Commentary for Avodah Zarah 79:19

The mishnah allows a Jew to buy a tarit fish which has not been minced from a non-Jew and eat it. The Talmud explains that the fish’s head and backbone must be recognizable. This is how we can be sure that a non-kosher fish did not get in there. The mishnah allowed “brine containing fish” as long as that fish was a specific fish, called the kalbit fish. This fish is a sign that the brine is kosher. In open barrels two of these fishes are necessary, but in closed barrels, one is sufficient.