Catholics are required to abstain from eating meat on Ash Wednesday and each Friday in Lent (including Good Friday). Fish is often used as a substitute for meat-based meals. But of course, with the popularity of vegetarian and vegan diets, there are many other solutions besides fish.
Historically, since about the second century of Christianity, Christians abstained from eating meat on Fridays as a kind of sacrifice and reminder that acknowledged Jesus’ sacrifice on the Cross, which we commemorate on Good Friday. It’s also why we proclaim the sorrowful mysteries of the rosary on Friday. About a century or two later, Lent came into being, as a season of intense preparation for Easter, so the fasting and abstinence was extended to much of Lent.
The Second Vatican Council simplified many Catholic customs and laws. There was too much of an emphasis on sin and sacrifice and some of the practices were rather involved. Many people believed that breaking Friday abstinence was a sin so serious it could land you in hell. They knew the whole thing had gotten out of hand.
So the bishops preserved fasting on Ash Wednesday and Good Friday (meaning to eat only one full meal for the day and then to merely sustain yourself for the rest of the day — two smaller meals that do not equal the one large meal) and abstaining from meat on Fridays during the more penitential time of Lent.
Some have said the bishops were in cahoots with the fishing industry, but there is little to no evidence to claim that as truth.