Is Swordfish Kosher?
The commentaries on the Shulchan Aruch say dag ha’cherev [literally, “the fish of the sword”] is kasher.
Why is it widely considered to be not kosher, then?
Because
around 60-70 years ago, they asked Rabbi [Moshe] Tendler if he could
make a list of which fish are kosher and which aren’t. Rabbi Tender
decided to list swordfish as a treife fish because he called up
an expert who told him scales on a swordfish are a different consistency
– or something like that – from those of other fish. So he decided it
was a treife fish.
But that’s absolutely not correct. The commentaries on the Shulchan Aruch say dag hacherev is kasher.
Professor [Shlomo] Sternberg, a big genius in learning and math,
published an essay maybe 20 years ago in which he writes that Rabbi
Soloveitchik asked him to conduct research on the status of swordfish.
He did. He showed Rabbi Soloveitchik the scales of a swordfish and Rav
Soloveitchik said, “It’s a kashere fish!”
Professsor Sternberg writes that he still has the envelope with the scales he showed Rav Soloveitchik in his Gemara Chullin.
If the commentaries on the Shuchan Aruch say dag ha’cherev is kosher, how can Rabbi Tendler claim it isn’t?
Rabbi Tendler claims “dag hacherev” is a different fish. It’s not true.
But Rabbi
Tendler did a service to the Orthodox Jewish community because at the
time there were Conservative rabbis who were giving hashgachas, so he laughed them out of existence and said they don’t know what they’re talking because [they were giving swordfish a hechsher when] swordfish is really treif. So the Orthodox realized you can’t rely on the Conservatives.
L’maaseh, the Conservatives were right on this issue, but Rabbi Tendler accomplished his goal.
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The Orthodox Union traditionally treats swordfish as non-kosher.
(See Sh’ailos and Teshuvos Tzitz Eliezer 9:40 who discusses a statement made by the Knesses HaGedolah about “cherev hadag” and explains why we cannot use the statement to permit swordfish.)
https://oukosher.org/blog/consumer-kosher/consumers-faqs-on-kosher-fish/
https://www.kashrut.com/articles/fish/