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 atreife 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 theShulchan 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 akashere 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 givinghashgachas, so he laughed them out of existence and said they don’t know what they’re talking because [they were giving swordfish ahechsher 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.