There are twenty-four existing sturgeon species worldwide. Five of them live in the Caspian Sea, and only three supply caviar. Caviar is the unfertilized eggs or roe of sturgeon preserved with salt. Most caviar is produced in Russia and Iran from fish taken from the Caspian Sea, the Black Sea, and the Sea of Azov. Caviar is graded according to the size of the eggs and the manner of processing. The grades are named for the types of sturgeon from which the eggs are taken. The species of sturgeon that produce caviar are, in order of size, beluga, osetra, and sevruga. These three provide 90 percent of the world's production of the most prized varieties of caviar.
Fresh-grained caviar is prepared from the full roe of the female sturgeon. The sturgeons are caught in nets and taken back to the fishery laboratory alive. There they are clubbed and anaesthetized, not killed, and the egg pockets are emptied. The fishermen carefully anaesthetize the fish by hitting them at a specific spot below the head. The roe must be taken while the fish are still alive. If the fish experience the stress of death, they release a chemical into the eggs that spoils the caviar by making it bitter.