Genesis 1:20-23 meaning

God continues to bring order and balance to nature. The waters and the air that was separated on the second day, are filled with their respective inhabitants on this day. God saw that all the creatures of the sea and all the creatures of the air that He created were good, and blessed them saying, "Be fruitful and multiply." In other words, to reproduce and become numerous. Day five of creation ends with God filling the earth with all that was needed for what was coming next in day six.

It is worth noting here that God made different "kinds." It is observable in nature that there is great diversity within "kinds." However, there is no "mixing" observed between the various kinds. There are many types or kinds of a dog, whose lineage can be observably traced. But we never consider a dog giving birth to a cat. This becomes important when God preserves the land animals from destruction in the flood of Noah. At that time, God preserves two of every "kind."