John Calvin Commentary Genesis 1:22

"And God blessed them, saying, Be fruitful, and multiply, and fill the waters in the seas, and let birds multiply on the earth." — Genesis 1:22 (ASV)

And God blessed them. He soon declares what the power of this blessing is. For God does not, like humans, pray for us to be blessed; rather, by the mere indication of His purpose, He brings about what humans seek through earnest pleading. Therefore, He blesses His creatures when He commands them to increase and grow; that is, He infuses fruitfulness into them by His word.

But it seems pointless for God to address fish and reptiles. I answer that this way of speaking was simply one that could be easily understood. For experience itself teaches that the power of the word spoken to the fish was not temporary; rather, being infused into their nature, it has taken root and constantly bears fruit.