What is the origin of the Christian fish?

Today, in popular culture (and even radio) the fish symbol has become synonymous with Christianity.  But what is the origin of this image?  What does a fish have to do with Jesus?  I've been reading a book called The Story of Christianity by Justo Gonzalez, and here's what he has to say on the subject, in a chapter about the art of Christians during the days of the Roman Empire:

"The fish was one of the earliest Christian symbols, and for that reason appears frequently in communion scenes as well as in other contexts.  The significance of the fish, apart from its connection with the miraculous feeding of the multitudes [in the gospels], was that the Greek word for fish--ichthys--could be used as an acrostic containing the initial letters of the phrase: "Jesus Christ, Son of God, Savior."  For this reason the fish appears, not only in representative art, but also in some of the most ancient Christian epitaphs.  Thus, for instance, the epitaph of Abercius, bishop of Hierapolis toward the end of the second century, says that faith nourished Abercius with "a fresh water fish, very large and pure, fished by an immaculate virgin" (Mary, or the church?).  And other similar epitaphs speak of "the divine race of the heavenly fish," and "the peace of the fish."

Now you know.