5th of Cheshvan, 5771
Can I buy fillet from a local fish store, which is not under rabbinical supervision? To answer this question, there are two issues we should bear in mind.
1. Recognizing the fish/fillet as a Kosher (tahor) type.
2. The knife.
1. Recognizing a kosher specie is a Mitzva Min haTora.
However, modern rabbis discussed if and when is possible to recognize a fish fillet.
I quote here two opinions.
1) According to the standard Ashkenazi custom (as written in the CRC web, under rabbi Gedalia Shwartz, Shlita) fish fillet can only be identifiable if it still has the skin. When the fillet does not have any skin left, you cannot identify the fish, therefore, you cannot buy it. This is for fresh or frozen fillet.
2)According to Sephardic custom (as taught by rabbi Eliyahu Ben Hayim , Shelita) if a person can recognize the fish fillet as a kosher specie, if he/she would be able to recognize this fish among ten other fillet, then is permitted to purchase that fillet, even though the fillet does not have any more skin. This is for fresh or for frozen fillet.
2. The knife.
1) Some authorities require to take your own knife to the fish store and to make sure they use that specific knife to cut the fish, in order to avoid any 'contamination' (noten ta'am) between Kosher and non Kosher fish by means of the knife.
2) Rabbi Ben Hayim allows using the sellers knife, provided:
a) The seller washes the knife with water from any eventual solid vestiges or traces (isur ba'ayn). The knife does not absorb (so there is no particles' 'contamination' ta'am) because the fish is cold.
b) at home, the fish must be washed with water to make sure there is no solid vestiges or traces of a non Kosher fish.
When buying frozen fillet from Costco or other reliable stores we can assume that the knives were clean.
Rabbi Yosef Bitton. YMJC 130 Steamboat Rd. Great Neck NY 11024