פירוש על עבודה זרה 79:2

R. Huna and R. Nahman argue over whether both the head and backbone need to be recognizable or whether one is sufficient. But R. Ukba argues that the kashrut of fish is determined by their having fins and scales, not by their heads or backbones. Abaye answers that while, of course, kosher fish need to have fins and scales, the fish can usually be told apart by their heads. If their heads are the same as unclean fish, then one will need to examine the fins and scales. But if we know the head belongs to a kosher fish, then the fish can be eaten.