Kosher Fish from a Regular Fish Shop
Bittul in Commercial Production
OU Fish Expert ORIGINAL
Rabbi Chaim Goldberg
Kosher fish have scales. Checking for the presence of scales is fairly simple. However, only scales that can be removed without tearing the skin signal Kosher fish. The fish is Kosher even if only a few scales are found, on limited locations on the fish.
We can rely upon the vendor's statement that skinned fish is a Kosher species, only when we are absolutely certain that the vendor is trustworthy or fearful of prosecution.
When purchasing fish from a store that also sells non-Kosher fish you may have seen or heard of some who bring their own Kosher knife. The concern is that there may be traces of non-Kosher residue on the knife. However one can just as easily [and this is a better idea as fishmongers all have a special affection for their own knives] ask them to wash their knife and board. If you forgot to ask or fear they were not properly cleaned, just wash the fish at home [OU suggests you scrape the surface of the fish with a knife edge which is strange since that would cause any non-Kosher residue to penetrate the fish]
Red fleshed fish are Kosher and Halacha accepts this as a reliable identifying mark. Therefore, skinless salmon, is Kosher, without Kosher certification since it is easily identified. Trout is similarly Kosher without certification by its flesh colour.
Even those who insist on bringing their own knife and cutting board to the fish-monger may rely upon commercially produced salmon. Even if a non-Kosher fish does get past their quality control, the gigantic volumes of production ensure that any non-Kosher fish and its residue becomes Bottel [nullified] in the vast malority of Kosher.
The classic illustration of this principle is found in the Shulchan Aruch's (YD 96:4) ruling that lemonade and salted fish may be acquired from a non-Jew, despite the use of non-Kosher knives. The Rama explains this is due to the large amount of product cut with the same knife.