1. If an animal or bird enters one's domain on Shabbos, he is not required to help it escape.
2. Whenever the Mishnah says "exempt" with regard to the laws of Shabbos, it means that the person is exempt from punishment or from the obligation to bring a Korban, but the act is nevertheless prohibited, with the exception of three cases where it means that the act is permitted.
3. One transgresses Shabbos when he wounds a Sheretz only when he wounds one of the eight Sheratzim listed in the Torah.
4. One is not liable for restraining an animal under his ownership or care, since it is already considered trapped.
5. It is forbidden to kill any living being on Shabbos.
A BIT MORE
1. He does not transgress trapping since he did nothing to trap the animal.
2. One of these three cases is the Mishnah which says, "If one traps a snake so that it not bite him, he is exempt." This means that he is permitted to trap the snake.
3. This is because the other Sheratzim and Remasim are not considered to have skin.
4. However, he is liable for wounding that animal.
5. Therefore, it is forbidden to take a fish out of the ocean on Shabbos and keep it out to the point where it is clearly going to die.