Commentary for Avodah Zarah 79:5

R. Papa said that the case of the barrel of fish which Rabban Shimon b. Gamaliel permitted was only when the pieces fit together such that we could see that they all came from one fish. But this is a bit too obvious to even need to be stated. The Talmud solves this minor problem by suggesting that we might have been concerned that this was just happenstance, that there was some unclean fish together with the clean fish. Therefore, according to R. Papa, we learn that we are not concerned about such a possibility. But if all of the pieces do not fit together, then we need to recognize the head and backbone of each and every fish.