Fish belong to the water by nature

"Let the waters bring forth." This shows you the natural kinship of swimming creatures with water, which is why fish perish when separated from water even for a short time. They do not have breathing to draw in this air; rather, what air is to land animals, water is to the swimming kind. And the reason is clear. In us, the lung lies within—a porous organ with many passages—which receives air through the expansion of the chest and fans and cools our inner heat. But in them, the expansion and contraction of the gills, receiving the water and letting it pass through, fulfills the function of breathing. Fish have their own lot, their own nature, a separate way of life, a distinct manner of living. For this reason, none of the swimming creatures allows itself to be tamed, nor does it endure at all the touch of a human hand.

"Let the waters bring forth creeping things that are living creatures according to kind" (Genesis 1:20). He commands that the first beginnings of each kind be brought forth now, as seeds of nature, so to speak; their abundance is stored up for the succession that follows, when they must increase and multiply. One kind is what are called shell-skinned creatures: mussels, scallops, sea snails, conches, and the countless varieties of oysters. Another kind besides these is what are called soft-shelled creatures: crayfish, crabs, and those similar to them. Another kind besides these is the soft-bodied creatures, so named because their flesh is soft and yielding: octopuses, cuttlefish, and those like them. And among these again there are countless differences. For sea serpents, moray eels, and eels—which are found in muddy rivers and lakes—approach the poisonous reptiles more closely than fish in the similarity of their nature. One kind is that of egg-laying creatures, and another that of creatures bearing live young. The dogfish-like creatures and the small sharks bear live young, and in general the so-called cartilaginous fish. And most of the sea-monsters bear live young: dolphins and seals, which are said to receive their young back into the belly and shelter them when they are still young and frightened by some cause.

"Let the waters bring forth according to kind." The sea-monster kind is one thing, and the kind of small fish is another. Again, among fish there are countless differences divided according to kinds, with their own names, different food, shape, size, and qualities of flesh—all separated from one another by the greatest differences and established in different forms. What tuna-watchers, then, can enumerate for us the differences of the kinds? Yet they say that they can report the number of fish in great schools. And who among those who have grown old along the shores and coasts can make known to us the account of all of them with precision? Those who fish the Indian Ocean know some creatures; those who hunt the Egyptian gulf know others; islanders know others; and the Maurusians know others. But all alike, small and great, that first command and that ineffable power brought forth. Many are the variations of ways of life; many also are the differences in the succession of each kind. Most fish do not brood over their eggs as birds do, nor do they build nests, nor do they raise their young with labor; but the water receives the egg that falls out and makes it a living creature.