Separation of Meat and Fish
Guidelines for consuming meat and fish at the same table, and the rules for cooking fish in a pot previously used for meat

Is it permissible to eat fish at the same table where my brother is currently eating meat?
It is true that when two people who know each other eat at the same table, one eating meat and the other dairy, the Sages were concerned they might come to eat them together and transgress the prohibition of consuming meat and dairy. This is ruled in the Shulchan Aruch (Yoreh De'ah 88:1), and in such a situation, they need to place a distinguishing item between them that is not normally present during meals.
However, in our case, where two people are eating at the same table - one meat and one fish - although one should not eat meat and fish together due to the danger involved as explained in the Shulchan Aruch (YD 116:2), nevertheless, when they know each other, there is no concern because people are more careful regarding matters of danger (as ruled in Yabia Omer Vol. 6, YD 9). Another reason why there is less concern with meat and fish compared to meat and dairy is that with meat and fish, one can eat them consecutively by washing hands, cleansing, and rinsing the mouth. Therefore, there's no concern they would eat them together without cleansing. This is unlike meat and dairy, which requires waiting six hours (Shem Chadash on Sefer HaYere'im 149).
Halachic conclusion: It is permissible for two people, even if they know each other, to eat at the same table - one fish and one meat - without placing any distinguishing item between them.My wife cooked meat and then cooked fish in the same pot. Are the fish permissible to eat?
We mentioned in the previous question the ruling of the Shulchan Aruch that one should not eat meat and fish together due to danger concerns. However, in our case, where the woman cooked meat and then, after washing the pot, cooked fish, we can say this doesn't fall under the category of danger because one is not eating meat and fish together. Rather, this is just a clean meat pot that now releases the taste of meat when cooking fish, and in such a case, there is no danger concern.
The source for this is from the Talmud in Tractate Chullin (111b), where it explains that fish cooked in a meat bowl that absorbed the taste of meat may be eaten with dairy. From here we learn that there is no prohibition due to danger concerns because absorption in a pot does not pose a danger. Furthermore, the Talmud brings several instances where Amoraim ate fish that were served in a meat bowl and were not concerned about danger.
The conclusion from all this is that there is no danger in the absorption of taste in vessels, and the only thing to be careful about is when actually eating the substance of meat and fish together. This is also written by the Taz (Yoreh De'ah 95:3) and ruled in Yabia Omer Vol. 4 (YD 4:1).
Halachic conclusion: It is permissible to cook or fry fish in a meat vessel even if meat was cooked in it that same day. The pot should be washed thoroughly (with cleaning materials), and there is no danger concern in this.
Rabbi Shai Amar is a rabbi in the Halacha department of Hidabroot
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