To summarize, fish that have a kaskeses are kosher. The definition of kaskeses is unique to kashrus, and scientific classifications of scales are not halachikly determinative.
In this article, we will discuss two methods how to practically determine if a fish is kosher.
The easiest way to determine if a fish is kosher, is by manually checking the fish for scales. Simply locate a scale on the side of the fish (preferably behind the gills, tail or fin – as mentioned by the Rama as a chumra to guarantee the scale did not fall off of another fish), grab it between your thumb and forefinger, and gently attempt to pull it out. One should note that scales are always attached to the fish on the side closer to the head. The reason is fairly obvious if you can imagine how a fish swims. If the scale would be able to swing on the side closer to the tail, the current pulling against the now exposed scale would inevitably rip it off as the fish swims. This would be similar to the effect of when one walks with an umbrella in a brisk wind, and does not point it in the direction of the blowing. The umbrella gets caught in the wind and blows inside out. So too, the current would get caught under the scale and rip it off, causing the fish to die due to infection.
After removing the scale, simply inspect the area where the scale came from for a rip in the skin. If the skin seems fairly undamaged, the fish is kosher. If the scale will not come out without the skin ripping, the scale is not a “kaskeses”. Generally speaking, it is fairly obvious whether ir nit the scale ripped. As a practical “aytzah” to get a sense of what skin normally looks like when a “kaskeses” is removed (and the skin does not rip) would be to inspect the skin of other fish which one knows to be kosher.
Though there is no requirement of “mesorah” on fish like there is on birds and animals, the Darchei Teshuva does describe the possibility of determining the kosher status of a fish based on mesorah. Specifically, one may bring a fish (whose “kaskeses” fell off or did not yet grow “kaskeses”) to someone familiar with the specific fish to determine if this is a species that the person has mesorah of it being a kosher fish. It is important to keep in mind that the “mesorah method” of determining kosher status is particularly useful when dealing with various types of mackerel, for example, as mackerels tend to lose their scales when removed from the water. Fish that lose their scales may have a single scale in the three areas mentioned earlier (behind the gills, tail and fin), though without a scale one could still recognize the fish based on its skin.
As with most matters relating to the permitting of a potential Torah prohibition, the person ruling on the fish must be both “halachicly” reliable and familiar with the issue at hand (in our case, the specific type of fish). One should note that a gentile working at a fish store is not qualified to confirm the kosher status of the fish.
The “mesorah method” is based on an idea mentioned in last week’s article, namely that the Gemara tells us that a fish that has not yet grown “kaskeses” or lost its “kaskeskes” is still a kosher specie. One should ask, even if theoretically true, how could one practically determine that the fish is kosher if there are no “kaskeses” on it now? The answer, says the Darchei Teshuva, is that one can recognize the specie based on its skin. There is no mention of someone with a “tvias ayin” on the flesh of a fish, which must be regarded as “kirvei dagim” and is forbidden.
Some have asked how big a piece of skin must be left on the fish for one to determine its status based on the “mesorah method”. Though I have not seen a specific size given, clearly the piece of skin must be big enough for someone to actually be able to say what it is. A few weeks ago, I received an inquiry from a smaller hashgacha organization, that wanted to know how they could accept as kosher fish whose skin had been completely removed except for a small (scaleless) patch, when their mashgiach could not properly identify the fish. I answered that they could not. The only way to accept the fish is by having someone familiar with the specie accept the delivery, and a mashgiach who is not familiar with the specific fish is not acceptable. Consider the following mashul (parable). Suppose a person, r”l, is blind. Halachicly, the person is “ne’eman” to testify in Beis Din. One would not, however, ask the person to confirm which of two identical pieces of meat has a hashgacha printed on the package. Here too, a person who does not have mesorah on the particular fish in question may not be relied upon to confirm the kosher status of the fish by a patch of skin. Such a person could only attempt to remove a scale from the fish, as described above.
Some hashgacha organizations allow for salmon to be accepted without skin at all. The justification behind this policy is that there are no known fish whose flesh resembles the red/pink of a salmon, making the flesh color a “siman muvhak”. Again, this heter would only apply to a case where the mashgiach accepting the fish knows what a salmon is supposed to look like.
Many of us are “zoche” to live in areas where we don’t much think about which fish are kosher or not, as we could not imagine the local “heimish” supermarket selling a non-kosher species. Some of us live in parts of the world where kosher meat is difficult to acquire, and buying fish from the local store is the easiest way to properly feed our families. Though it may seem odd at first, those people have at least one advantage over their brethren living in Jewish neighborhoods. They have the chance to teach themselves and their children how to determine if a fish is kosher, often having no other option. It would be unfortunate if those of us who can easily acquire a kosher fish would lose out on the opportunity to know how to be “mavchin bein hatamei u’bein hatahor”, to be able to distinguish between the pure and the impure.