TRICAS: BLUE SHARK AND ITS PREY SPECIES 



digestive states of anchovies recovered from wild 

 sharks. Three healthy, active sharks were accli- 

 mated for 24 h in large seawater holding tanks 

 (14°-16°C) at Marineland of the Pacific, and then 

 fed marked anchovies and mcU'ket squid. Stomach 

 contents were examined at 6, 12, and 24 h after 

 feeding and the digestion rates recorded. 



Short-term movements of sharks were moni- 

 tored in the fall and winter seasons by telemetric 

 instrumentation similar to those of Ferrel et al. 

 1 1974 1 and Nelson (1974). Transmitters were 

 applied externally to free-swimming sharks with 

 stainless-steel darts. Effective transmission range 

 was approximately 2 km under good conditions 

 but depended largely upon ambient noise from 

 waves, wind, and biological sources. Some trans- 

 mitters included a depth sensor for a record of 

 vertical movements. Signals were tracked using a 

 tuneable ultrasonic receiver and a staff-mounted 

 directional hydrophone. These trackings supple- 

 ment the spring through fall trackings of Sciar- 

 rotta and Nelson il977i. 



The feeding behavior of blue sharks among 

 spawning squid was studied in January 1976. Just 

 before sunset, squid schools were detected near the 

 bottom (30-40 m deep) using a recording Fathome- 

 ter'' and the work boat anchored directly above. A 

 1,500-W light was then suspended over the water. 

 Squid typically converged beneath the light and 

 formed a large surface school at which sharks usu- 

 ally appeared and began to feed. 



Orientation and feeding responses of sharks to 

 moving prey were documented during baiting ses- 

 sions at offshore stations. In these tests, a dead 

 anchovy, attached to a light fishing line was cast 

 beyond the bait-attracted sharks and then re- 

 trieved back towards the boat. All field observa- 

 tions of shark and prey activities were made from 

 the boat, using scuba and/or by snorkeling. 



RESULTS 



Sharks were captured during all months of the 

 1-yr study. Of the 81 sharks sampled, 9¥( had 

 recognizable food items in their stomachs. The 

 northern anchovy, Engraulis mordax, was the 

 predominant prey item for sharks in the study 

 area while other small fishes occurred at much 

 lower frequencies (Figure 3). 



Although sharks fed on a wide variety of 

 cephalopods, an analysis of relative importance 

 (Table 1) showed L. opalescens and squid of the 

 genus Histioteuthis as the most common and sub- 

 stantial cephalopod prey. Monthly analysis re- 

 vealed important shifts between these prey items 



ivngnalhui ealitarniei 

 rioihurui lymmttrtiu 

 Soualui ocanlhioi 

 Chroma punt.iipinfn\ 

 Cypirluiiti culilornicu 



Loligo opaltitrnt 



Chitottulhii, caly 



Onythoirulhii borrt 

 (Xythoe tuheitulalu 

 Octapoiruihii dtkl 

 (ktopui vp 

 Va/nftyrolfulhii mlr 

 MjMigoltultUi (D/iOi 





Lv^unutid jmphipiiiJ 

 Rtnilla holhhtn 



PhyllOipodii lotiry 

 i\g^ (Chluruphvul 



FIGL'RE 3. — Stomach contents of 81 blue sharks sampled during 

 the year. Occurrence = percent of the 81 individuals containing 

 that prey species. Inset gives a summary by broader food 

 categories. 



Table l. — Annual relative importance of identified cephalopod 

 prey in the diet of blue sharks near Santa Catalina Island. Calif. 

 Impwrtance was estimated as an index of relative importance 

 (//?/) inaccordwithPinkasetal- 1 1971)://?/ = uV +V)F,whereN 

 (numerical percent) is the percent of individuals of that species 

 among all individual cephalopods recovered; V (volumetric per- 

 cent) is the percent volume represented by that species of all 

 cephalopods recovered. andF (frequency) is the percent of indi- 

 vidual shark stomachs containing that prey species. 



■*Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service. NOAA, 



177 



