अजानन् रक्षणार्थाय शफर्या: स मनो दधे ॥ १५ ॥
śapharyāḥ sa mano dadhe
tam — unto the fish; ātmanaḥ — personal; anugraha-artham — to show favor; prītyā — very much pleased; matsya-vapuḥ-dharam — the Supreme Personality of Godhead, who had assumed the form of a fish; ajānan — without knowledge of this; rakṣaṇa-arthāya — just to give protection; śapharyāḥ — of the fish; saḥ — the King; manaḥ — mind; dadhe — decided.
To please himself, King Satyavrata, not knowing that the fish was the Supreme Personality of Godhead, decided with great pleasure to give the fish protection.
Here is an example of giving service to the Supreme Personality of Godhead even without knowledge. Such service is called ajñāta-sukṛti. King Satyavrata wanted to show his own mercy, not knowing that the fish was Lord Viṣṇu. By such unknowing devotional service, one is favored by the Supreme Personality of Godhead. Service rendered to the Supreme Lord, knowingly or unknowingly, never goes in vain.