The Gemorah infers that one must see the kosher fish being swallowed otherwise it might be a Nidmeh (and really the offspring of the non-Kosher fish). The Gemorah then asks it still might be a Nidmeh and the kosher fish was digested. The Gemorah answers that it was found in a position inside the mother which negates it being a Nidmeh. The final answer the Gemorah gives is that one can rely on the improbability of it being a nidmeh.
Rashi comments on this answer that now it is not necessary to see the fish swallowed (as is implied by the Gemorah). Why do we need to see the fish swallowed according to the first answers?
Mordechai Shifman
it is not necessary to see the fish swallowed (as is implied by the Gemorah)
The Gemara implies that we only permit a kosher fish that is found in a non-kosher fish if we see it swallowed, from the working of the Mishnah. (First Rashi in the Mishnah.) This is why the first two answers of the Gemara state take this for granted. but Rav Ashi comes up with a new way to read the Mishnah (-it does not mean that we saw the fish swallowed, but that is as certain that this fish was swallowed as if we saw it happen -Rashi DH Rav Ashi). This allows him to take the more simplistic (and halachic) approach that we do not have to see the fish swallowed. (The words of SHITA MEKUBETZES #14 add much meaning to the Gemara, as we mentioned in the Background to the Daf, look at it.)
Yours, Mordecai