1. A representation of a fish, common as a talisman or amulet, and regarded in ancient times as a symbol of fertility.
2. The Greek word for fish was also an acronym for Iesous CHristos, THeou Yios, Soter (Jesus Christ, God’s Son, Saviour) and it is said to be the secret symbol used by early Christians to communicate with each other.

They did this by drawing a fish-like sign or symbol: This symbol is found on many seals, rings, urns, and tombstones from the early Christian period; and for some, it was a mystical charm.