Notice the use of imperative language for a blessing. The imperative language does not necessitate that something is a command in the Old Testament. See commentary on verse 28.
Targum Onkelos: And the Lord blessed them, saying, Spread abroad and become many, and fill the waters of the seas; and let the fowl become many on the earth.
Rashi: And…blessed them: Because [people] decimate them and hunt them and eat them, they required a blessing; and the beasts also required a blessing, but because the serpent was destined to be cursed, He did not bless them, lest he [the serpent] be included. — [from Gen. Rabbah 11:3, Midrash Tadshe 1]
Be fruitful: [The word פְּרוּ is derived from פְּרִי, fruit, meaning produce fruits.
and multiply: If He had said only, “Be fruitful,” one would beget one and no more.“And multiply” was therefore said so that one could beget many.
Rambam: God decreed a blessing on all the animals that each should give birth to many of its kind and that the sea animals should fill the waters in the seas while the birds, although their origin was in the sea, should reproduce on the land.
Ibn Ezra: God blessed them and said, “Because I have blessed you, you will be fruitful and multiply. You will fill the water in the seas and the birds will multiply on the land.”