Gemara

(Rav Berona): One may buy Kirvei Dagim (fish innards) and Ubran (fish eggs) only from a Mumcheh (expert).

Question (Ula): This implies that we are concerned lest the eggs are from a Tamei fish. Tamei fish give birth to fish. They do not lay eggs!

Answer #1 (R. Dosta'i d'Min Biri): We must delete 'fish eggs' from Rav Berona's teaching.

Answer #2 (R. Zeira): The teaching is correct. Tamei fish also lay eggs, which hatch inside the mother. Tahor eggs hatch outside the mother.

Question: Why do we need a Mumcheh? A Beraisa teaches that Simanim of Tahor fish are like those of Tahor eggs.

Objection: The Simanim of Tahor fish are fins and scales!

Correction: Rather, the Simanim of Tahor fish eggs are like those of Tahor bird eggs. One end of a Tahor egg is spherical, and the other is pointy. Both ends of a Tamei egg are spherical, or both are pointy.

The yolk of a Tamei egg is outside, and the white is inside. The yolk of a Tahor egg is inside, and the white is outside.

Answer (Rava): The case is, the eggs are dissolved.

Question (against R. Dosta'i): The Beraisa discusses Simanim of Tahor fish eggs. (This shows that also Tamei fish have eggs!)

Answer: R. Zeira corrected the Beraisa to say so. R. Dosta'i can correct it to say 'Simanim of Tahor fish innards are like those of Tahor eggs... '

Question: If there is no Mumcheh, what should one do?

Answer #1 (Rav Yehudah): If the seller says 'I salted them myself', one may rely on him (he would be afraid to lie). He ruled like this in practice.

Answer #2 (Rav Nachman): The seller must say 'these are the fish, and these are the innards.'

Chulin 63b (Beraisa): We may buy (bird) eggs from Nochrim.

(Shmuel's father): The case is, the Nochri said they are from a particular Tahor species.

Question: Why do we rely on him? We should inspect the Simanim!

Answer #1: The yolk and white were beaten together in a bowl.

Objection: We may not buy such eggs from a Nochri!

Answer #2 (R. Zeira): The Simanim to distinguish Tamei eggs from Tahor are not mid'Oraisa, so we do not rely on them.

Support: If the Simanim were mid'Oraisa, we would be able to resolve the eight doubtful species (62b) by inspecting their eggs!

If eggs have Simanei Taharah, we may trust even a Nochri if he says 'they are from Tahor species Ploni. We may not rely on Simanim alone, for raven eggs resemble dove eggs (perhaps other Tamei eggs resemble Tahor eggs).

Rishonim

Rif: Fish innards and Me'ayim (intestines) may be bought only from a Mumcheh. If there is no Mumcheh, the seller must say 'I salted them.' Simanim of fish innards are like those of eggs. If one end is spherical, and the other is pointy, it is Tahor. If both ends are spherical, or both are pointy, it is Tamei.

Rosh (2:43): R. Tam observed that here we did not say that we cannot rely on Simanim of eggs because they are not mid'Oraisa, like we said in Chulin. This shows that Simanim of fish eggs are mid'Oraisa. One may rely on this to permit the Burbuta fish. We do not find scales on it; perhaps they fell off. However, perhaps we did not answer that Simanim of fish eggs are not mid'Oraisa, for it would not explain why one must buy from a Mumcheh. For bird eggs, it suffices for the seller to say 'they are from Tahor species Ploni.' We answered that we discuss dissolved fish eggs. It suffices if the seller salted it himself, but this is rare, therefore, we say that a Mumcheh is required. If Simanim of fish eggs are mid'Oraisa, why do we equate them to Simanim of eggs, which are only mid'Rabanan? (We should say vice-versa!) Perhaps this is because eggs are more common and are better known. The Yerushalmi connotes that Simanim of fish eggs are not mid'Oraisa. It mentions a Tahor fish whose eggs appear to be Tamei.

