Biology and Ecological Niches in the Gulf of the Farallones 



White Sharks 



Scot Anderson 



Each fall, white sharks (Carcharodon car- 

 charias) are observed around the Farallon 

 Islands, preying on seals and sea lions (pin- 

 nipeds) (see chapter on Marine Mammals). 

 The shark's dark back and light underbelly 

 blend with the surrounding environment, 

 hiding the shark from unsuspecting pinni- 

 peds. White sharks spend a great deal of 

 time searching for food; it may be weeks or 

 a month between feeding opportunities. 



The peak in predation, in the fall months, 

 is related to the number of young (1- and 

 2-year-old) northern elephant seals arriving 

 at the islands at this time. These immature 

 seals are the preferred prey of the white 

 shark at the Farallon Islands. Observations 

 there indicate that the white sharks eat 

 young elephant seals seven times more fre- 

 quently than they eat other pinnipeds, such 

 as California sea lions and harbor seals. 



Immature elephant seals arrive at the Far- 

 allon Islands to come ashore beginning in 

 September and continuing through Novem- 

 ber. The small pocket beaches and surge 

 channels around the islands offer undis- 

 turbed "haul out" sites and resting areas. 

 Unlike California sea lions, which are the 

 most numerous pinnipeds on the islands, 

 elephant seals cannot climb high up on the 

 islands' rocky shores. Therefore, they are 

 restricted to the pocket beaches and surge 



channels. The fall haul out of elephant seals 

 is commonly their first visit to the islands. 

 Many of these seals are from other colonies, 

 such as those at the Channel Islands or 

 Point Ano Nuevo to the south, and they may 

 be unaware of white sharks patrolling the 

 waters around the Farallon Islands. 



Studies of white sharks in the Gulf of 

 the Farallones begun in 1987 have revealed 

 the remarkable way in which these predators 

 are able to locate and catch pinnipeds. 

 Using photographs of dorsal and tail fins, 

 and underwater videotapes of entire sharks, 

 it has been possible to identify individual 

 white sharks and follow their movements 

 in the gulf over many years. Several of the 

 larger sharks have been seen in the same 

 areas for more than 5 years. When individual 

 sharks are identified, they are always seen in 

 the same area. However, some of the sharks 

 are regular visitors, whereas others are only 

 intermittently seen in the gulf. 



By 1995, monitoring of individual white 

 sharks around the Farallon Islands, using 

 small transmitters swallowed by the animals, 

 had documented their movement patterns 

 and internal temperatures. The movement 

 patterns indicate that each shark has an area 

 that it covers by zigzagging back and forth. 

 These sharks are not swimming aimlessly; 

 they increase their odds of finding pinnipeds 



by looking in familiar areas where they 

 have previously been successful. The largest 

 sharks covered the smallest home ranges and 

 the smallest shark tracked covered the larg- 

 est home range. The larger, older sharks 

 seem to be more experienced and therefore 

 know where to search. 



Monitoring found that white sharks in 

 the Gulf of the Farallones maintain a con- 

 stant body temperature of nearly 80F in the 

 cold (54 to 57F) water of the gulf. This 

 allows them to move quickly and to capture 

 warm-blooded marine mammals, such as 

 pinnipeds. Most of the world's predatory 

 sharks cannot increase their body tempera- 

 ture above the ambient water temperature, 

 and so their range is limited to warmer trop- 

 ical waters. 



High tides and large swells affect the 

 rate of white shark predation on pinnipeds 

 in the waters around the Farallon Islands. 

 Such conditions force many of the seals 

 to move from their haul-out space. This 

 displacement may explain why significantly 

 more attacks on elephant seals sometimes 

 two or three in one day are observed when 

 large swells are combined with high tides. 



Despite the white shark's fearsome rep- 

 utation and Hollywood image, attacks on 

 humans are relatively rare. In northern and 

 central California, there is on average one or 



two white shark attacks on humans per year 

 and about one fatality per decade (chances 

 of drowning in the State's coastal waters are 

 at least 100 times greater). In most of these 

 instances, the shark has bitten only once and 

 then released the person immediately. Those 

 bitten usually survive if they can make it 

 to shore, so swimming alone is not advised. 

 Some evidence suggests that white sharks do 

 not like the "taste" of people but sometimes 

 mistake them for there favorite prey, pin- 

 nipeds. Therefore, it is not advisable to swim 

 or surf near colonies of pinnipeds, where 

 white sharks may be actively feeding. Areas 

 where there has been a history of shark 

 attacks should also be avoided. For these 

 reasons, sport divers avoid the waters around 

 the Farallon Islands. 



The population of white sharks off Cal- 

 ifornia's coast is probably small, having 

 perhaps a few hundred to a few thousand 

 adults. White sharks are important predators 

 in the State's marine ecosystems. In 1994, 

 with the support of scientists, fishermen, 

 surfers, divers, and others, the State of Cali- 

 fornia placed the white shark on the list of 

 species protected in its waters. 



52 Biology and Ecological Niches in the Gulf of the Farallones 



