20 Then God said, “Let the waters abound with an abundance of living creatures, and let birds fly above the earth across the face of the firmament of the heavens.” 21 So God created great sea creatures and every living thing that moves, with which the waters abounded, according to their kind, and every winged bird according to its kind. And God saw that it was good. 22 And God blessed them, saying, “Be fruitful and multiply, and fill the waters in the seas, and let birds multiply on the earth.” 23 So the evening and the morning were the fifth day.
John Wesley's Notes on the Old Testament
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. Gen_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, etc. - 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, Gen_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.