How do you know if a fish is kosher?

The two signs

The Torah says: “These you may eat of all that live in water: anything with fins and scales” (Leviticus 11:9). The Mishnah and the Talmud explain that any fish with scales also has fins, so in practice one looks for scales. The scales must be removable by hand or with a knife – the kind on most bony fish. Fish with tiny embedded scales or plates, like sturgeon, are not accepted by most authorities.

In practice

A whole fish can be checked by anyone who knows what to look for. A fillet without skin cannot, so buy from a supervised store or one that leaves a patch of skin with scales. Smoked, canned and frozen fish need certification for the species and for the equipment. Fish does not require slaughter or salting, and its blood is permitted.

Common kosher fish

Salmon, trout, tuna, cod, haddock, hake, sole, flounder, halibut, carp, tilapia, herring, sardines and anchovies (with supervision). Not kosher: all shellfish and crustaceans, squid and octopus, eel, catfish, monkfish, shark and swordfish (debated, generally not).

Roe, sauces and sushi

Fish eggs follow the fish: red caviar from salmon is kosher, true caviar from sturgeon is not accepted by most authorities. Anchovy paste, fish sauce and surimi (“imitation crab”) need certification because they may contain non-kosher species. At a sushi bar the fish itself can be kosher while the knives, the rice vinegar or the “crab” are not, so only a supervised sushi counter is acceptable.