“God blessed them, saying, ‘Be fruitful and multiply, and fill the waters in the seas, and let birds multiply on the earth.’” (Genesis 1:22, NASB)
This is the first blessing of God. The word “blessed” would be more pointedly translated as “endowed”, i. e. gave the ability to reproduce. God clearly separates these living creatures from the plants created on day three by this blessing. The verb tense is such that the blessing is continuous, not a single act.
God is the origin of the ability to have offspring. This is revisited many times in the Bible (Genesis 29:31, Genesis 30:22, Psalm 113:9, Luke 1:7,13). God alone has the ability to grant life. Here we find the first mention of it in the creatures of the sea and air. Again, the abundance is stressed. This time it is a direct command from God that the seas be filled.