What was the reason for Lord Vishnu's incarnation as Matsya Avatar?

 Satyavrata, the king of pre-ancient Dravida and a devotee of Vishnu, was washing his hands in a river when a little fish swam into his hands and pleaded with him to save its life. He put it in a jar, which it soon outgrew. He then moved it to a tank, a river, and then finally the ocean, but to no avail. The fish then revealed himself to be Vishnu and told him that a deluge would occur within seven days that would destroy all life. The fish instructed Satyavrata to build an ark to take "all medicinal herbs, all the varieties of seeds, and accompanied by the seven saints", along with the serpent Vasuki and other animals, as the mare who lived at the bottom of the ocean would open her mouth to release a poisonous fire at the end of Kali Yug. This fire would burn the whole universe, including Gods, constellations and everything. The seven clouds of doomsday would then flood the earth until everything was a single ocean.

As the time of the flood approached, Manu's ark was complete. When the flood swept over the land, Manu asked Vishnu why mankind had to meet such a deadly fate. Matsya Vishnu told Manu that he was the only moral man alive and that he would be the father of the future generations of men. Matsya killed Hayagriva and returned the Vedas to Brahma. Then he tied himself to Manu's ark using Vasuki as a rope and protected them from the storm and the floods. When the storms ended and the water subsided, Matsya Vishnu left Manu and the others at the Himalayas, where they could begin human civilization again.