I was told that centuries ago the current Pope had fishing vessels which came into port on Friday. Hence the command to eat fish and no other meat on Fridays was monetarily advantageous to his business interests.

Christians have abstained from meat on Fridays since the first century in honor of Our Blessed Lord's Passion and Death on that day. In 1966 Pope Paul VI issued his famous encyclical on Fasting and Abstinence. In the Encyclical he gave permission to individual Bishops conferences to allow people to substitute some other penance in their country for their Friday abstinence. But, on the whole, Catholics have been eating fish on Fridays for about twenty centuries.