Question: May one lay a mouse trap on Shabbat?
Answer: In the previous Halachot we have discussed that one of the forbidden works on Shabbat is trapping; one who traps an animal on Shabbat is liable for the Torah prohibition of Shabbat desecration. We have written that if one traps animals which are usually trapped, such as, birds, fowl, fish, and the like, one is liable by Torah law. If the creature is not of a species that is usually trapped, such as flies and mosquitos, the prohibition to trap them on Shabbat is only rabbinic.
If Trapping a Mouse is a Torah Prohibition
Regarding mice, the Mishnah (Shabbat 107a) states that one transgresses the Torah prohibition of trapping on Shabbat when trapping any one of the eight swarming creatures mentioned in the Torah (which include: A weasel, mouse, great lizard, gecko, land-crocodile, lizard, sand-lizard, and chameleon), for these creatures have skins and hides and people would commonly trap these creatures in order to make user of their skins. Thus, we see that trapping a mouse on Shabbat is a Torah prohibition.
The Opinions of the Magen Avraham and Maran zt”l
The Magen Avraham (Chapter 316, Subsection 9) writes that just as one is forbidden to set traps to trap any other animal on Shabbat, one may likewise not lay a mouse trap on Shabbat. Other great Acharonim rule likewise.
Nevertheless, Maran Rabbeinu Ovadia Yosef zt”l writes in his Chazon Ovadia-Shabbat, Part 5 (page 119) that if one traps mice in one’s home, one certainly has no intent to use any part of the mouse’s body and one is therefore not liable according to Torah law, for this can be considered “a work the direct outcome of which is not necessary” which is only considered a rabbinically forbidden work. He proceeds to quote the Geonim who rule likewise. After discussing the words of various Poskim, Maran zt”l concludes that it seems that there is no prohibition to lay mouse traps on Shabbat since the mouse is not trapped immediately and only after some time in addition to the fact that trapping mice is not a Torah prohibition. On the other hand, in his Responsa Yabia Omer (Volume 3, Chapter 20) he quotes the Tosafot and Rabbeinu Chaim Ohr Zarua who rules stringently on this matter and forbid setting any trap for any kind of animal on Shabbat. This includes trapping harmful mice on Shabbat and these opinions maintain that one may not place traps to catch them on Shabbat based on the edict of our Sages.
Thus, halachically speaking, it seems that one should act stringently and not lay mouse traps on Shabbat. However, to lay them before the onset of Shabbat is not prohibited whatsoever.