It's a special type of Omega 3 you can only get from animals by the way. Omega 3 is generally available in a wide range of vegetable oils as well, but it's not the same. Another good source is grass-fed beef.
>eating farmed salmon
>not pronouncing it "samon"
Mercury toxicity is a result of mercury's effect of deactivating selenium. Equal amounts of selenium will be deactivated by certain amounts of mercury. Salmon is an excellent source of selenium and has far more of it than mercury.