Food Habits of the white sharks

In most areas where white sharks occur, pinnipeds such as these California sea lions (Zalophus californianus), make up the bulk of the white shark's diet 
© D. A. Sutton, Amer. Soc. Mammalogists Library

The white shark is a macropredator, known to be active during the daytime. Its most important prey items are marine mammals (including, seals, sea lions, elephant seals, dolphins) and fishes (including other sharks and rays). Marine reptiles are sporadically ingested, mostly sea turtles. Marine birds and sea otters are almost exclusively rejected as prey. These animals are commonly found having suffered injuries from encounters with white sharks, but are rarely ingested.Predatory behavior is usually divided into five stages; detection, identification, approach, subjugation, and consumption. However, these stages, especially the first and second, are poorly understood in white sharks. The patterns of prey detection and identification in white sharks have been investigated by the use of experimental targets, baits, and other objects in which they are "offered" to the sharks. The results of these experiments reveal that when white sharks have a choice between a square target and a fusiform, seal-shaped target, they select the shape that is more common in their natural environment. Indeed, the choice made in nature is usually whether to respond to a single potential prey item rather than choosing between two of them. When only a single object was presented, it was invariably investigated. Some scientists believe diver and surfer silhouettes, when viewed from below, resemble those of pinnipeds and that this misidentification on behalf of the shark is the cause of most white shark attacks on humans. However, the fact that white sharks attack inanimate objects of a variety of shapes, colors and sizes, none of which resemble those of a marine mammal, refute the well-known hypothesis of "mistaken identity". Researchers suggest that white sharks often strike unfamiliar objects to determine their potential as food. In this case, it would seem that grasping an unfamiliar object would be the shark's only reliable method of determining palatability.
Based on underwater observations, scientists described some approach patterns. Most sharks used an "underwater approach" in which the shark swam just below the surface until it was approximately 1 m (3.3 ft) from its intended prey and then attacked by deflecting the head upward and emerging out of the water. The white shark also presented a "surface-charge" which consisted of a rapid rush with the body partially above the surface. In rare cases, whites performed an "inverted approach" in which they swam with the ventral side up. Although the majority of approaches are horizontally oriented, vertical approaches are nonetheless common. White sharks readily engage in vertical swimming during feeding activities, sometimes swimming perpendicular to the surface in direct and rapid pursuit of floating objects. There are benefits of using the vertical approach to capture prey positioned near the surface. Firstly, a predator attacking from below is more difficult for the prey to see, while at the same time, the shark has a better view of its prey positioned overhead. In addition, fleeing (rapid movement away from an approaching predator) is probably the most common escape tactic used by animals under attack. Considering these situations, extended escape in the direction opposite the vertically approaching shark is virtually impossible. The propensity for vertical swimming was observed in small white sharks approximately 220 cm (86 in) in length. Scientists believe that the development of this behavior precedes physical changes, such as broadening of the teeth, believed to be adaptations for feeding on large marine mammals.
Few hypotheses about the consumption patterns of white sharks have been made based on observations under natural conditions.

White shark scavenging a dead humpback whale 
©Tobey Curtis

One of these hypotheses, the "bite, spit and wait" theory, is composed of three elements. Initially, the white shark seizes its prey and releases it intact; secondly, the shark waits until the prey lapses into a state of shock or bleeds to death; finally, the white shark returns to feed on the dead or dying animal. However, recent studies do not support this hypothesis. Scientists believe that these sharks may not release potential prey to permit them to die but, rather, let them go in response to their defensive behavior or unsuitability as food. Some evidence suggests that white sharks decide a prey's palatability while it is lodged in the shark's mouth. Researchers also believe that white sharks may prefer animals rich in energy, such as marine mammals, in favor of less fatty, energy-poor prey. This is supported by some observations of aggregations of white sharks selectively feeding on the blubber but not the muscle layers of mysticete whales. This behavior seems based upon a size-hierarchy, where large sharks dominate in the feeding.
A behavior pattern described as "repetitive aerial gaping" was observed in white sharks of southern Australia. The sharks were seen with their heads out of the water, mouths at or above the surface, rolling onto their side and opening and closing their mouth in a moderately slow, rhythmic, partial gape while swimming slowly along the surface. The most notable difference between this behavior and normal surface feeding is that the repetitive aerial gaping is not oriented toward food or possible targets. White sharks also scavenge from fishermen's nets and longlines and take all manners of hooked fish. This propensity often results in their own accidental entrapment.