The Greek word for fish is (Ichthys) |
 
(Iesous Christos Theou Yios Soter) Jesus Christ, Son of God, the Saviour |
Therefore the fish is symbol for Jesus Christ.
This symbol may have been suggested also by the miraculous multiplication
of the loaves and
fishes or the repast of the seven Disciples, after the Resurrection,
on the shore of the Sea of Galilee (John 21:9)