Soon after the cosmic creation, the entire universe was inundated with water. At that time Lord Krishna incarnated as a gigantic fish to protect the Vedas. Therefore Manu addresses Lord Matsya as mukhyatama, the first incarnation to appear. Fish are generally considered a mixture of the modes of ignorance and passion, but we must understand that every incarnation of the Supreme Personality of Godhead is completely transcendental.

(Srimad-Bhagavatam 5.18.25 purport)