Once, at the end of the previous kalpa, a demon named Hayagrīva wanted to take the Vedic knowledge away from Lord Brahmā at the time of annihilation. Therefore the Supreme Personality of Godhead took the incarnation of a fish at the beginning of the period of Svāyambhuva Manu and saved the Vedas. During the reign of Cākṣuṣa Manu there was a king named Satyavrata, who was a great pious ruler. To save him, the Lord appeared as the fish incarnation for a second time. King Satyavrata later became the son of the sun-god and was known as Śrāddhadeva. He was established as Manu by the Supreme Personality of Godhead.
To receive the favor of the Supreme Personality of Godhead, King Satyavrata engaged in the austerity of subsisting only by drinking water. Once, while performing this austerity on the bank of the Kṛtamālā River and offering oblations of water with the palm of his hand, he found a small fish. The fish appealed to the King for protection, asking the King to keep Him in a safe place. Although the King did not know that the small fish was the Supreme Personality of Godhead Himself, as a king he gave shelter to the fish and kept Him in a water jug. The fish, being the Supreme Personality of Godhead, wanted to show His potency to King Satyavrata, and thus He immediately expanded His body in such a way that He could no longer be kept in the jug of water. The King then put the fish in a big well, but the well was also too small. Then the King put the fish in a lake, but the lake was also unsuitable. Finally the King put the fish in the sea, but even the sea could not accommodate Him. Thus the King understood that the fish was no one else but the Supreme Personality of Godhead, and he requested the Lord to describe His incarnation as a fish. The Personality of Godhead, being pleased with the King, informed him that within a week there would be an inundation throughout the universe and that the fish incarnation would protect the King, along with the ṛṣis, herbs, seeds and other living entities, in a boat, which would be attached to the fish's horn. After saying this, the Lord disappeared. King Satyavrata offered respectful obeisances to the Supreme Lord and continued to meditate upon Him. In due course of time, annihilation took place, and the King saw a boat coming near. After getting aboard with learned brāhmaṇas and saintly persons, he offered prayers to worship the Supreme Personality of Godhead. The Supreme Lord is situated in everyone's heart, and thus he taught Mahārāja Satyavrata and the saintly persons about Vedic knowledge from the core of the heart. King Satyavrata took his next birth as Vaivasvata Manu, who is mentioned in Bhagavad-gītā. Vivasvān manave prāha: the sun-god spoke the science of Bhagavad-gītā to his son Manu. Because of being the son of Vivasvān, this Manu is known as Vaivasvata Manu.