Let’s start with the common Anti-Catholic explanation ~ Many years ago a fisherman discovered how to catch more fish and was unable to sell the extra fish. It just so happened that the fisherman's wife was the mistress of the pope. So she held back her favors to the pope until he made it a rule for all those in his kingdom eat fish on Friday.
You would not believe how many times I have heard people tell this story…as you can imagine; it is not true.
In the first century, all Jews fasted on Mondays and Thursdays. The original Christians, many who were converted Jews, were used to the fasting as a spiritual discipline; to distinguish themselves from the Jewish custom they moved the fast days to Wednesdays and Fridays, because Judas engineered Jesus' arrest on a Wednesday and Jesus was crucified on a Friday. The Wednesday and Friday fasts were a universal Christian custom in ancient times. Later on the Friday fast became the norm in our church. In ancient times, during Lent, people fasted on every day except Sunday. All 40 days!
Most often that fast took the form of avoiding meat in the diet. In those days, meat was a luxury food. You either had to buy it in a market or you had to own enough land to keep cattle. On the other hand, anyone could grow vegetables or forage for them, and anyone could catch a fish in a lake or a stream. You could buy better fish and vegetables, but the point is that you could eat without money if you were poor. So meat was rich people's food and fish was poor people's food. That is why the most common form of fasting was to omit meat and eat fish, to be poor and humble.
Why fish? A culinary argument…as a generic culinary and butchery term: "meat" refers to the muscular flesh of a mammal. This is the definition most commonly applied by governments in meat product regulation and food labeling, and in religious rites and rituals. Edible birds and fish/seafood are not "meat" under this application but are treated separately from mammals. Likewise, amphibians and reptiles, not to mention the "meat" of edible insects, arachnids, and so on.
The abstaining from meat during Lent became official practice in the Middle Ages, when the Church declared fasts in honor of all sorts of events. These fasts were declared for religious reasons or for political reasons. Sometimes the underlying reason was, to try to make the winter food supply stretch until spring. Remember, in the Middle Ages, there were no grocery stores. Whatever had come in at harvest was all everyone in town had to eat until the next year. Enforced fasts were a way of stretching the food supply out a little longer, and preventing people from eating up the livestock they would need the next year, and eating up the seed they would need for planting in the Spring.