Genesis 1:22

"God blessed them and said, 'Be fruitful and increase in number and fill the water in the seas, and let the birds increase on the earth.'"  Genesis 1:22

In the Interlinear Bible "increase in number" can be translated "become many or great," but it does still seem to be encouraging the water creatures to multiply and fill the waters.  So whatever creatures are in the waters and whatever birds or flying creatures that God just created, he's now telling them to go thrive through out the earth.  And they did.  It's not hard to find birds or water creatures.