Q:How do we identify a kosher fish?
A:The Torah says that the simanim of kosher fish are “snapir v’ kaskeses”. However the Gemara tells us that all fish that have “kaskeses” have “snapir”, so in practice, all one needs to determine that a fish is kosher is that it has kaskeses!
Q:So what exactly is kaskeses?
A:“Kaskeses” is generally translated as scales. Nonetheless, not all scales are considered kaskeses. This is because the Ramban, in his commentary on the Torah tells us “kaskeses” are scales that can be easily removed by hand or with a knife without tearing the skin. Scales that are embedded in a fish (or are not visible to the naked eye) are not “kaskeses”. The Ramban’s definition is universally accepted, and in fact the Rema rules that those scales that cannot be easily removed (according to the parameters that will be discussed below) cannot be called “kaskeses”.
Q: I heard there a several different scientific classifications of scales. Which are kaskeses?
A: Though scientists categorize scales by certain characteristics, the Torah is only concerned with whether or not a scale can be easily removed without tearing the skin, irrespective of its shape, color or size. From the Torah’s perspective, the various scientific classifications of scales are irrelevant. Statements made by certain “experts” about certain types of scales always being kosher are not true.
Q: What are some examples of fish with scales that are not kosher?
A: Sturgeon definitely has scales , but it is not kosher. Its scales are classified as “ganoid”, which means that they are covered with ganoin (similar in texture to fingernails) and cannot be removed without tearing the skin. Burbot has cycloid scales (one of the types often referred to as “always kosher” ) yet because they are embedded, this fish is not kosher. Sand lances may have tiny scales, but since they are not visible, this fish is not kosher.
Q- How can I know if a fish is kosher?
A: To check if a fish is kosher, one must ascertain that its scales could be properly removed. Scales are attached on the side to the fish on that side of the scale which is closer to the head and are not attached on the other side of the scale which is closer to the tail. To remove the scales, one must grasp that side that is not attached and gently pluck it off from the side of the fish. If removing the scale did not damage the skin, then the fish is kosher.
Q- My local fish store is not under Rabbinic supervision, and it sells fillets without skin. How could I tell if the fish they are selling are kosher?
A: You cannot! Even if the fish is halibut, whitefish or carp (all kosher fish), once the skin is removed it is impossible to identify, and it cannot be assumed to be kosher. In determining the kosher status of fish, identifying the species is critical.
There are two ways to identify a kosher fish:
1) By removing a kosher scale from the skin.
2) By recognizing the fish as being from a kosher species. One can only recognize a fish species if the skin is still intact. It is generally impossible, even for a “maven”, to identify fish without skin. The exception to this rule is that the Orthodox Union accepts salmon and red trout fillets without skin, as there is no non-kosher fish whose flesh resembles that of a salmon or red trout.
For example, let us say that you want to purchase tilapia. You heard that tilapia is a kosher fish, and the friendly counterperson assures you this scale-less fillet is tilapia. You simply cannot rely on this person, unless he is both observant in Torah and mitzvos and is familiar with the laws of kosher fish. Now let us say that a tilapia-eating friend (who is halachicly reliable) comes to the store with you and recognizes a fish in the display case whose scales have been removed (but the skin is still intact) as tilapia. Even though its scales are not present, you may eat this fish because a halachicly reliable person has positively identified it as kosher. Therefore, one can only purchase skinless fillets from a store under reliable Rabbinic supervision.
Q: What if only a patch of the skin is left on a fillet?
A: If you can have someone (halachicly reliable) confirm the identity of a fish based on a patch of skin, this would be sufficient.