1) HALACHAH: FISH EGGS
OPINIONS: The Beraisa states that a sign that an egg comes from a Kosher bird is that one end is round and the other end is pointed. If both ends are round or both are pointed, then that is a sign that the egg comes from a non-Kosher bird and is prohibited. The Beraisa earlier (63b) teaches that the same way in which eggs are verified as Kosher, fish eggs are verified as Kosher.
The Halachah regarding fish eggs is recorded by the SHULCHAN ARUCH (YD 83:8), who writes that if both ends are the same, the egg certainly is not Kosher. If one end is pointed and one is round, then the buyer must ask the Jewish seller about the origin of the eggs. If he says that he personally took them from a Kosher fish, he is believed. If he simply says that they are Kosher, he is not believed unless he is known to be very trustworthy in areas of Kashrus.
The Shulchan Aruch adds that the common practice, however, is different. In practice, a person is allowed to buy red fish eggs, even from Nochrim, in any form, while black fish eggs may not be eaten at all.
What is the basis for this practice, and is it the commonly-followed practice today?
(a) The source for the custom is the words of the BEIS YOSEF himself. The Beis Yosef explains that it appears that the earlier generations must have maintained that the only reason why the seller must attest that he took the fish eggs out of the fish himself is to permit even certain black fish eggs. In the earlier generations, they must have known that there simply were no types of non-Kosher, red fish eggs. In order to determine that one did not make a mistake among the variations of black eggs, they prohibited consumption of all black fish eggs. The Beis Yosef comments that even though he heard that there indeed is a non-Kosher fish that lays red fish eggs, upon being salted they turn black. Therefore, the rule is that no red fish egg in the world can be non-Kosher if it remains red after being salted. The SHULCHAN GAVO'AH (YD 83:23) upholds the ruling of the Beis Yosef.
(b) The SHACH (YD 83:27) quotes the ISUR V'HETER who says that "we may no longer rely on the signs for fish eggs, since we are no longer experts." The Shach explains that it is clear that the Isur v'Heter is ruling not to rely on the Shulchan Aruch's practice regarding external signs. While the Shulchan Aruch writes that everything depends on color, the Isur v'Heter is saying that we follow the original Halachah, which depends on the believability of the seller. The PRI MEGADIM in SIFSEI DA'AS concurs with the Shach's understanding of the Isur v'Heter, but he says that it is possible that we follow the Shulchan Aruch's stringent practice never to eat black fish eggs.
The DARCHEI TESHUVAH (83:80) quotes many Acharonim, including the PRI CHADASH, YA'AVETZ, and the CHOCHMAS ADAM, who rule that one should not rely on the custom of the Shulchan Aruch, but one should buy fish eggs based only on the believability of the Jewish seller. The ARUCH HA'SHULCHAN (YD 83:50) mentions that many Acharonim objected strongly to the lenient practice of the Shulchan Aruch, and he notes that this is a strange leniency. He explains that had this custom been clearly transmitted to us from the Ge'onim, then we could have relied on such a leniency. However, it is impossible to rely on a lenient custom that developed with no clear Halachic source, especially a custom that involves a Torah prohibition. He concludes that, therefore, one may not eat fish eggs that have come from afar, whether they are red or black. (Y. MONTROSE)
2) THE SOURCE FOR THE PROHIBITION AGAINST EATING AN EGG OF A NON-KOSHER BIRD
QUESTION: Chizkiyah teaches that the source that eggs of a non-Kosher bird are prohibited is the verse, "v'Es Bas ha'Ya'anah" (Vayikra 11:16). By referring to the bird as the "Bas ha'Ya'anah," the Torah alludes to its egg and teaches that its egg is also prohibited.
Why is a special source needed to teach that the egg of a non-Kosher bird is prohibited? There is a general principle that "Kol ha'Yotzei Min ha'Tamei Tamei" -- anything that comes from a non-Kosher animal is also not Kosher, such as milk that comes from a non-Kosher animal (Bechoros 7b). This principle should prohibit the egg of a non-Kosher animal.
(a) TOSFOS (64a, DH she'Im Rikmah v'Achlah) suggests the following answer. Even though eggs that come from a Kosher bird should be prohibited as Ever Min ha'Chai since they emerged from a living animal, the Torah nevertheless permits them. When the Torah teaches the Mitzvah of Shilu'ach ha'Ken, it shows that eggs are not prohibited as Ever Min ha'Chai. One might have thought that just as the Torah excludes eggs from the prohibition of Ever Min ha'Chai, it also excludes them from all other prohibitions, such as "Kol ha'Yotzei Min ha'Tamei Tamei," and thus even the eggs of non-Kosher birds should be permissible. For this reason a new source is needed to teach that the eggs of non-Kosher birds are prohibited.
(b) The RAMBAN and RE'AH answer that even though the eggs of a non-Kosher bird indeed are prohibited because of the general principle of "Kol ha'Yotzei Min ha'Tamei Tamei," a new source is necessary to teach that one who eats such an egg is punished with Malkus. (Z. Wainstein)