The fish outline is a logical symbol for the early Christian church to adopt.
Fish play a major roll in the gospels.
Jonah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth. And when they had done this, they caught a great number of fish, and their net was breaking. So they cast, and now they were not able to draw it in because of the multitude of fish. The symbol was simple to draw and was often used among Christians as a type of password during times of persecution by the Roman government. In modern times, the fish outline symbol is experiencing a comeback.