Whenever there is a decline of religion and a rise of irreligion, I incarnate myself.  To protect the good, to destroy the wicked, and to re-establish religious principles, I appear in every age.
Bhagavad Gita (4.7-8)

According to Hindu mythology there are ten AVATARS, or incarnations of Vishnu. The first incarnation was Matsya, a fish.

Once a king named Satyavrata was performing a sacred ceremony besides a river.  While scooping water from the river he caught a tiny fish in the palm of his hand. The fish begged him not to throw it back into the river where it would be eaten by larger fish. The king felt sorry for the little fish and took it home to his palace where he put it in a small bowl. Next morning the fish had outgrown the bowl and begged the king to put it in something larger. The king then transferred it into a pond, but it very quickly outgrew that too, so he put it into a small lake. Within no time the fish had outgrown the lake and had to be put into larger lake in the kingdom.  Soon, however, it had grown so big that even this was not large enough and the amazing fish had to be put in the ocean.

By this time the king had concluded that the fish must be a divine appearance of God. He offered prayers and asked it why it had taken this form. Matsya, the fish incarnation of Vishnu, that in seven days a huge devastation would engulf the world. He advised the king to gather samples of all kinds of living creatures, all herbs and seeds, and the seven great sages. He promised that he would save them all in a large boat.
As Matsya had predicted, huge clouds gathered in all directions and ceaselessly poured water on land and sea. Soon the ocean overflowed onto the land and a large mysterious boat appeared floating across the waves. Satyavrata and the seven sages brought all the species of flora and fauna aboard the boat. Matsya, who had grown into a golden fish of inconceivable size then appeared in the ocean. The boat was tied to Matsya’s horn, using the giant serpent Vasuki, the Lord guided them across the waters of devastation. For countless years, while darkness covered the worlds, they wandered waiting for the waters to subside. During this journey Vishnu-Matsya gave spiritual instructions to the King and the sages, preserving the Vedic wisdom for future generations and saving all the species of life.

(Main source: Vedic Ecology by Ranchor Prime)