The Catholic Church's requirement to fast from meat on Fridays is Canon 1251. The Latin word translated "meat", however, is "caro", which means flesh. Fish is distinguishable from flesh meat by several standards.

• Experientially, a freshly caught fish, when sliced open, will not bleed.
• Scientifically, most fish are cold-blooded and all are aquatic.
• Historically, fish was poor person food; it was so easy to catch and market in the Mediterranean and the Levant that it had been a staple long before Jesus showed up. Slaughtering a sheep, goat, or calf, on the other hand, was a big expense since each of those animals produced other things of value (wool, dairy) if kept alive.

Notice in the Gospels that Jesus and the disciples eat fish with some frequency, but are never depicted eating another animal. When Jesus feeds the 5000, it's with a young boy's lunch: five loaves and two fishes.