RELATIONSHIPS OF THE BLUE SHARK, PRIONACE GLAUCA, AND 

 ITS PREY SPECIES NEAR SANTA CATALINA ISLAND, CALIFORNIA' 



Timothy C. Tricas^ 



ABSTRACT 



Small fishes and cephalopods associated with both pelagic and inshore habitats composed the major 

 prey for the blue shark. Prtnnace glauca. near Santa Catalina Island, Calif. The northern anchovy. 

 Engraulis mnrdax, was the predominant prey for sharks in the immediate study area while at least 13 

 species of pelagic cephalopods constituted major prey for sharks in more distant oceanic waters. Inshore 

 species taken by sharks included pipei\sh.Syngnatbus californiensis: jack mackerel, Trachurus sym- 

 metricus; and blacksmith. Chrornis punctipinnis. In addition, sharks moved inshore to feed on winter 

 .spawning schools of market squid. Loligo opalesci'ns. Digestive rate studies and telemetric monitoring 

 of activity patterns indicate that sharks forage in waters near the surface from around midnight 

 through dawn. Diel activities of prey species were examined and show that most prey dispersed in the 

 upper water column at night and refuged during the day either by schooling (anchovies and jack 

 mackerel ) or by retreating to deeper waters (pelagic cephalopods*. Field observations of shark feeding 

 behavior indicate that predatory modes vary in response to prey behavior. 



Thehlue shark. Prionace glauca (Carcharhinidaei 

 (Figure 1), is a pelagic carnivore cosmopolitan in 

 tropical and warm temperate seas. Because of its 

 pelagic habits, the majority of ecological studies 

 on this species have been predicated on data from 

 sharks captured by sport and commercial 

 fisheries. As a result data has been largely qual- 

 itative, and the shark's role as a predator in the 

 epipelagic habitat has remained unclear. 



The importance of small fish as prey items for 

 blue sharks has been described by Couch (1862), 

 Lo Bianco (1909). Bigelow and Schroeder (1948), 

 Strasburg ( 1958 ). LeBrasseur ( 1 964 ), Bane ( 1968 ), 

 Stevens (1973), and others. These prey generally 

 are schooling species common in productive coast- 

 al waters. Cephalopods were also reported as 

 major prey but little information is available on 

 specific identifications (see Stevens 1973: Clarke 

 and Stevens 1974). 



Although blue sharks have been observed feed- 

 ing on dead or wounded cetaceans (Bigelow and 

 Schroeder 1948; Cousteau and Cousteau 1970) 

 there is little indication that they habitually prey 

 on live, healthy marine mammals. The occurrence 



'Based on a portion of a thesis submitted in partial fulfillment 

 of the requirements for the M.A. degree in the Department of 

 Biology. California State University. Long Beach. Calif Con- 

 tribution no. 27 from the Catalina Marine Science Center, Uni- 

 versity of Southern California. 



^Department of Biology, California State University, Long 

 Beach, Calif; present address: Department of Zoology, Univer- 

 sity of Hawaii at Manoa, Honolulu, HI 96822. 



of mammalian tissue in the diet of blue sharks is 

 rare (Strasburg 1958; Stevens 1973), and such 

 feeding is most likely directed to dead mammals or 

 those in poor health. Air/sea disasters have re- 

 sulted in attacks on humans by blue sharks (see 

 Schultz and Malin 1963; Fitch^) but these cases 

 usually involved injured persons or corpses. 



Standard tagging programs (Weeks 1974; Casey 

 1976; Stevens 1976) and telemetric trackings 

 (Sciarrotta and Nelson 1977) have provided some 

 information on large-scale movements of blue 

 sharks but relatively little is known of their orien- 

 tation mechanisms and predatory behavior. 



Despite the profusion of descriptive reports, 

 there still exists a great need for quantitative data 

 on ecological relationships between the blue shark 

 and its prey species. With these ideas in mind, I 

 undertook this study within a limited geographic 

 area to 1 ) provide a quantitative assessment of the 

 diet of blue sharks near Catalina Island, 2) estab- 

 lish temporal and/or geographical shifts in food 

 habits, and 3) describe behavioral interactions be- 

 tween the blue shark and its prey species. 



METHODS 



The study area was located north of the Isthmus, 

 Santa Catalina Island, Calif. (Figure 2). Beds of 



Manuscript accepted Julv 1978. 



FISHERY BULLETIN: VOL. 77. NO. 1, 1979. 



^J. E. Fitch, California Department of Fish and Game, Opera- 

 tions Research Branch, 350 Golden Shore, Long Beach, CA 

 90802, pers. commun. May 1976. 



175 



