Q All my teachers tell me that swordfish is not kosher, but my grandfather insists that his family ate it when he was a kid. Can you explain this?

- S.F., Tel Aviv

A I first heard about the controversy over the kashrut of swordfish as a college student, when a local Orthodox rabbi told me that the Conservative movement might be correct that it should be kosher. I forgot about the remark, since I was never a big fish consumer (I only started eating tuna a few years ago). Yet a recently acquired taste for fish (aliya will do that to you), plus a fascinating article by Ari Zivotofsky of Bar-Ilan University (B.D.D. 19), from which this column will heavily draw, has resparked my interest.

While the Torah specifies that kosher fish require both scales and fins (Leviticus 11:9-10), an ancient tradition codified by Halacha asserts that all fish with scales necessarily have fins (Nida 51b, YD 83:3). As such, much halachic literature focused on defining halachic scales, a complex project since these coverings vary greatly in different fish. Among other criteria, kosher fish must contain scales attached to their body which can be peeled without damaging the fish's skin (Rama YD 83:1). Scales that shed when fish mature or leave the water, or alternatively, that develop only later in life, were also deemed acceptable.

Nonetheless, it remains difficult to identify which fish possess the biological traits that match these halachic criteria. For starters, there are myriads of fish which must be carefully examined by competent authorities.