The piscatorii ludi are celebrated, according to Sext. Pompeius Festus (de Muller, 238), in the month of June.
Fishermen's games, as they are called, are held annually in the month of June on the farther side of the Tiber; they are celebrated by the City Praeton on behalf of the fishermen of the Tiber, whose catch is not taken to the market but to the Area of Vulcan, because that sort of live fish is given to that god as a substitute for human souls.
The Area of Vulcan was a large open space at the foot of the slope of the Capitol on the side of the Forum. It was one of the most ancient religious centers of Rome, but never contained a temple of Vulcan, only an altar or shrine of the diety. Apparently the fish were offered to Vulcan by being thrown alive into the fire; for Varro tells us that at the festival of Vulcan (the Volcanalia on August 23) people threw live animals into the fire as substitutes for themselves (De lingua Latina, 6.20) The sacrifice was seemingly a sort of fire insurance; the conflagrations which often raged in Rome were doubtless ascribed to the wrath of the fire-god Vulcan, and people thought to ransom their own lives from the fire by throwing live animals or fish into it. If, as we gather from Ovid, the day on which the fish were caught was a festival of Tiber, the river-god must have resented the use made of his creatures on his holy day. In any case the employment of substitutes to save people from the danger of fire furnishes a parallel to the Argei interpreted as substitutes to save poeple from the danger of water [Cicero, De Natura Deorum, 3.20.52].

The worship of the Tiber (Tiberinus) is said to have been instituted by Romulus; certainly it would seem to have been very ancient at Rome, since the river-god was mentioned in the litany of the pontiffs and in the prayers of the augurs. Yet the notices and monuments of the worship which have come down to us are few and far between. An inscription, found near the river, records that a certain Sex. Atusius was the first man to dedicate to the Tiber an altar which he had vowed while he was on military service. Another inscription, found near the Tiber at rome, records a dedication to Tiber, "the father of all waters," by the Emperors Diocletian and Maximian on the occasion of their completing the cleansing and repair of the aqueducts. And oddly enough in this dedication the emperors coupled with the Tiber "the men of old, the inventors of admirable engineering works."According to Ovid, the fishermen's games in honour of the Tiber were held in the Field of Mars, but according to Festus they were held on the farther side of the river.

On the third morn after the Nones it is said that Phoebe chases away (the grandson of) Lycaon, and the Bear has none behind her to fear. Then I remember that I saw games held on the sward of the Field of Mars, and that they were named thine, O smooth Tiber. The day is a festival for those who draw their dripping lines and hide their bronze hooks under little baits.

Ovid, Festi 6.235-241