Adult white sharks feed on bony fishes, such as snapper, mackerel and tuna, other sharks and marine mammals. Juveniles are mainly piscivorous. In South Australia, the main pinniped prey are Australian Sea-Lions and New Zealand Fur Seals which inhabit the islands around the Gulf waters. Dolphins are also known to fall prey to white sharks, most commonly the bottlenose (Tursiops truncatus) and the common dolphin (Delphinus delphinus). The carcasses of large whales are readily scavenged.
Based on measurements involving the rate at which a great white elevates its muscle temperature, it is suggested that this species can go six weeks without food after consumption of 45kg of whale blubber. In this regard, it is estimated that a single elephant seal pup will sustain a shark for three months. Thus, the white shark needs only eat infrequently when feeding on marine mammal prey.
Being large and rich in blubber, seals and sea lions are highly attractive as prey to white sharks. However, since they are highly agile in the water, powerful and tend to live in vigilant groups, the white shark relies on stealth and ambush to capture these prey items. The great white, when pinniped hunting, employs a number of strategies. They stalk surface-swimming pinnipeds, ensuring they are not detected by relying on the camouflage of their dark dorsal surface against the dark, rocky sea floor. They mostly target young, lone seals, presumably because they are more vulnerable with decreased vigilance, and are easier to catch and overpower due to their small size and lack of experience in predator avoidance. They are also seen to attack seals as they return to the island from sea, possibly as they would be tired, well fed and slower to respond to an attack.
Predation usually occurs during the day, the start of which coincides with the onset of daylight and the predation rate then drops rapidly as the light diminishes (Klimley et al, 1992). It has been found in California that attacks peak in the early morning and then abate slowly until late afternoon after which there is a rapid decrease in predatory activity (Klimley, 1994). Predation rates at the crepuscular periods (dawn an dusk) are about half that of those observed at other times of the day (Klimley et al, 1992). This diurnal feeding behaviour is consistent with the high density of cone receptors present in the shark’s retina (Gruber and Cohen, 1985). Although rods are present in the shark’s eye, they are not adapted for low light conditions as they are not oriented for maximum light reflection, so hunting during this time would be ineffective for the animal (Klimley et al, 1992).
Attacks are initiated offshore (25-450m) rather than close to the island to decrease the chance of escape onto land by the target and so that the water depth is sufficient to allow a forceful and fast vertical strike (Klimley et al, 1992). Seals are attacked whilst at or approaching the surface to decrease the chances of detection and to limit the avenues of escape. The strike is from a vertical position, the shark using its strong musculature to accelerate from below thus minimising its profile during the attack whilst attaining the force necessary to immobilise and kill its victim.
When chasing a seal engaged in evasive zig-zagging behaviour at the surface, the great white is able to perform a lateral snap of the jaws in order to catch the prey with the lateroposterior teeth at the fulcrum of the jaw where the bite is the most powerful. The prey is usually quickly consumed (less than 2 minutes after the initial strike) to ensure it is not consumed by another member of the species.
Being an opportunistic predator, the great white is known to scavenge on carcasses, particularly those of dead and dying whales. Scavenging offers maximum caloric intake with minimal energy expenditure since no energy is used in pursuit and capture of prey prior to feeding.