But I'm not sure.

The fish symbol was not, however, represented exclusively with symbols of Baptism and the Eucharist; quite frequently it is found associated with such other symbols as the dove, the anchor, and the monogram of Christ. The monuments, too, on which it appears, from the first to the fourth century, include frescoes, sculptured representations, rings, seals, gilded glasses, as well as enkolpia of various materials.

http://www.psalm40.org/ixoye.html


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Note added at 8 mins (2006-04-23 15:02:11 GMT)
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By the Monogram of Christ is ordinarily understood the abbreviation of Christ's name formed by combining the first two letters of the Greek form (see Greek word 1, in table above) thus (see Monogram a); this monogram was also known as the Chrismon. There are, however, besides this type of monogram, two other monograms of Christ -- one of His name, Jesus, the other of both His names together. The most common form (that first alluded to), was adopted by Constantine the Great on his military standards.

http://www.newadvent.org/cathen/10488a.htm