The Matsya Avatara
Skandha. VIII. Chap. 24
Towards the end of the previous Kalpa, Brahmâ was falling asleep and the Vedas fell from his mouth. The Asura Hayagrîva took them up. Seeing this Vishnu became a small fish. King Satyavrata was making Tarpana (i.e. offering libations of water), when the fish found its way into his hands. He threw it into the river. The fish implored the King to preserve him. So he took it home and placed it in a small waterpot. The fish increased in size so much that all tanks and rivers were tried, but they could not contain it. At last the king took the fish to the sea, but it implored him not to throw it away into the sea. The king then said: — “This fish must be the Deity Himself, otherwise how could it grow so large?” The fish then addressed the king thus: “On the seventh day from this, the Trilokî shall be plunged into the Pralaya waters. Then a big Ark shall come to thee. Take all plants, all seeds, all animals, and the seven Rishis with you and get into that ark. When the wind shakes that ark, tie it with a serpent to myself. I will remain with that ark in the Pralaya Ocean till the awakening of Brahmâ. I will manifest supreme wisdom in thee.”
So saying the fish disappeared and on the seventh day the Pralaya waters deluged the Trilokî. Satya Vrata did as he was told. He got the highest wisdom from the Fish Incarnation.
That Satya Vrata is Srâddhadeva, our present Manu.