20 And God said, Let the waters bring forth abundantly the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven.
21 And God created great whales, and every living creature that moveth, which the waters brought forth abundantly, after their kind, and every winged fowl after his 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 fowl multiply in the earth.
23 And the evening and the morning were the fifth day.
On the fifth day, God created the fish and all the creatures of the water, and the birds and all the creatures of the air. He commanded them, as He commands all of His creation, to be "fruitful and multiply!"
He wanted His creation to fill the whole world.
But, there is another aspect to this command.
Did you ever stop to consider what would happen once the whole world was filled? Has God ever rescinded that command? When the world is populated abundantly, is over-population going to be an issue?
I believe that God is God. He makes room for everyone. I don't believe, at the time, that over-population would have EVER been an issue, had Adam not made the fall.
Now it is a test of faith. Do we continue to "be fruitful and multiply"? I say, yes. God has never rescinded His command. He never changes His mind. He is the same yesterday, today, and always. And, He uses man's mistakes to complete His perfect Will!