One of the sages, Abaye, permitted the consumption of small fish taken from the river Bav in Babylon without required they be examined to see if they are kosher or unkosher, for only pure fish grow in that river. The scholars asked: Why do only pure fish grow there? Answer: Since the river Bav is a river whose waters flow fast, impure fish cannot live there, for they have no spines. The scholars asked: Yet in other rivers whose waters flow fast we do find impure fish living. They were given a different answer: Since the waters of the river Bav are salty, impure fish with no scales cannot live there. The scholars asked: Are there not many salty rivers in which impure fish without scales live? They were given a different answer: The riverbed of the Bav is inhospitable to the spawning of impure fish from its mud, and is hospitable only to pure fish. Another sage, Ravina, added that in his era the authorities moved rivers and merged on with the next. Since other rivers, in which impure fish live, now flow into the Bav, one must forbid the fish of the river Bav unless it is clear that the fish is pure following an examination of its scales.
(Babylonian Talmud, Tractate Sukkah 18a)