Charles Ellicott Commentary Leviticus 11:9

"These may ye eat of all that are in the waters: whatsoever hath fins and scales in the waters, in the seas, and in the rivers, that may ye eat." — Leviticus 11:9 (ASV)

These you shall eat. —Water animals, which, as we have seen, constitute the second division of the animal kingdom, now follow the land animals. They are discussed in Leviticus 11:9-12. Like the clean quadrupeds, saltwater and freshwater fish must comply with two conditions to bring them into the class of clean: they must have both scales and fins.

It will be seen that in the case of the quadrupeds, not only are two criteria given by which the clean animals may be distinguished from the unclean, but the law is also illustrated by adducing ten land animals of the former kind and four of the latter . In the case before us, however, not a single typical fish is given by name, and the law itself is expressed in the briefest and most generic manner possible.

It was evidently left to those on whom the administration of the law devolved to define it more minutely so that it might be observed in practical life. Hence, the following expanded definitions became established during the Second Temple period:

1. All fishes with scales invariably also have fins, but fishes that have fins do not always have scales. Any fish, therefore, or even a piece of one exposed by itself for sale in the market, that exhibits scales may be eaten, for it is to be taken for granted that it had fins, or that the fins cannot be seen because of their extraordinary smallness. On the other hand, a fish with fins may exist without scales and therefore is unclean.
2. Clean fishes have a complete vertebral column, but the unclean have simply single joints, united by a gelatinous cord. To the former class belong: (a) “the soft fins,” such as the salmon and trout, the capelin and grayling, the herring, the anchovy and the sardine, the pike and carp families, the cod, the hake and the haddock, the sole, the turbot, and the plaice; and (b) “the spiny fins,” such as the perch, the mackerel, and the tunny. To the latter class belong the shark tribe, the sturgeons with their caviar, the lamprey, and the nine-eyed eel.
3. The head of clean fishes is more or less broad, while that of the unclean kinds is more or less pointed at the end, like the eel, the mammalian species, etc.
4. The swimming bladder of clean fishes is rounded at one end and pointed at the other, while that of unclean fishes is either rounded or pointed at both extremities alike.

It is in allusion to this law that we are told in the parable of the fisherman, which is taken from Jewish life, that when they drew to shore the net with every kind of fishes, the fishermen sat down (i.e., to examine the clean and the unclean), and gathered the good (i.e., the clean) into the vessels, but cast the bad (i.e., the unclean) away (Matthew 13:48). Orthodox Jews to this day strictly observe these regulations and abhor eating those fishes that are enumerated under the four above-named criteria as unclean. Moreover, it is to be remarked that fishes without scales are also still regarded in Egypt as unwholesome, and that the Romans would not permit them to be offered in sacrifice.