Ictiomancy was the practice of divination through inspection of fish entrails, as well as their eating habits.
It is a very old divination art that was practiced in ancient Greece and Rome.
For instance, Plinius told of divination using fish in Lycia and Mina.
The practice in those times involved playing a flute thrice with the intention of attracting fish to Sirius, the fountain of Apollo, although this was not very common. If the fish did come close and gather, then meat was offered them, and it was considered a good omen if they ate it right away – and a bad one if it was rejected.
Another form of ictiomancy was to inspect fish entrails after offering them in sacrifice. For example, at the Hierapolis and Lycia oracles, there were sacred fish, destined exclusively for this practice.