QUESTION: Rav Huna says [Bava Basra 21b] that if a person who lives in a semi-public alley ("Mavoy") sets up a mill to grind flour, he has the right to prevent a neighbor from building a second mill. He can claim that the second neighbor will take away his customers (and thus his livelihood) if he sets up a second mill.
The Gemara attempts to prove Rav Huna's ruling from the law that if a fisherman has already found the hole in which a fish lives, he has the right to prevent other fishermen from spreading their nets, up to a radius of one Parsah from the hole, even though he has not yet caught the fish. This law seems to support Rav Huna's ruling that a person has the right to prevent others from taking away his potential livelihood.
TOSFOS (DH Marchikin) asks that the right of a fisherman seems to contradict the principle taught by RABEINU TAM concerning the Halachah of "Ani ha'Mehapech b'Chararah" (see Kidushin 59a). The Halachah of "Ani ha'Mehapech b'Chararah" states that if one person is trying to buy a certain object or obtain a certain job and a second person preempts him, the preemptor is considered a "Rasha." Rabeinu Tam maintains that if the second person preempts the first in picking up an object of Hefker, he has done nothing wrong and he is not considered a Rasha; since he would not have been able to acquire the object elsewhere, he is entitled to take it first.
According to Rabeinu Tam, why are the other fishermen not allowed to spread their nets? Even if one fisherman clearly began to attempt to trap a certain fish, the other fishermen should be permitted to trap it because the fish is Hefker, and they will not be able to find such a fish elsewhere.
TOSFOS in Kidushin (59a, DH Ani) answers in the name of RABEINU MEIR (the father of Rabeinu Tam) that in the Gemara's case of the fishermen, the first fisherman lowered into the water a dead fish which attracted all of the other fish. Since the first fisherman has performed an action to attract the other fish, his colleagues are not allowed to take those fish even though they are Hefker.
The KETZOS HA'CHOSHEN (273:4) challenges this answer from another Gemara in Kidushin. The Gemara in Kidushin (25b, according to the explanation of Tosfos there) explains how it is possible to acquire an elephant with a Kinyan of "Hagbahah" (lifting it up): the purchaser lifts tree branches above the elephant's head so that the elephant jumps up to eat them. Since the elephant raises itself as a result of the purchaser's action, it is considered as though the purchaser has done "Hagbahah" and lifted the animal, and thus he fully acquires the animal.
The Ketzos ha'Choshen reasons that according to this explanation, a fisherman who uses a dead fish to attract other fish makes a Kinyan on the fish. Just as the Kinyan of "Hagbahah" is accomplished when the purchaser causes the elephant to lift itself, a Kinyan of "Meshichah" should also occur when the fisherman places a dead fish in the water that causes other fish to move toward it. The fisherman should be considered to own the fish as a result. Why, then, does Tosfos say merely that if other fishermen take the fish, they are infringing on the first fisherman's livelihood? Since the first fisherman has already acquired the fish, the other fishermen who take them are committing an act of outright theft!
ANSWER: The NESIVOS HA'MISHPAT (273:3) answers that there is a distinction between a direct action and an indirect action on the part of the purchaser. In the case of the elephant, the purchaser can acquire the elephant only if he actually holds the tree branches at the time the elephant jumps up to eat them. If he has already let go of the branches, he only indirectly causes the animal to jump up, and he is not considered to be performing a Kinyan of "Hagbahah."
The Gemara in Kidushin (22b) says that if a person calls to an animal and thus causes the animal to come to him, he is considered as though he pulled the animal with his hands and it is a valid Kinyan of "Meshichah." That is because the owner did something which directly caused the animal to move toward him. In contrast, when the first fisherman places a dead fish in the water and then goes away, he does not directly cause the fish to come, and therefore he does not fully acquire the fish that come. If someone else then takes the fish, it is not considered an act of theft because the first fisherman did not own the fish in the first place. Rather, the person who takes away the fish is considered a "Rasha," for he encroaches on the first person's livelihood when he preempts him and captures the fish.