Picks from the Past: October 2006

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White sharks have a number of markings that may serve a social purpose. The pectoral fins, for instance, feature black tips on the undersurface and white patches on the trailing edge. Both markings are all but concealed when the sharks swim normally, but are flashed during certain social interactions. And a white patch that covers the base of the lower lobe of the shark’s two-pronged tail may be important when one shark follows another. But if those markings help white sharks signal to one another, they may also make the sharks more visible to their prey. And if so, the trade-off between camouflage and social signaling demonstrates the importance of social interactions among white sharks.

Complex social behaviors and predatory strategies imply intelligence. White sharks can certainly learn. The average shark at Seal Island catches its seal on 47 percent of its attempts. Older white sharks, however, hunt farther from the Launch Pad and enjoy much higher success rates than youngsters do. Certain white sharks at Seal Island that employ predatory tactics all their own catch their seals nearly 80 percent of the time. For example, most white sharks give up if a seal escapes, but a large female we call Rasta (for her extremely mellow disposition toward people and boats) is a relentless pursuer, and she can precisely anticipate a seal’s movements. She almost always claims her mark, and seems to have honed her hunting skills to a sharp edge through trial-and-error learning.

We are also learning that white sharks are highly curious creatures that systematically escalate their explorations from the visual to the tactile. Typically, they nip and nibble to investigate with their teeth and gums, which are remarkably dexterous and much more sensitive than their skin. Intriguingly, highly scarred individuals are always fearless when they make “tactile explorations” of our vessel, lines, and cages. By contrast, unscarred sharks are uniformly timid in their investigations. Some white sharks are so skittish that they flinch and veer away when they notice the smallest change in their environment. When such sharks resume their investigations, they do so from a greater distance. In fact, over the years we have observed remarkable consistency in the personalities of individual sharks. In addition to hunting style and degree of timidity, sharks are also consistent in such traits as their angle and direction of approach to an object of interest.

Any discussion of white sharks must acknowledge their occasional, though much-publicized, “attacks” on people. The vast majority of them, however, bear no resemblance to shark attacks on prey. The attacks on people are slow and deliberate, and the resulting wounds are relatively minor compared with the wounds inflicted on prey. About 85 percent of the victims survive. Deaths do occur from blood loss, but there are very few verified cases in which a white shark actually consumed a person. Clearly, we are not on their menu.

Klimley suggests that, compared with blubbery marine mammals, people are simply too muscular to constitute a worthwhile meal. Our view is different: we believe that white sharks probably bite people not to eat them but to satisfy their curiosity. Fortunately, the shark’s investigation of a person is usually interrupted by the victim’s brave companions.

For all the fear white sharks inspire, it is ironic that people probably pose the single greatest threat to white sharks. People kill them for sport and trophies, and hunt them to reduce their populations near swimming and surfing beaches. In addition, there’s a flourishing and lucrative black market in white-shark jaws, teeth, and fins, even though such trade is illegal under international law. White sharks take between nine and sixteen years to reach maturity, and females give birth to just two to ten pups every two or three years. Such a life in the slow lane makes the white shark extremely vulnerable to even moderate levels of fishing.

In recent studies, electronic tags attached to individual white sharks and monitored by satellites have shown that the animals can swim thousands of miles a year. One individual swam from Mossel Bay, South Africa, to Exmouth, Western Australia, and back—a round trip of 12,420 miles—in just nine months. Such long-distance swimming may take white sharks through the territorial waters of several nations, making the sharks hard to protect (not to mention hard to study). Yet a better understanding of their habitat needs, their movement patterns, their role in the marine ecosystem, and their social lives is critical to the species’ survival.

As September approaches, the white sharks’ hunting season at Seal Island draws to a close. Soon most of them will depart, remaining abroad until their return next May. The Cape fur seal pups that have survived this long have become experienced in the deadly dance between predator and prey. They are bigger, stronger, wiser—and thus much harder to catch. The handful of white sharks that remain in False Bay year-round probably shift to feeding on fishes such as yellowtail tuna, bull rays, and smaller sharks. In effect, they seasonally switch feeding strategies from energy maximization to numbers maximization.

Next May we, too, will return. But fieldwork always has its surprises, and we cannot predict what the white sharks of Seal Island will have in store for us.