The meanings, origins and ancient traditions surrounding Christian symbols date back to early times when the majority of ordinary people were not able to read or write and printing was unknown. Many were 'borrowed' or drawn from early pre-Christian traditions. No Fish is mentioned by name either in the Old or in the New Testament. Fish abounded in the Mediterranean and in the lakes of the Jordan, so that the Hebrews were no doubt acquainted with many species. Two of the villages on the shores of the Sea of Galilee derived their names from their Fisheries, Bethsaida (the "house of Fish") on the east and on the west. There is probably no other sheet of water in the world of equal dimensions that contains such a variety and profusion of Fish. About thirty-seven different kinds have been found. 

The Definition and Meaning of the Ichthus / Fish as a Catholic Christian Symbol
Catholic Christian symbolism in art provides a clear graphic illustration which represents people or items of religious significance. What is the definition and the meaning of the Ichthus / Fish symbol? Ichthus is the Greek word for fish (ΙΧΘΥΣ). The initials of the word Ichthus are also used as a Christian acronym of the following Greek words:

I=Jesus
Ch=Christ
Th=Theou (God's)
U=Uios (Son)
S=Soter (Savior)

Using the Ichthus acronym IChThUS means "Jesus Christ, God's Son, Savior".

The Ichthus / Fish was chosen as an early secret Symbol indicating that the person who knew the meaning of the Ichthus was a Christian and safe to associate with. Being a Christian was extremely dangerous for hundreds of years following the life of Jesus due to the persecutions of the pagan Roman Emperors.

The sign of the Ichthus
The sign of the Ichthus was simple to draw consisting basically of  two arcs:

It was not necessary to point the fish in any particular direction, the basic symbol was sufficient:

Reference to the Ichthus / Fish Christian Symbol in the Bible
The Easton Bible Dictionary provides the following definition, meaning and emblem for the Fish  in the Bible.

Fish are called called 'dag' by the Hebrews, a word denoting great fecundity (Gen. 9:2; Num. 11:22; Jonah 2:1, 10).

There was a regular Fish-market apparently in Jerusalem (2 Chr. 33:14; Neh. 3:3; 12:39; Zeph. 1:10), as there was a Fish-gate which was probably contiguous to it.

Ichthus / Fish Christian Symbol

• The Importance of Catholic Symbolism or Icons
• Religious Information, Meaning and Definition of the Ichthus / Fish Catholic Art Symbolism
• Ichthus / Fish Christian and Religious symbolism
• Symbolism and early religious meaning in art of the Ichthus / Fish Christian Symbol
• Ancient history, biblical meanings and practices of the church
• Facts, Information, Meanings and Definitions about symbols
• Significance and representations of the Ichthus / Fish Christian Symbolism