Avodah Zarah 39A

התם ידיע ממשו, הכא לא ידיע ממשן.

The Talmud explains: There, the substance of the wine is a recognizable component of the fish stew; here, its substance is not a recognizable component of the pickled vegetables.

וטרית טרופה, וציר שאין בה דגה וכו׳. מאי חילק? אמר רב נחמן בר אבא אמר רב: זו סולתנית, ומפני מה אסורה? מפני שערבונה עולה עמה.

§ The Mishnah teaches: And minced tarit fish, and brine that does not have a kilbit fish floating in it, and ḥilak are all prohibited. The Talmud asks: What is ḥilak? Rav Naḥman bar Abba says that Rav says: This is sultanit, a type of small fish that is generally caught before its scales have developed. And for what reason is it prohibited? It is because its size causes it to be intermingled with other fish, and as a result sultanit rises out of the water with non-kosher fish when it is caught.

תנו רבנן: אין לו עכשיו ועתיד לגדל לאחר זמן, כגון הסולתנית (והעפיץ) [והעפיין] ⁠— הרי זה מותר. יש לו עכשיו ועתיד להשיר בשעה שעולה מן הים, כגון אקונס ואפונס, כטספטייס ואכספטייס ואטונס ⁠— מותר.

A baraita states: If a fish does not currently possess scales but will grow them after a period of time, such as the sultanit and afiyatz fish, it is permitted. Likewise, if it has scales now but will shed them when it is caught and rises from the sea, such as akunas and afuna, ketasfatiyas and akhsaftiyas and otanas fish, it is permitted.

אכריז רבי אבהו בקיסרי: קירבי דגים ועוברן ניקחין מכל אדם. חזקתן אינן באים אלא מפלוסא ואספמיא, כי הא דאמר אביי: האי צחנתא דבב נהרא שריא.

R' Abbahu announced in Caesarea: Fish entrails and their eggs may be purchased from any person, as the presumption with regard to them is that they come only from Pelusium [Pilusa] and Spain [Aspamya], and non-kosher fish are not found in those areas. This is similar to that which Abaye says: These small fish [tzaḥanta] of the Bav River are permitted, as non-kosher fish are not found in that river.