About a fish symbol
The first Christians used the fish symbol as a secret password to recognize each other. The symbol can be understand as a cryptogram. In Greek the word fish is written as ΙΧΘΥΣ (Ichtys), which stands for Jesus, Christ, Son of God, Messiah.
When a Christian met a foreigner, he draw a half of the fish symbol. If the foreigner completed the symbol by drawing the second half, both of them knew there were Christians. Nowadays the fish logo on a car, on a business card, (and the fish image on a website :-) :-) ) are based on this tradition. The symbol is still in use today to identify the active Christian (more on wikipedia).
We can find stories about fish and fishing in the Bible. Some of Jesus disciples were fishers. We can find a call: “I will make you to become fishers of men”. When the disciples spent the whole night by fishing in the Tiberias lake without catching anything, until Jesus came to them telling to throw a net to the right side of the ship. Despite of the catch was huge, number of fish 153, the net did not rip. The number of the caught fish may symbolize the number of nations known that day. It may also indicate missionary activity of the church.