John Calvin Commentary Genesis 1:21

"And God created the great sea-monsters, and every living creature that moveth, wherewith the waters swarmed, after their kind, and every winged bird after its kind: and God saw that it was good." — Genesis 1:21 (ASV)

And God created. A question arises here from the word created. For we have previously contended that because the world was created, it was made from nothing; but now Moses says that things formed from other matter were created. Those who truly and properly assert that the fish were created because the waters were in no way sufficient or suitable for their production are only resorting to a subterfuge. For, meanwhile, the fact would remain that the material from which they were made existed before, which, strictly speaking, the word created does not permit.

I therefore do not limit the creation spoken of here to the work of the fifth day, but rather suppose it refers to that shapeless and confused mass, which was the fountain of the whole world. God then, it is said, created whales (balaenas) and other fish, not that the beginning of their creation is to be reckoned from the moment in which they received their form, but because they are included in the universal matter which was made from nothing.

So, with respect to species, form only was then added to them; but 'creation' is nevertheless a term truly used regarding both the whole and the parts. The word commonly translated 'whales' (cetos vel cete) might, in my judgment, be appropriately translated as thynnus or tunny fish, as corresponding to the Hebrew word thaninim.

When he says, the waters brought forth, he proceeds to commend the efficacy of the word, which the waters obey so promptly that, though lifeless in themselves, they suddenly teem with living offspring. Yet the language of Moses expresses more: namely, that innumerable fish are daily produced from the waters, because that word of God, by which He once commanded it, is continually in force.