The Torah gives two signs that determine if a fish is kosher—fins and scales. In order for a fish to be kosher, it must have both of these signs. According to the Talmud, any fish that has scales also has fins. Thus, if one knows that a particular fish has scales, it is considered a kosher fish.
As with the other laws of kosher, the Torah doesn't give a reason as to why only a fish with these signs is considered kosher. These laws are considered a chok (a decree beyond comprehension).
Definition of Scales
Certain fish have scales while in the water, but they shed their scales when caughtIn order to render a fish kosher, the scales must be visible to the naked eye and they must be easy to remove from the skin of the fish, either by hand or with an instrument. If the scales can be only be removed after soaking the fish in scalding water, there are differing views as to whether the fish is considered kosher. Sturgeon is one such fish, and, in practice, it is not considered kosher.
If a fish is not completely covered in scales – it only has several scales – it is still considered kosher.
If a young fish belongs to a species which develops scales when they mature, it is kosher even if it has not yet developed them. Conversely, if a type of fish has scales when it is young but not when it matures, there are differing views as to whether or not it is kosher. A swordfish is one such fish, and the accepted rule is that it is not kosher.
Certain fish have scales while in the water, but they shed their scales when caught and brought to dry land. These fish are considered kosher. (I have heard that the Spanish mackerel is one such fish.)
Partial List of Kosher Fish:
- Many (but not all) tuna
- Many species of sardines
Partial List of Non-Kosher Fish:
- Freshwater cod
- Seafood (see below)
See our Kosher Fish List for a more exhaustive listing of kosher and non-kosher fish.
Purchasing Kosher Fish
One should ask the fish monger to thoroughly wash the knife and boardWhen purchasing fish from a store that does not have kosher supervision, it is necessary to personally see the scales on the fish, or at least the indentations in the skin where the scales were before they were removed. It is not sufficient that the sign in the store identifies it as a kosher species of fish. If the fish is filleted, and one sees scales on one of the pieces, the entire fish can be considered kosher if the pieces "match."
I have heard that one may eat salmon even if one does not see the scales on the fish because the distinct color is sufficient evidence that it is salmon.
When purchasing fish from a store that also sells non-kosher fish, one should ask the fish monger to thoroughly wash the knife and board that he will use to fillet the fish. Some people prefer to bring their own knife and board for him to use.
If the fish monger filleted a kosher fish with a knife that was not properly washed, one must wash the area that was cut and scrape it off with a knife in order to remove any non-kosher residue.
Any sea creature that does not have fins or scales is not kosher, regardless of whether it is scientifically classified as a fish or whether it actually resembles a fish. This means that whales, prawns, shellfish, crabs, octopus, lobster, and shrimp are all not kosher.
A Fish Inside a Fish
If a kosher fish is found inside the belly of a non-kosher fish, it is kosher. If a non-kosher fish is found in the belly of a kosher fish, it is not kosher.
Caviar / Roe
The eggs of a non-kosher fish are not kosher, while the eggs of a kosher fish are kosher. In order to establish that the roe is kosher, one must know that it came from a kosher fish. Therefore, caviar should not be purchased unless it has a reliable kosher certification.
When Are Fish Considered Dead?
It is forbidden to eat a fish while it's aliveIt is not necessary to slaughter fish in a ritual manner, because as soon as a fish is removed from the water it is considered slaughtered. It is, however, forbidden to eat a fish while it's alive.
If a fish dies in the water, it may be eaten.
Fish blood is kosher. Nevertheless, it is forbidden to drink fish blood if it is removed from the fish, because others may confuse it with animal blood. However, if there are scales in the blood, it is permissible, as it is clearly not animal blood.
Fish and Meat (or Dairy)
According to the Talmud, it is harmful for one's health to eat fish and meat together. See Fish with Meat or Dairy for more information.
Some argue that, for health reasons, fish should also not be eaten with cheese. See the above link for more information.