All-over body sweat—the cooling waters of the eccrine glands—is different from flop sweat. Had Tester done what my friends at the Monell Chemical Senses Center did to me—gathered the pungent armpit exudations of a human under stress—his results might have been different. The sharks might have detected the scent of distress, of easy pickings, and gone into attack mode.
That is precisely what happens when a shark’s preferred prey falls under stress. The shark senses a no-hassle meal and closes in to attack. Tester harassed a bucket of groupers by “threatening them with a moving stick” (elsewhere referenced as “poking”). Pumping water from the bucket—scientific nomenclature: “distressed grouper water”—into the shark tank provoked a “violent hunting response.” Since the prey were outside the tank, we know it wasn’t the sight or sound of grouper pandemonium that set off the sharks’ predatory moves. It had to be some chemical exuded through the groupers’ skin or gills. And not just any grouper scent would do the trick. When “quiescent grouper water” was introduced into the tank, the sharks paid little heed.
Fish blood and fish guts—two blaring sensory trumpeters of piscine distress—also trigger vigorous hunting moves. So powerful is the chemical signal, Baldridge found, that sharks could be roused to devour a rat—not normally an item of gustatory interest—if its fur were coated with “mullet blend” (whole mullets blenderized with a little water). In a different study, sharks were inspired to attack a kitchen sponge that had been dipped in a bowl of fish body fluids. “Sharks,” wrote Baldridge, “will strike essentially anything that has been treated with fish ‘juice.’”
That includes spearfishers. In particular peril are those who swim around with the day’s catch hanging from their belts or trailing from lines. At the time Baldridge ran his analysis, the Shark Attack File had logged 225 incidents that mentioned the presence of wounded fish and/or fish blood or guts. “Sharks,” marveled Tester, “are able to track down and converge on a distressed fish (such as a live fish suspended from a hook through the jawbone but otherwise uninjured) with uncanny speed and accuracy.”
Spearfishing probably serves to explain why 17 percent of the Shark Attack File victims were wearing wetsuits. The original theory put forth was that the sharks mistook people in black wetsuits for seals. Perhaps that happens too, but where spearfishing was involved, it’s more likely that the wetsuit’s accessories—the spear and the belt of oozing fish—drew the shark.
Dead fish also ring the dinner bell. Tester exposed blacktip and gray sharks to a sushi bar of fish flesh: tuna, eel, grouper, snapper, parrot fish, giant clam, octopus, squid, and lobster. All of them he classed as attractants. Sharks prefer to take no risks. They prefer to go after a meal that’s not going to put up a fight. Injured is good. Dead is better.
Which makes you wonder about the alleged shark-repellent qualities of decomposed shark flesh. Tester wondered, too. He secured some “alleged shark repellent” from a fisherman, another sample from a fisheries lab, and a sample his team prepared on their own by leaving hammerhead and tiger shark flesh outside in the tropical heat for a week. No repellent effects were observed. On the contrary, it sometimes functioned as an attractant. “Our results… seem to be at variance with those of Springer…. No convincing explanation can be made.” Tester perhaps unaware of the powerful attractant effect of kickbacks from shark-processing plants.
As with fish, so with humans. Over and over, in the shark attack reports of World War II, corpses took the hit. A floating sailor could dispatch a curious shark by hitting it or churning the water with his legs. (Baldridge observed that even a kick to a shark’s nose from the rear leg of a swimming rat was enough to cause a “startled response and rapid departure from the vicinity.”) “The sharks were going after dead men,” said a survivor quoted in a popular book about the 1945 sinking of the USS Indianapolis, an event that often comes up in discussions of military shark attacks. “Honestly, in the entire 110 hours I was in the water,” recalls Navy Captain Lewis L. Haynes, in an oral history conducted by the US Navy Bureau of Medicine and Surgery, “I did not see a man attacked by a shark….” They seemed to have been, he said, “satisfied with the dead.” Haynes says fifty-six mutilated bodies were recovered, but there’s nothing to suggest that any more than a few of them were bitten into while alive.
Why, then, do sharks hang around life rafts? For what’s underneath. Schools of fish loiter there, either for the shade or to feed on smaller marine life that gathers to take the shade on the raft’s underside. Recalled one World War II sailor: “Larger fish came to feed on those minnows, then larger ones to get them; finally the boys with the peculiar dorsal fins arrived to see what the fuss was about.” Here’s one more, just because I like it: “The shark submerged and swam directly under the raft…. We all sat very quiet,… and the radar man abandoned the idea of defecating over the side for fear of capsizing. The shark repeated this behavior several times but at no time seemed concerned with us.”
And so it continues to be. I know of only one recorded instance in recent history of a shark’s biting Navy personnel. In 2009, a bull shark took off the hand and foot—in one bite—of an Australian clearance diver during a counter-terrorism exercise in Sydney Harbor. I asked Naval Special Warfare Command communications specialist Joe Kane about sharks attacking Navy SEALs. “You’re coming at this the wrong way,” he said. “The question is not, Do Navy SEALs need shark repellent? The question is, Do sharks need Navy SEAL repellent?”
The modern US Navy has no formal shark-attack curriculum. One diver recalls being told to descend slowly and take cover on the bottom should he sense a threat. A 1964 Air Force training film called Shark Defense advises downed aviators to blow a stream of bubbles or yell into the water. I asked veteran shark videographer Robert Cantrell what he thought of this advice. Cantrell has swum among sharks, cageless, for three decades. This is a man who will apply the adjective “nippy” to a group of excited blue sharks. His answer, an answer Baldridge and Tester often came up with, is that it depends on the kind of shark. Screaming into the water may briefly deflect a bull shark, Cantrell notes, but not a tiger shark. Bubbles scare blue sharks, but other species ignore them.
The last Air Force suggestion was a puzzler: “Tearing up paper into small pieces and scattering them all around.” I suppose it was meant as a means of distracting the shark—or maybe just the sailor, now absorbed in the challenge of locating sheets of paper while afloat at sea. On one of Cantrell’s expeditions, he threw some stale bagels overboard. Tiger sharks swam over immediately; bull sharks ignored them. Cantrell’s main advice to the diver who encounters a shark? “Enjoy the experience.”
Let us turn now to the question on many a sailor’s mind: Is it true that human blood draws sharks? The results of Baldridge’s and Tester’s experiments are inconsistent. Sometimes the sharks behaved as though attracted to the blood; other times they avoided the test area. Tester wondered whether the freshness of the blood was a factor. In his own experiments, blacktip sharks and greys were strongly attracted to blood less than one or two days old—at concentrations as weak as .01 parts per million of seawater. But Baldridge’s analysis of the Shark Attack File data belie this finding. In only 19 of 1,115 cases was the victim bleeding at the time of the attack. “It is difficult,” he concluded, “to accept the concept that human blood is highly attractive and exciting to sharks in general when so many shark attack victims have been struck a single blow and then left without further assault even though they were then bleeding profusely from massive wounds.”