Or HaChaim on Genesis 1:22/אור החיים על בראשית א׳:כ״ב

1And He blessed: The reason that God needed to bless that which swarmed from the waters - which He did not do with the creations that came out of the earth - and that, even among those that were born of the water themselves, He brought jealousy by not blessing the swarms of birds; it appears that this must come from an unavoidable reason, [and it] is according to that which I have explained, that that which God commanded the waters to swarm (with creatures) was only for that time and not for afterwards. [Hence,] for the survival of the specie, He commanded that they be fruitful and multiply, since, [had He] not [done] this, the specie would have ended. And given that according to nature, the reproductive facility of the creations that God created, [comes from] the element of fire; as the nature of cold - it is just the opposite - it destroys the reproductive facility. And go and learn form natural science; also from [the] words of [the Sages], of blessed memory, (Shabbat 156b) that God said to Avraham, "what is your thinking [about this], that Saturn lies etc., as it is written (Isaiah 41:2), 'who called righteousness to His feet'" (the Hebrew word, tsedek means both Saturn and righteousness); look there at their words. And according to this, fish do not have the composition that is set in nature for creatures to reproduce and so, therefore God went back with His glorious proclamation and blessed them. "Saying" the explanation [of which] is that the intention of the blessing with which He blessed them is to say that they would be able to reproduce, and that the coldness of the water would not prevent them [from doing so]; and that they would not need to go out of the water to become warmed by the earth, like the chickens (Beitzah 7a) that are [sometimes] fertilized by the ground, but rather they would give off their fruit in the water.

2And this is [why] it states, "and fill the waters, in the seas;" [as] it is difficult, why is is it needed to state "in the seas" and not suffice with stating, "and fill the waters;" but rather the intention is that within the seas, there will they swarm, and that is the blessing of the Blesser, may He be blessed. And it finishes [by] stating, "and the birds will multiply in the earth," since it will increase in the way of all the earth and [this] is correct.