Any fish that has fins and scales is kosher, as it says:
“These you may eat from all that are in the water: anything in the water that has fins and scales, whether in the seas or in the rivers — those you may eat. But anything that does not have fins and scales, whether in the seas or in the rivers, among all the creeping creatures of the water and among all living beings in the water, they are detestable to you. They shall remain detestable to you; you shall not eat of their flesh, and their carcasses you shall detest” (Vayikra 11:9–11; see also Devarim 14:9–10).
Fins extend from the sides of the fish and help it swim. Scales grow over the fish’s skin, forming an additional protective layer. Each scale is attached to the skin at one end, while its other end lies against the skin without being attached. A fish is kosher even if it has only one fin and one scale (Ḥullin 59a; SA YD 83:1), provided it is certain that the scale was attached to this fish and did not come from another fish and stick to it (Shakh YD 83:1).
Although the Torah mentions both fins and scales, the Sages taught that any fish with scales also has fins. In practice, then, scales are the decisive criterion for determining whether a fish is kosher (Ḥullin 66b; SA YD 83:3).
A fish does not need to have visible scales at every stage of its life. Some fish grow scales only later; others have scales when young and shed them as they mature; and still others lose their scales when removed from the water. All of these fish are kosher, because any fish that has scales at some point is considered kosher (Ḥullin 66a; SA YD 83:1).
The scales must, however, be visible to the naked eye. If they are very thin or difficult to see because of the fish’s coloring, one may wrap the fish in a cloth and then inspect the cloth; if a scale is found there, the fish is kosher (Avoda Zara 39a; SA YD 83:2).
Finally, the scales must be true scales. True fish scales are attached to the skin, but loosely enough that they can be peeled off by hand or with a utensil while the skin underneath remains intact. If they must be cut away, and the skin is damaged in the process, they are not scales but part of the skin, and the fish is non-kosher (Rema YD 83:1). For this reason, many authorities prohibit swordfish, because the scale-like formations visible on it are part of its skin; see the Herḥavot.
Fish do not require sheḥita, and the prohibition against eating a limb from a living animal does not apply to them. It is therefore permissible to cut a piece from a live fish and eat it. One may not, however, bite directly into a live fish. Doing so is forbidden both because of bal teshaktzu, the prohibition against repulsive conduct (Rema YD 13:1), and because it is cruel (Da’at Kohen 12).
