The Filet-O-Fish is a fish sandwich sold by McDonald's since 1963. It contains a breaded fish patty made from Pollock or Hoki.
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Alaska pollock is commonly used in the fast food industry, for example the fish filet at both McDonald's and Burger King are also made from Alaska pollock.
The Alaskan pollock is said to be "the largest remaining source of palatable fish in the world." Atlantic pollock is largely considered to be a white fish, although it is a fairly strongly flavored one. Alaska pollock has a much milder taste, whiter color and lower oil content. (Pollock, Wikipedia)