Predation

The major predators on elephant seals are white sharks, Carcharodon carcharias , and killer whales, Orca orcinus (Laws 1953b ; Ainley et al. 1981; Le Boeuf, Riedman, and Keyes 1982). Shark attacks on northern elephant seals have been observed throughout the breeding range. White sharks kill elephant seals near the surface, victimizing both sexes and all age groups, including

the largest bulls. At the Farallon Islands, near the mouth of San Francisco Bay, white shark predation is restricted to juveniles during the fall haul-out and is localized in the zone of shallow water (< 11 m) within 0.4 km of the island (Klimley et al. 1992). At Año Nuevo, peak attacks on elephant seals occur in December and January, and breeding age males are most frequently victimized (Le Boeuf, Riedman, and Keyes 1982). The contribution of white shark predation to total mortality at sea is unclear, but the diving behavior of elephant seals in the high-risk zone, moving to and from the Año Nuevo rookery over the continental shelf, suggests selection for predator avoidance (Le Boeuf and Crocker 1993). Predation by killer whales has been documented less frequently than predation by sharks (Laws 1953b ; M. Pierson, pers. observ.). Leopard seals, Hydrurga leptonyx , will occasionally take a southern elephant seal but cannot be classed as an important predator (Laws 1953b ).

The cookiecutter shark of the genus Isistius bites out circular chunks of skin and blubber the size of a tennis ball from northern elephant seals in the southern part of their range, but since the craterlike injuries are not lethal, this is more a matter of parasitism than predation (Le Boeuf, McCosker, and Hewitt 1987).

Historically, man has been the major predator on both species of elephant seals, killing them for their oil. The history of sealing in the Southern Hemisphere is reviewed by Laws (this volume) and W. N. Bonner (1982); the decline in numbers and the virtual annihilation of northern elephant seals due to sealing is reviewed by G. A. Bartholomew and C. L. Hubbs (1960) and B. C. Busch (1985) and addressed in chapter 2 of this volume.