Not being a believer in reinventing the wheel, I did some research and found several artices on the subject. The best one (in my opinion) is the following:
"This reply is based on several sources in halacha [the entire body of Jewish law and tradition, including the laws of the Bible, Talmud and oral law]. Most notably, extensive research by Rabbi Moshe Tendler, Chairman and Professor of the Biology Department at Yeshiva University, March, 1966.
"The halachic [status] of swordfish (xiphias gladius):it is a non-kosher fish.It has been presented to halachic giants of our generation such as Rabbi Moshe Feinstein and from previous generations such as Rabbi Itzel of Ponovitz,and they have all concluded that it is not kosher.
"The biblical term for scaled kosher fish is 'kaskeses.' Nachmanides comments in Leviticus XI:9 that based on Biblical and Talmudic sources this refers to a type of structure like the nail of a human which can be removed from the skin of the fish by hand or with a knife. But if it be affixed to the skin and not separated therefrom at all (i.e. no free margins), then the bearer of these 'scales' may not be eaten.
"Ichthyologists recognize four types of fish scale. The kosher variety of scales are cycloid (round) and ctenoid (comblike). The ganoid scale found on sturgeon, or the placoid scale of the shark are specifically excluded from the Biblical term kaskeses since they are not 'removable' scales without tearing the skin from the flesh. Even an educated layman would not see any similarity between the heavy bony plates of the sturgeon or the needle-like projections on the shark skin and the classic kosher scale of the whitefish or carp.
"In Fishery leaflet #531, U.S. Dept. of Interior, Fish & Wildlife Service, Bureau of Commercial Fisheries, Wash. D.C., it states 'swordfish during early juvenile stage of life (up to 8 inches long), have "scales" that are markedly specialized and rather unique. They are in the form of bony tubercules or expanded compressed platelike bodies. These scales are rough, having spinous projections at the surface and they do not overlap one another as the scales in most fish do. With growth the scales disappear and the adult fish including those sold commercially have no scales.'
"The Talmud (Chulin 66A) lists but two exceptions to the absolute requirements of having visible scales:
a) fish that shed their scales when netted, like the mackerel. A black cloth placed in the net beforehand can verify this;
b) fish that have scales developing later in the life cycle; consequently the juvenile fish that lack scales may be eaten since they DO have scales at maturity.
"Nowhere in the Talmud or in responsa literature is there any reference or precedent to such a deviant: a fish that has scales as a juvenile but not as an adult. Therefore, based on the above, swordfish (xiphias gladius) do not possess scales of the kosher variety and is a non-kosher fish" (www.koshersupermarket.com/rabbidavis.htm).
Because of the scientific evidence listed here, we've listed the swordfish with other unclean animals. Based on an article byCecil E. Maranville, from thewww.ucg.orgwebsite.
Technically, swordfish do have scales, but these scales cannot be removed without taking fish flesh with it. Hence they do not qualify as true scales, as coverings. The swordfish are therefore not considered kosher. It is obviously not proper to serve swordfish to anyone.
Thank you for such a mindful and sensitive question.
According to the Conservative Movement, swordfish is considered kosher.This follows from the biblical injunction that fish (unspecified types) with fins and scales are considered kosher (Lev. 11:9; Deut. 14:9).Later, the sages of the Mishnah declare that any species of fish with fins and scales is kosher (Chullin 3:7).
Then, the Talmud follows the Mishnah by specifying particular fish that may have been questionable, including swordfish (B. Talmud, Chullin 66b).Some Orthodox authorities do not necessarily agree that the fish in the Talmud is indeed the swordfish that we know and to which we are referring now.Others debate whether or not the swordfish actually does qualify as having scales, since swordfish lose their scales at a certain point in their development and maturation.And still others may say that, even if it is deemed that they have scales, it is impossible to remove them according to kosher standards.In all, cases the Conservative Movement argues both through Halakhah (Jewish law) and science that a swordfish qualifies as both a fish with scales and it is kosher.
The second part of your question, however, (i.e., would it be proper to serve to friends who keep kosher?) is a question of the value of friendship and pluralism.In this regard, it is important to be open, honest, humble, and respectful.That is, you may want to open your invitation with your personal position on Kashrut and whether you hold that swordfish is kosher.Your friend may wish to know understand why and you should do your best to explain.At this point, you must also be willing to listen to your friend in an open manner – with tolerance and appreciation of their position, even if it differs from your own.Or, on the other hand, you may wish to pose it as a question, for example: do you eat swordfish; does it comply with your standards of Kashrut?
Jewish law, Halakhah, is a wonderful gift through which we are given the opportunity to raise our own inherent divinity.It offers us a way of living a holy life in relationship with God and the world around us.It should never, however, be used as a “weapon” or a leverage of spiritual supremacy.It should bring Jews closer together in friendship rather than as an obstacle to friendship.Sometimes Jews understand the law differently, yet we should never mistake the letter of the law for the spirit of the law.At times, this demands tolerance and love, while maintaining our own integrity.Therefore, since the point of sharing a meal together is actually sharing a meal together, if you find that serving swordfish would counter the values of a friendly host – even if it is in full accord with your own observance – it is probably best to serve salmon (or something else with which everyone will be comfortable).
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