TRICAS BLUE SHARK AND ITS PREY SPECIES 



snri'grass, Phyllospadix torreyi, habitats along the 

 shore of the island. The occurrence of pipefish at 

 the surface in the San Pedro Channel at night and 

 the fact that sharks containing freshly ingested 

 pipefish were captured 2-5 km from the island 

 imply that this prey was most likely taken in wa- 

 ters away from inshore kelp and surfgrass 

 habitats. 



Freshly ingested blacksmith, Chroniis 

 punctipinnis, were recovered from a shark cap- 

 tured near Ship Rock at noon. At Catalina, this 

 planktivorous damselfish formed midwater feed- 

 ing aggregations at the outer edges of the kelp 

 forest during the day, and at times ranged sea- 

 ward up to 100 m from the nearest kelp. At dusk, 

 blacksmith retreated to the protection of rocks and 

 crevices (see Quast 1968; Hobson 1976). Blue 

 sharks frequented waters near exposed kelp 

 stands at Ship Rock and have been reported chas- 

 ing and feeding on blacksmith during the day 

 (Sciarrotta and Nelson 1977; Given'^). 



With the exception of Mastlgotcufhis pyrodes, 

 Vampyrott'uthis infernaliti, and nonspawning 

 Loligo opalescens, all of the cephalopod prey 

 species (or their congeners for which data are 

 available) occur near the surface at night through 

 vertical ascent from greater depths or by normal 

 epipelagic distribution (Roper and Young 1975; 

 Tricas 1977). Mastigoteuthis pyrodes (mesopelag- 

 ic) and V. infernalis (bathypelagic) occasionally 

 migrate to the lower limits of the epipelagic zone 

 at night i Roper and Young 1975). 



In their study of blue shark movements near 

 Catalina, Sciarrotta and Nelson (1977) described 

 evening-twilight shoreward movements of sharks 

 from late March through early June and 

 suggested the change in movement patterns as a 

 response to seasonal increases of inshore spawn- 

 ing squid and decreases in availability of pelagic 

 fishes offshore. Such movements, however, may 

 not be strictly food related. For example, daily 

 inshore-offshore migrations of sharks (late March 

 through early June) would not be synchronous 

 with the cold-water winter peak (December 

 through February) of inshore squid spawning ac- 

 tivity near the Isthmus. Also, some sharks ob- 

 served during this study fed among spawning 

 squid schools throughout the day and therefore did 

 not exhibit the diel inshore-offshore movement 



^R. Given, Catalina Marine Science Center, P.O. Bo.x 398, 

 Avalon, CA 90704. pers. commun. July 1977. 



pattern. Furthermore, sharks fed upon anchovies 

 m offshore waters throughout the year and there 

 is no indication that the availability of anchovies 

 or jack mackerel to blue sharks significantly 

 changed over the course of this study. 



Detection of prey by sharks is often dependent 

 on the reception of abnormal or unusual stimuli 

 such as low-frequency vibrations of struggling or 

 fleeing fishes (Nelson and Gruber 1963; Nelson 

 and Johnson 1972). In addition, olfaction plays 

 a well-documented role in location of injured, 

 stressed, or bleeding prey (Tester 1963; Hobson 

 1963). Ultimately, however, vision (Gilbert 1963) 

 and possibly electroreception (Kalmijn 1971) are 

 the principal senses used immediately prior to at- 

 tack. For blue sharks in a normal nocturnal feed- 

 ing mode, it is probable that search images are 

 formed for a general size rather than for a particu- 

 lar species. Pipefish, for example, were relatively 

 small in biomass, but represented a length charac- 

 teristic of other prey species. Similarly, most 

 cephalopods in the diet fell within the common 

 prey size range (e.g., 5-25 cm TL). Bioluminescent 

 trails of darting anchovies and other small fish and 

 squid were frequently seen while snorkeling at 

 night in offshore waters and likewise would be 

 readily visible to sharks. Also, the majority of 

 cephalopod species taken by sharks possessed 

 photophores. Bioluminescence associated with 

 prey movements and light organs may represent 

 significant predatory cues for sharks at night. 



ACKNOWLEDGMENTS 



Thanks to D. R. Nelson for his assistance and to 

 R. Given and R. Zimmer of the Catalina Marine 

 Science Center. F. G. Hochberg, Santa Barbara 

 Museum of Natural History, and L. Pinkas, 

 California Department of Fish and Game, pro- 

 vided helpful suggestions and access to their 

 cephalopod collections. J. Goldsmith, Marineland 

 of the Pacific, provided sharks and facilities for the 

 digestion rate studies. Thanks to C. Shoemaker for 

 her help in the field, J. McKibben for his technical 

 assistance, and H. Izuta Tricas for her help in 

 preparation of the manuscript. E. S. Hobson and J. 

 C. Quast constructively criticized the manuscript. 

 Special thanks to F. Banting and C. Best for their 

 contribution that made this work possible. Finan- 

 cial support was granted by the Office of Naval 

 Research through contract N00014-75-C-0204, 

 under project NR- 104-062, for the program of 

 shark research of which this study is a part. 



181 



