And God {r} blessed them, saying, Be fruitful, and multiply, and fill the waters in the seas, and let fowl multiply in the earth.

(r) That is, by the virtue of his word he gave power to his creatures to reproduce.

Wesley's Notes

1:20-23 Each day hitherto hath produced very excellent beings, but we do not read of the creation of any living creature till the fifth day. The work of creation not only proceeded gradually from one thing to another, but advanced gradually from that which was less excellent, to that which was more so. 'Twas on the fifth day that the fish and fowl were created, and both out of the waters. Observe,
1. The making of the fish and fowl at first. Ge 1:20,21 God commanded them to be produced, he said, Let the waters bring forth abundantly - The fish in the waters, and the fowl out of them. This command he himself executed, God created great whales, and c. - Insects which are as various as any species of animals, and their structure as curious, were part of this day's work, some of them being allied to the fish, and others to the fowl. Notice is here taken of the various species of fish and fowl, each after their kind; and of the great numbers of both that were produced, for the waters brought forth abundantly; and in particular of great whales the largest of fishes, whose bulk and strength, are remarkable proofs of the power and greatness of the Creator. Observe, 2, The blessing of them in order to their continuance. Life is a wasting thing, its strength is not the strength of stones; therefore the wise Creator not only made the individuals, but provided for the propagating of the several species, Ge 1:22. God blessed them, saying, Be fruitful, and multiply - Fruitfullness is the effect of God's blessing, and must be ascribed to it; the multiplying of the fish and fowl from year to year, is still the fruit of this blessing here.

Matthew Henry's Concise Commentary

1:20-25 God commanded the fish and fowl to be produced. This command he himself executed. Insects, which are more numerous than the birds and beasts, and as curious, seem to have been part of this day's work. The Creator's wisdom and power are to be admired as much in an ant as in an elephant. The power of God's providence preserves all things, and fruitfulness is the effect of his blessing.