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"Certain fish are a good source of omega 3’s, plus protein. Fish, especially larger fish like tuna, swordfish and marlin, store the mercury which we then ingest. The chart below lists in order of best to worst the best fish to eat in terms of the greatest amount of omega 3 with the lowest amount of mercury. Farmed salmon may have significantly lower amounts of omega 3 compared to wild salmon because the amount of omega 3 in the fish depends to a large extent on the quality of its diet.
omega 3 g/100g mercury mg/kg omega3/mercury
fresh wild salmon 2.7 0.05 54.0
canned sardines 1.57 0.04 39.3
canned/smoked salmon 1.54 0.04 38.5
mackerel fresh 1.93 0.54 35.7
herring (kipper) 1.31 0.04 32.8
trout 1.15 0.06 19.2
fresh tuna 1.5 0.4 3.8
cod 0.25 0.11 2.3
fresh sole 0.1 0.05 2.0
canned tuna 0.37 0.19 1.9
marlin *2 1.1 1.8
swordfish *2 1.4 1.4
Larger fish like tuna tend to be higher in mercury so don’t eat tuna more than once a week. Since canned tuna has much less of the omega-3’s left after the canning process, stick with fresh tuna. Marlin and swordfish and best consumed once a month max. Eat salmon, small mackerel or sardines instead. Generally speaking, the larger the fish the more the mercury.
However, there are ways to detoxify mercury thus reducing its effects on you. The most potent mercury detoxifier is selenium. A daily intake of 50 to 100mcg may provide significant protection. Seafood provides selenium, as do seeds and nuts. While Brazil nuts are supposed to be high, some analyses show very little. Make sure your multivitamin is providing at least an additional 30mcg."