Beitzah, Chapter 3, Mishnah 2. The Mishnah continues to deal with the laws of trapping, only that now its subject is animals that have already been caught.
The law of the traps:
If a person set traps for animals, birds or fish before Yom Tov, and on Yom Tov he found creatures caught in them - "ein nitalin mehen b'Yom Tov" - they may not be taken from them on Yom Tov, unless he knows clearly that they were caught while it was still day, before the festival began.
The reason is this: a creature that was caught on Yom Tov itself is muktzeh, since at the onset of Yom Tov it was not prepared and available. Anything that requires trapping in order to obtain it is by definition muktzeh, because the trapping itself is forbidden on Yom Tov, as we learned in the first Mishnah.
A doubtful case of "prepared":
The prohibition is not limited to a case where it is known for certain that the creature was caught on Yom Tov. Even when the matter is in doubt - and it is not known whether it was in the trap before Yom Tov or not - this is a case of safek muchan, a doubt as to whether it was prepared, and it too is forbidden.
The Gemara teaches that a line is missing from the Mishnah, and the correct text should read: "v'Rabban Gamliel matir" - and Rabban Gamliel permits. In Rabban Gamliel's opinion, in a case of safek muchan - where it is possible that the creature was caught before Yom Tov - it is permitted to benefit from it.
The incident of the non-Jew who brought fish to Rabban Gamliel:
The Mishnah relates an incident involving a non-Jew who brought fish to Rabban Gamliel on Yom Tov, and it was not clear whether they had been caught before the festival or on Yom Tov itself. If they were caught on Yom Tov, they were not prepared and available at the onset of the festival and they are muktzeh; and if they were caught beforehand, they are not muktzeh.
Rabban Gamliel said: "mutarin hem, ela she'ein retzoni l'kabel heimenu" - they are permitted, but it is not my wish to accept from him. By strict law they are permitted, for according to his view, unlike the first Tanna, a case of safek muchan is permitted; and nevertheless he does not wish to accept the gift from that person. Why?
Rashi: That particular non-Jew was not agreeable to him, and therefore he did not want to accept a gift from him.
Other commentators: Rabban Gamliel was stringent with himself in this matter, even though according to his own view the halachah did not require it of him.
In summary: In this Mishnah we learned that creatures found in traps on Yom Tov may not be moved or used because of muktzeh, unless it is known that they were caught while it was still day; and that even a case of safek muchan is forbidden according to the first Tanna, while Rabban Gamliel permits it. We also examined the incident of the non-Jew who brought fish to Rabban Gamliel, and the two approaches to explaining his refusal to accept them.