<Li> IU = International units </Li> <Tr> <Td_colspan="2"> Percentages are roughly approximated using US recommendations for adults . Source: McDonald's </Td> </Tr> <P> The Filet - O - Fish, otherwise known as the Fish - O - Filet, is a fish sandwich sold by the international fast food restaurant chain McDonald's . It was invented in 1962 by Lou Groen, a McDonald's franchise owner in Cincinnati, Ohio, in response to falling hamburger sales on Fridays resulting from the Roman Catholic practice of abstaining from meat on Fridays . While the fish composition of the sandwich has changed through the years to satisfy taste and supply shortcomings, the framework of its ingredients have remained constant; a fried breaded fish fillet, a steamed bun, tartar sauce and pasteurized processed American cheese . </P> <P> As of December 2014, the US Filet - O - Fish contains a battered, fried fish fillet made from Alaska pollock . In the Republic of Ireland either hoki or Alaska pollock may be served . In New Zealand and the United Kingdom Filet - O - Fish contains hoki instead of Alaska pollock . McDonald's Canada, United States, United Kingdom, Australia, New Zealand, Portugal, Czech Republic, The Netherlands and Hong Kong use a half slice of cheese in each Filet - O - Fish sandwich . </P>

When did mcdonald's start serving filet o fish