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Q: When it comes to fish, such as salmon, does that need to be purchased at a kosher supermarket or can salmon also be purchased at a general supermarket?
A: There are two issues when purchasing raw, fresh fish.
Firstly, one must be sure the fish is, in fact, a kosher species. As a rule one may not rely on the name of the fish to make that determination as (a) in some cases multiple fish are referred to by the same name and (b) there is considerable fraud in the fish industry with one fish being passed off as another. Therefore, the only reliable method of determining that a fish is from a kosher species is by inspecting its scales to be sure that they are the type that can be removed from the fish without ripping any flesh. If the fish has no scales or you are unsure how to determine if the scales are “kosher”, there is no way to know that the fish is from a kosher species. One notable exception is salmon, where the flesh-color is unique and is considered a clear identifying mark of the kosher, salmon fish.
Secondly, the knives used to scale, eviscerate, fillet and/or cut the fish may have been previously used for non-kosher fish. If that were true, it is possible that some residue of the non-kosher fish is still on the knife and will transfer to the kosher fish. To avoid this issue it would be best to purchase cut fish from a kosher fish store, or at least to ask the store employees to clean the knife and work on a clean piece of butcher-paper. If neither of those are possible, there is basis to permit the purchase of packaged, pre-cut fish with the assumption that the store employees used the knife to cut many slices from the same kosher fish, and the non-kosher residue is likely not on the piece you chose. The worst-case scenario would be if the store would use a dirty knife to cut just one piece of kosher fish for you; in that case, you would have to scrub clean any surfaces that had been cut.
Of course, the above only applies to raw fish; fish which is cooked, smoked or otherwise processed requires proper kosher certification.