This is a good question however, the first verse in Chapter 6 of the Gospel of John states that the crowd had followed Jesus to the Sea of Galilee which is where Peter and some of the other Apostles made their living fishing before being called by Jesus to follow him. In Verse 9 of this chapter, the Apostle Andrew came over to Jesus saying that there was a boy there who had 5 loaves of barley bread and 2 small fish. Jesus then gave instructions to have the crowd sit down after which he performed the miracle of the loaves and fishes. Given that this event took place by the sea I don't see a connection between this and people later substituting fish for meat on Fridays. I still see the best evidence for substituting fish for meat on Fridays was the fact that meat is associated with warm-blooded animals while fish are cold-blooded. Throughout history, people have eaten both meat and fish but I have never seen fish being considered meat - even today fish is referred to as "seafood" and is usually sold in the Seafood department of stores rather than the meat department. Further, in times past before the rise of supermarkets meat was sold in butcher shops, fish in fish markets (or by fishermen at the seashore) and vegetables in a green grocer or similar market. Further, both meat and seafood both tend to be more expensive to produce which makes them more expensive than fruits and vegetables. This meant that the rule to abstain from meat probably affected mostly wealthier people who, being better educated and better connected, were able to latch onto the loophole when the Church required that they abstain from consumption of "meat" rather than from "flesh" as "flesh" would have included the flesh of any living creature regardless of whether the flesh was from a warm or a cold-blooded creature.