פירוש על עבודה זרה 77:5

The Talmud tries to understand why Abaye is so sure that unclean small fish do not live in the river Bab-Nahara. The answer is that the conditions simply are not conducive for their growth. The rapid water and its saltiness are not a problem, but the mud is. Ravina revokes Abaye’s leniency—since two other rivers now join the Bab-Nahara, we cannot assume that the fish is kosher.