Ge 1:20 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.

Ge 1:22 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.

The purpose of Moses here in verses 20 to 23 is to show that God was the one who created these creatures. In pagan cultures, these creatures were worshiped, instead of being viewed as mere creatures.

Moses in effect says here: "Don't be worshiping these creatures. Know that they are creatures of the God who delivered you from Egypt."

Moses also shows that fertility and abudance is the result of the blessing of God. They don't have to worhip some other god in order to be blessed with abundance. God Himself is the one who blesses with abudundance.

In Egypt, they worshipped fertility gods. And Moses here shoots that idea down.