Have you heard of cat-paw potage—the costly delicacy made by trapping servals, caracals, cheetahs, leopards, cougars, lynxes, bobcats, jaguars, jaguarundis, and ocelots, slicing off their paws (sometimes when they’re still alive), and leaving or landfilling everything that remains?No, because it doesn’t exist. Nor could it. Civilized people wouldn’t stand for it. Yet most fishing nations, including the United States, tolerate or promote the exact analogue at sea, in which sharks are separated from their fins and, because shark meat is difficult to preserve, usually discarded. Killing sharks takes some doing, so they’re often tossed overboard alive. They tend to wriggle along the bottom until they starve or, if they’re lucky, drown or bleed to death. Finless sharks have been caught on cut bait, the only thing they can chase down. Severed fins are dried and used in shark-fin soup, a traditional Asian dish served to signal wealth and celebrate lofty occasions. You can buy a...