Beitzah, Chapter 3, Mishnah 1. This mishnah deals with the laws of trapping - catching living creatures on Yom Tov, and with the distinction between trapping that is forbidden and trapping that is permitted.
"Ein tzadin dagim min ha'beivarin b'Yom Tov":
One may not catch fish from their fish ponds on Yom Tov. Several approaches are given for the reason behind this prohibition:
Rashi: The reason is that this could have been done before Yom Tov.
Some understand that trapping resembles harvesting: a person is harvesting the fish from the place they come from and where they grew, and that is forbidden on Yom Tov.
The Rosh holds that the reason the Sages forbade harvesting and grinding is that these are labors normally done in large quantity, and not only for the needs of Yom Tov itself. In the same way, this was how people would customarily catch fish.
Furthermore, the mishnah says: "V'ein notnin lifneihem mezonos" - one may not place food before the fish. As to the reason for this:
Tosafos: Because a person may come to catch them.
An additional reason: It involves unnecessary exertion, since the fish are able to obtain their food on their own within the pond.
"Aval tzadin chayah v'of min ha'beivarin":
One may catch wild animals and birds that are found within enclosures - in a closed or fenced-in area. Since they are set aside in that area to be eaten, they are considered prepared, and it is easier for a person to catch them. And consequently "v'notnin lifneihem mezonos": since it is permitted to take them, it is also permitted to place food before them - and especially according to the approach of Tosafos, that the concern was that one might come to trap them, and here the trapping itself is permitted.
The words of Rabban Shimon ben Gamliel:
Rabban Shimon ben Gamliel qualifies this and says: "Lo kol ha'beivarin shavin" - not all fenced-in areas are the same. The commentators understand that he does not come to disagree, but rather to qualify and explain the words of the Tanna Kamma. And this is the rule he established:
"Kol ha'mechusar tzeidah": Even though the animal is inside a fenced-in area, an act of trapping is still required in order to obtain it - such as when a person says, "Let us go get a trap and catch it." In such a case it is forbidden, and one may not catch it on Yom Tov.
"V'she'eino mechusar tzeidah": When it can be obtained easily, in a single grab and at one time, and there is no need to devise schemes but it is enough to jump and grab it - this is permitted, and one may catch it on Yom Tov.
In summary: In this mishnah we learned that one may not catch fish from the ponds and may not place food before them, whereas animals and birds in enclosures may be caught and one may place food before them. Rabban Shimon ben Gamliel qualifies that everything depends on the degree of effort required: whatever still requires an act of trapping is forbidden, and whatever does not require an act of trapping is permitted.