2,306. Buying Kosher Fish From a Non-Kosher Vendor

Maachalos Assuros 3:23

If a non-Jew brings a tub of open brine barrels and there’s a kosher fish in one of them, all of them are permitted. If they’re closed and someone opens one, finding a kosher fish, and then he opens a second, again finding a kosher fish, then all of them are permitted so long as the head and spine of the fish are there so that the species may be identified as kosher. This is why we don’t purchase crushed, salted fish from non-Jews, though if the head and spine of a fish is identifiable, it may be purchased from a non-Jew even crushed.

Maachalos Assuros 3:24

If a non-Jew brings a cask of uniformly cut fish pieces and it’s clear that they came from one fish, then they’re all permitted if one can find scales on one of the pieces.