I found this to be interesting.

Fish is an universal symbol and is used in many cultures. But between the first christianism had very much more significance and interest. The word "fish" in greek can be translitered "IXZUS" and his letters are the initials of the frase IESOUS XRISTÓS ZHEOUS UYOS SOTER what means "Jesus Christ Son of God Saviour" this is the reason that first christian adopt the fish like a password for recognise between them.