And God blessed them, saying,
Be fruitful, and multiply, and fill the waters in the seas, and let fowl multiply in the earth.
All Commentaries on Genesis 1:22 Go To Genesis 1
Augustine of Hippo
God wanted the blessing to have the power of fecundity, which is revealed in the succession of offspring. Thus, though the animals were made weak and mortal, they might by that blessing preserve their kind by giving birth.