National Geographic : 2016 Aug
122 across that might be good to eat. So they glide through the water with their long, winglike pectoral fins, and when they come across a po- tential food source—sailors flailing around a shipwreck, a dead whale, a school of tuna—they lock in to check it out. If you’re the only food around, the oceanic whitetip is going to be a very dangerous shark. Otherwise, it’s apt to be mostly unnerving. One of the most interesting anecdotes about the behavior of oceanic whitetips has nothing to do with shipwrecks or divers, though. In the 1950s, fishery researchers in the Gulf of Mexico were surprised when they opened up the stom- achs of whitetips and found five- and 10-pound tuna in them, because the sharks aren’t fast enough to chase down small tuna. Then one day they saw a large group of whitetips swimming through a school of tuna, at the surface, with their mouths open. “No attempt was made by the sharks to chase after or snap at the hun- dreds of tuna,” the researchers reported. “The whitetips were merely waiting and ready for those moments when tunas would accidentally swim or leap right into their mouths.” Of course, it’s doubtful anyone would be able to observe behavior like that now, and the great irony is that the researchers who recorded the spectacle were helping pave the way for its end. “They were out there to see what kinds of commercial fisheries could be developed in U.S. waters,” says Julia Baum, a marine ecologist who compared the data from the 1950s with more recent longline catch data to gauge the change in oceanic whitetip populations in the Gulf. “They were setting out these longlines for tuna, and the sharks were just everywhere,” eating the tuna on the hooks and getting hooked on the lines themselves. “They didn’t know if they’d be able to develop commercial tuna fisheries because the sharks were so numerous.” The fishermen came up with two solutions: shoot the sharks before they ate the hooked tuna, and set separate lines to catch the sharks, the fins of which, they realized, were worth money. Perhaps enough money to justify catching them. And together, these two forces— a callous disregard for sharks and a growing demand for shark fin soup in Asia—have dec- imated global shark populations in the past several decades and have taken a particularly steep toll on oceanic whitetips. Baum’s research led her to conclude in 2004 that whitetip pop- ulations had fallen by as much as 99 percent in the Gulf of Mexico, and though her study has critics, other research has found similarly dramatic declines in the Atlantic and Pacific. It became so clear by 2010 that oceanic whitetips were in trouble that the five major in- ternational fishery organizations that oversee swordfish and tuna fishing forbade vessels from keeping any oceanic whitetips they caught— the only shark species so far to receive that n Grant Brian Skerry’s fieldwork was funded in part by your National Geographic Society membership.