NOTES 



PREDATION BY SHARKS ON PINNIPEDS AT 

 THE FARALLON ISLANDS' 



What we know about mortality in pinnipeds has 

 largely been derived indirectly. For example, 

 pinnipeds or parts thereof have occasionally been 

 found in shark stomachs. Sharks have thus be- 

 come known as pinniped predators (e.g., Gogan^ ), 

 but, since few direct observations of shark/pinni- 

 ped interactions exist, we do not know the extent 

 of such predation. The present paper summarizes 

 observations of shark/pinniped encounters at the 

 Farallon Islands between 1970 and 1979. We 

 relate the frequency of observed encounters to 

 annual and seasonal changes in pinniped popula- 

 tion, and the marine climate, and assess the effect 

 of shark-bite injury on the reproductive perfor- 

 mance of seals. 



Methods 



The South Farallon Islands, San Francisco 

 County Calif, (lat. 37.4° N, long. 123.0° W), lie at 

 the inward edge of the California Current, 30 km 



west of the California coast. Southeast Farallon, 

 West End, and accompanying rocks compose the 

 South Farallones and in all are about 44 ha 

 (Figure 1). Over 250,000 seabirds of 12 species 

 breed there (Ainley and Lewis 1974). Pinnipeds 

 reach a peak of 2,500 animals — three species 

 breed and occur there year-round: harbor seal, 

 Phoca vitulina, northern elephant seal, Mirounga 

 angustirostris , and northern sea lion, Eumetopias 

 jubatus; a fourth, California sea lion, Zalophus 

 californianus , the most numerous of Farallon 

 pinnipeds, occurs most abundantly in spring, 

 but few breed there; and a fifth, northern fur 

 seal, Callorhinus ur sinus, occasionally hauls out 

 (Pierotti et al. 1977; Ainley et al.^). 



Since 1968, the Point Reyes Bird Observatory 

 has maintained a year-round research station on 

 Southeast Farallon. On a rotating but continual 

 schedule at least two biologists, plus several 

 volunteer workers, have operated the station. 

 Every day, weather permitting, a census of birds 

 and a general visual survey of inshore waters was 

 made. Beginning in 1970 elephant seals were 



'Contribution 169 of the Point Reyes Bird Observatory. 



^Gogan, P. J. P. 1977. A review of the population ecology 

 of the northern elephant seal, Mirounga angustirostris. Un- 

 pub. manuscr, 68 p. Natl. Mar. Mammal Lab., NMFS, NOAA, 

 7600 Sandpoint Way, NE., Seattle, WA 98115. 



^Ainley, D. G., H. R. Huber, R. P Henderson, T. J. Lewis, and 

 S. H. Morrell. 1976. Studies of marine mammals at the 

 Farallon Islands, California, 1975-76. Final report, Marine 

 Mammal Commission (Contract No. MM5AC027), Wash., D.C., 

 available Natl. Tech. Inf. Serv., Springfield, VA 22151 as 

 PB 2-266249, 32 p. 



HAUL-OUT AREAS 

 SEA LIONS 



El ELEPHANT 

 SEALS 



SHARK 

 ATTACKS A 

 SIGHTINGS • 



EAST ••• 



LANDING AAA 

 AA 



SEA PIGEON 



GULCH A A 



• ••AA 

 AAAA 



HEAD 



FIGURE 1.— South Farallon Islands, central 

 California, and location of observed shark/ 

 pinniped encounters and major haul-out areas 

 of sea lions and elephant seals. 



FISHERY BULLETIN: VOL. 78. NO. 4, 1981. 



941 



