Avodah Zarah 40A

שוטטות בו. השתא כילבית אחת אמרת שרי, שתי כילביות מבעיא? לא קשיא: כאן בפתוחות, כאן בסתומות.

floating in it. The Talmud asks: Now that you have said that brine which has only one kilbit in it is permitted, is it necessary to teach that brine that has two kilbiyot in it is also permitted? The Talmud explains that it is not difficult: Here, two kilbiyot are required to permit the brine, as it is contained in open jugs, but there, only one is required, since it is in sealed jugs.

איתמר: רב הונא אמר: עד (שתהא) [שיהא] ראש ושדרה ניכר, רב נחמן אמר: או ראש או שדרה. מתיב רב עוקבא בר חמא: ובדגים כל שיש לו סנפיר וקשקשת! אמר אביי: כי תניא ההיא בארא ופלמודא, דדמו רישייהו לטמאים.

§ It was stated that Rav Huna says: Tarit is not permitted for consumption until its head and spine are recognizable. Rav Naḥman says: It is permitted if either the head or the spine is recognizable. Rav Ukva bar Ḥama raises an objection from a Mishnah (Ḥullin 59a): And with regard to fish, any that has fins and scales is permitted. Evidently, the kashrut of fish is determined only by its fins and scales, not its head or its spine. Abaye said: When that Mishnah is taught, it is in reference to the ara’a and falmuda fish, whose heads are similar to the heads of non-kosher fish.

אמר רב יהודה משמיה דעולא: מחלוקת לטבל בצירן, אבל בגופן ⁠— דברי הכל אסור עד שיהא ראש ושדרה ניכר. אמר רבי זירא: מריש הוה מטבילנא בצירן, כיון דשמענא להא דאמר רב יהודה משמיה דעולא: מחלוקת לטבל בצירן, אבל בגופן ⁠— דברי הכל אסור עד שיהא ראש ושדרה ניכר, בצירן נמי לא מטבילנא.

The Talmud analyzes the dispute between Rav Huna and Rav Naḥman. Rav Yehuda said in the name of Ulla: The dispute applies to dipping foods in their brine, but with regard to eating their actual bodies, everyone agrees that it is prohibited until both the head and the spine are recognizable. The Talmud notes that R' Zeira said: Originally, I would dip foods in their brine, but once I heard that which Rav Yehuda said in the name of Ulla: The dispute is only with regard to dipping foods in their brine, but with regard to eating their actual bodies everyone agrees that it is prohibited until both the head and the spine are recognizable; I too no longer dip foods in their brine.