The earliest and most famous source to explicitly declare the swordfish kasher was the renowned Sephardic posek, Rabbi Chaim ben Yisrael Benvenisti (1603-73), the Knesset haGedolah. His statement became central to all later halakhic discussion concerning the swordfish. He wrote:
It is a widespread custom among all Jews to eat the swordfish (dag ba’al ha-Cherev), known in vernacular as fishei ispada, even though it does not have scales because it is said that when it comes out of the water, due to its anger, it shakes and the scales are thrown off.
He further reports that his grandfather had confirmed this by experimentation. Thus, the Knesset haGedolah acknowledges that a landed swordfish appears to be scaleless; nonetheless, he rules that it is kasher.
For example, it is mentioned by the Peri Meggadim (Siftei Da‘at, 83:2); the ‘Ikre Dinim (a.k.a. Ikre Ha-tsa‘ir, Daniel Tirni, of Florence, Italy), YD 8:4; the Darke Teshuvah, YD, 83:17; the Hida (Haim Yosef David Azul’ai, of Hevron, Israel [1724-1806]), MaChzike Berakha, YD, 83:3; the Zivche Tzedek, YD, 83:8; the Kaf ha-Chaim, YD, 83:9; and Kemach Solet, p. 95, no. 24 (written by Rabbi Yehudah Eli, originally published in Solonika, 5558[1798], republished 1998).
He explains that it loses its scales during the fight and that this was proven to a skeptical vizier by the Knesset haGedolah's grandfather, Rabbi Moses Benvenisti, who placed a black garment in the fishing net and, on landing the fish, pointed out the many scales on the garment. [Me'am Lo'ez on Lev. 11:9-12; page 122 in the standard Hebrew edition.] Even though all of the authorities just cited clearly base their rulings on the Knesset haGedolah, nonetheless, they each may be counted as independent sources, and not just as students, because each authority independently supported the Knesset haGedolah’s ruling. More importantly, each confirmed the widespread custom of the swordfish being eaten at his time and in his place.
The confusion about the presence or absence of scales on adult swordfish owes to the thickening of the dermis above the scale as larval and juvenile swordfish grow. The result is that only the tips of the scale spines protrude in adults. Similarly, a spearfish such as the blue marlin, is also kasher and its scales are covered with a fine layer of skin which make them easily scraped off the body. The scales on tuna are so minute as to be nearly invisible, or are absent. Tuna loses its scales when caught/landed the same as the swordfish does. But nobody argues the kashrut of the tuna. Scales are often fractured and abraded when the fish are caught and processed by the fishery. More complex adult scales do not replace scales of larval swordfish. Scales may be replaced if lost, but no scales appeared on the surface of the epidermis where they might be sloughed off as when the fish is caught/landed. And the marlin has exactly these same characteristics as the swordfish and tuna and it is permitted as well.
Our Sages have already explicitly declared swordfish and tuna kosher in the same sentence and for the same reason. It is consensus, not opinion. Gemara will opine like 'rabbi so-and-so said'. But gemara will consense like 'Our Rabbis taught'. And this is precisely the statement made in Avodah Zarah 39a "Our Rabbis taught: Those species of fish, which have fins and scales at the time; but shed them when drawn out of the water, as for example, colias, scomber, swordfish, anthias, and tunny are permitted."
Help us stop the spread of misinformation on the status of swordfish. The rabbis of the Talmud concluded swordfish is kasher and we have a long mesorah of eating swordfish. Enough is enough. Stand up for the truth.