Ran (DH Omar): Rashi says that a Mumcheh is one who is skilled to discern. Others say that it is someone trustworthy. The Yerushalmi supports this. Amei ha'Aretz were suspected to buy fish innards from Nochrim b'Chezkas that they are Tehorim, but really they are Teme'im. The Sugya does not discuss Me'ayim, only innards and eggs. The Rif relies on R. Dosta'i, who deletes 'eggs' from Rav Berona's teaching. This is why the Rif discusses innards and intestines. He permits buying fish eggs from anyone, for Tamei fish do not have eggs. This is astounding. We should rely on R. Zeira, who says that both have eggs, and we need not change the text of Rav Berona! Also, we should be stringent to follow R. Zeira and require Simanim for fish eggs. The Rambam holds like this. The Rif said Stam that fish innards may be bought only from a Mumcheh, even if they were not dissolved. He learns from R. Zeira, who says that Simanei Beitzim are not mid'Oraisa. If so, one must buy from a Mumcheh even if they were not dissolved, unlike Ravina. Even though Ravina is Basra, the Rif relies on the Sugya in Chulin, which is in its place (Chulin discusses Simanei Kashrus). According to both R. Tam and the Rif, if a whole fish egg is shaped like a Tahor egg, one may rely on a Nochri who says that it is of Tahor species Ploni, like we rely on Nochrim for bird eggs. Some say that this is only for bird eggs, which are recognized for each species. We rely on a Nochri even when there is a Tamei specific that resembles a Tahor species. However, the Rambam says that we rely only on a Yisrael. He explains that the Beraisa discusses a Yisrael seller. However, the Sugya there proves that we discuss a Nochri seller. It says that if the egg is scrambled, we may not buy it from Nochrim!

Rambam (Hilchos Ma'achalos Asuros 3:20): The Simanim of fish eggs are like the Simanim of bird eggs. If both ends of an egg are spherical, or both are pointy, it is surely Tamei. If one end is spherical, and the other is pointy, one asks the Yisrael seller. If he says 'I salted it, and it is Tahor', we rely on him. If he says only that it is Tahor, he is believed only if he is established b'Kashrus.

Poskim

Shulchan Aruch (YD 83:8): If both ends of a fish egg are Kadim (Rema - spherical and thick like jugs), or (both are) pointy, it is Tamei.

Rema: The same applies to innards.

Shulchan Aruch (ibid): If one end is spherical, and the other is pointy, one asks the Yisrael who sells them. If he says 'I salted them, and I took them from a Tahor fish', one may eat based on his words. If he said 'they are Tehorim', he is not believed, unless he was established to be Kosher.

Beis Yosef (DH v'Chasav): The Rashba said that Ravina knew R. Zeira's question from the Safek species.Why wasn't he concerned for it? If you will say that we rely on Ravina because he is Basra, if fish eggs and bird eggs are exactly the same, why didn't the Gemara in Avodah Zarah ask from the Safek species? Rather, only Simanim of fish eggs are mid'Oraisa. It seems that Rashi and R. Chananel agree. The Magid Mishneh says that this does not suffice to be lenient about a mid'Oraisa law; we are more stringent about fish eggs to require the seller to say that he salted them because they normally come from afar, and the seller can evade responsibility (if it will be found that they are Teme'im).

Shach (26): Even one established to be Kosher is not believed to say that an egg is Tahor if it has Simanei Teme'im, for he contradicts the Torah. Maharshal and all Poskim say so. The Levush says otherwise; Chas v'Shalom to be lenient about this.

Note: The Rosh cited a Yerushalmi that discussed a Tahor fish whose eggs appear Tamei! The Shach must say that the Bavli disagrees.

Shulchan Aruch (ibid): Nowadays the custom is to buy fish eggs Stam, whether whole or dissolved, even from a Nochri, as long as they are red. We do not eat black eggs at all.

Beis Yosef (DH v'Achshav): Early researchers found that no red eggs are from Tamei species. We require a Mumcheh, or for the seller to say 'I salted them' only for black eggs, which are also from Tamei species. To avoid all Safek, they forbade all black eggs. I heard that there is a Tamei fish with red eggs, but they blacken when they are salted.

Hagahos ha'SMA: Based on this, one may buy unsalted red eggs only if they are shaped like Tehorim and the seller says 'I salted them.'

Machatzis ha'Shekel (DH b'Gavna, citing Pri Chodosh 26): Had the Beis Yosef brought a tradition for this, perhaps we would accept it. However, he invented this idea to explain the custom. We cannot rely on him without a proof to permit all red eggs. Why is this Chidush not in the Bavli, Yerushalmi or any Rishon?! We do not bring a proof from an errant custom. Rather, one may eat only what letter of the law permits. May Hash-m pardon the Beis Yosef for forbidding the Mutar and permitting the Asur!

Aruch ha'Shulchan (50): Surely one may not be lenient about red eggs like the Beis Yosef.

Igros Moshe (YD 2:14 DH Hinei): We do not rely on Simanim not mentioned in the Gemara or Rishonim.

Yabi'a Omer (6 YD 6): Most Acharonim accepted the Beis Yosef. If we know that black eggs are from a Tahor fish, there is no problem of Mar'is Ayin