I could not find a single citation to prove that the symbol was actually used during the persecutions. However there is proof that the symbol was in use during the early years of the Church.
From The Creed: The Apostolic Faith in Contemporary Theology by Berard L. Marthaler
By adding the title savior to the formula put into the mouth of the Ethiopian eunuch, (Acts 8:36-38) tge early Christians constructed a popular acrostic which is still seen in churches…Thet used the first letter of each of the Greek words for “Jesus Christ, Son of God, Savior” (Iesous Christos Theou Yios Soter) and thus spelled the Greek word fot “fish”, (ICHTYS). Tertullian, the first Christian Latin author of consequence, writing about the year 200 says that the baptized “after the example of our ICTHUS,” are like little fishes (pisciculi) who are born in the water and cannot thrive apart from the water. The fish appears as a common symbol of Christ in literature, inscriptions, and art already in the second century. Because of its sacramental significance it is associated with the Eucharist as well as with Baptism. The fish is pictured as food in banquet scenes in Roman catacombs, and other eucharistic associations are found in funeral inscriptions— Christ, "fish of the living., p. 80.