mercury by selenium. The good news about this is that for many of the fish where mercury contamination is of concern -- particularly large coldwater ocean fish -- there is a high level of selenium. Selenium binds mercury and in essence converts it to a non-toxic form. Sorta like those amalgam fillings most folks over 35-40 (and younger but less common) have had*. The good news is if a fish has high selenium, it's a low mercury toxicity risk. The bad news is if the fish has a high mercury content, there's less of the beneficial "free" selenium you need.
I went looking for mackerel since it was brought up in comments and found this "pamphlet". Thought I'd share: Selenium and Mercury - Fishing for Answers
I feel much better eating my fatty fishies, farmed or otherwise. Meanwhile Joel Salatin made me feel pretty bad about my real food based diet. Maybe I'll share some thoughts on his intro video (free download with registration ... unfortunately missed his presentation yesterday).
* I was a relatively lucky one and only had 3 of these fillings. They were in back molars and a few years back I got to considering replacing these 25+ year old fillings. So me being me, I looked into this thoroughly before "fixing what wasn't broke". Much of the Mercola-style alarmism (man does that man manage to take hyperbole to new levels!) about amalgams is based on the same crapola alarmism he feeds you regarding a chlorine atom in a molecule. The mercury in those fillings is for all intent and purposes inert. It might as well be gold. Drilling these out is more potentially harmful to you than having them there, because what mercury you could be potentially exposed to is magnified several-fold by creating and dispersing "dust". Your dentist should take appropriate precautions to prevent such exposure, and then all is okey dokey. My dentist convinced me that my lower molar fillings were "leaking" (not mercury, but that there was evidence of leakage at the edges and were no longer protecting the underlying tooth) and I had those replaced with tooth-colored composites. There was some cosmetic consideration. In retrospect I probably would have let them be until real signs of failure, because I "feel" the composites that replaced them, and many of the chemicals in those fillings can potentially be as or more toxic than miniscule amounts of mercury. My last amalgam will probably go with me to the grave, it's small and in a back upper molar, closer to my brain which Mercola tells me I should worry over. In any case, if anyone ever has questions about the safety of dental implants or dental materials, this IS my graduate work (materials science, metallurgy specifically, and corrosion!) wheelhouse, and while not the subject of this blog, I'm happy to answer questions you may have. Cosmetic dentistry "for your health" of course, is rife with BS, and $$$$$$$$$$$ are well, that's why I put so many $ there. You could redo your whole face for the cost of replacing one tooth!! Look long and hard before you leap -- especially if leaping is an option (e.g. veneers for teeth straightening)