jñātvā tad dānavendrasya
hayagrīvasya ceṣṭitam
dadhāra śapharī-rūpaṁ
bhagavān harir īśvaraḥ
jñātvā - after understanding; tat - that; dānava-indrasya - of the great demon; hayagrīvasya - of Hayagrīva; ceṣṭitam - activity; dadhāra - accepted; śapharī-rūpam - the form of a fish; bhagavān - the Supreme Personality of Godhead; hariḥ - the Lord; īśvaraḥ - the supreme controller.
Because everything was inundated by water, to save the Vedas it was necessary for the Lord to assume the form of a fish.