The fish symbol was used extensively in pre-Christian culture as a fertility symbol. For example, it is associated with Aphrodite in ancient Greece, and her other aspects in other religions, such as Delphine, Pelagia, and with the son of the Sea Goddess Atargatis, Ichthys. It is also the creature that swallowed the penis of Osirus, when he was ripped apart, and was a representation of the vulva of Isis when the point is pointing downwards.
Sir James George Frazer notes in his work, "Adonis, Attis, Osiris: Studies in the History of Oriental Religion" (Part Four of "The Golden Bough"
that for some in India, the fish was thought to hold deceased souls, and that as part of a fertility ritual certain fish are eaten in the belief that the soul will be reincarnated in a newborn child.
Also, shrines dating from the sixth millennium BC contain this symbol, and are associated with a Pagan religion found in the ancient Danube river basin, used to represent the Great Mother, and in this form is referred to as the
Vesica Piscis.
Ancient Christians used it as a sign designating safe haven, and to signal other Christians. What better symbol could one use, than one your 'enemy' utilized?