334 



Fishery Bulletin 89(2). 1991 



The number of prey taxa eaten appears 

 to be closely linked to the abundance and 

 availability of northern anchovy. When an- 

 chovy were abundant near San Nicolas 

 Island, sea lions evidently ate them in 

 preference to other species. Both seasonal 

 and annual patterns indicate that when the 

 frequency of occurrence of anchovy in the 

 diet is high, the number of prey species con- 

 sumed is low. When northern anchovy are 

 not available the breadth of the diet in- 

 creased, with 3 to 4 other species being 

 eaten. 



Jack mackerel were displaced northward 

 out of the Southern California Bight in 1984 

 when ocean surface currents changed and 

 warm water associated with the 1982/83 El 

 Nino-Southern Oscillation intruded into that 

 area from Baja California (Mason 1989); the 

 presence of jack mackerel in sea lion scats 

 declined substantially then. Pelagic red 

 crabs were transported into the Southern 

 California Bight during this warm-water in- 

 trusion and they were frequently eaten by 

 sea lions in 1983 and, to a lesser extent, 

 through 1985 when they disappeared from 

 their diet. 



The dietary and behavioral flexibility of 

 California sea lions in response to move- 

 ments and availability of prey and to en- 

 vironmental perturbations (e.g., El Nino 

 Southern Oscillation) may be one of the most 

 important factors contributing to the con- 

 sistent increase in their abundance during 

 the past several decades. Relative to other 

 locations, it appears that northern anchovy 

 may be one of the most important prey of 

 California sea lions near San Nicolas Island. 

 This regional phenomenon is due to the 

 proximity of San Nicolas Island to large 

 spawning aggregations of northern anchovy 

 compared with other locations where 

 dietary studies of California sea lion have 

 been made. 



m 

 o 

 <r 

 m 

 a. 



1981 



100 



80 - 



60 



40 



20 







100 



80 



60 



40 



20 

 



1982 1983 



Octopus 



1984 1985 1986 





<i <i I ft ft 3 is ft ft i\S ft ft SiS ft ft <i 

 Pacific Mackerel i 



ft i 



r-n 



ja 



100 

 80 - 

 60 

 40 

 20 

 



100 

 80 

 60 

 40 

 20 

 



100 

 80 

 60 

 40 

 20 

 



100 

 80 

 60 

 40 

 20 

 



100 

 80 

 60 

 40 

 20 

 



100 

 80 

 60 

 40 

 20 

 



ft z<i ft ft i'z ft ft 2<i ft ft 2'i ft ft i\i ft ft % 

 pelagic F^ed Crab 



. .1, 1 , rTTTT-T* 



-l — q— i — i T i 

 ft <' t ft ft < 



du 



~r 



5 ft 3 < ' i ft ft 3'S ft 3 < 

 Market Squid 



"i — i — i — r~ 



i * I I 



fcWQ 



I 111 I III! I I 



III 



ft < 



■a ft ^ <ii ft ft <i!s ft ft <ii ft ft <i i ft ft < 

 Rockfish i r— "" 



rTj , n--r-> . rrrTTJy 



;T7f7V-n~1 



ft 3|s ft ft I|l ft ft 3|s ft ft 3]s ft ft 3] 5 ft ft 5 



Jabk Mackerel 



r~p—p 



i .' i 



cfi < '5 « IS < 



zftaZ'iftoii'iftfti'iftftZ 



i Pacific 

 'Whiting 



 ■vtTH J .' . 



TT-T 

 ft<\tftft<]tftft<\tftft< 



I ' I ' I ' I ' I I 



i ft $ <ii ft $ < 



Anjchovy 



r-T Lmm- 



I 3;i ft ft !;i ft ft 3;i ft ft ^;g ft ft 3;s ft ft I 



1981 ! 1982 1983 ! 1984 ! 1985 ! 1986 



Figure 2 



Seasonal frequency of occurrence (%) for eight prey taxa found in Califor- 

 nia sea lion scats collected at San Nicolas Island. California. 



Acknowledgments 



We thank R. Dow and staff in the Environmental Divi- 

 sion and the commands at SNI/OLF and PMTC of 

 Pt. Mugu Naval Air Station for accommodating our 

 research at San Nicolas Island. We also thank J. Wex- 

 ler and R. Folk for assistance in processing scat 

 samples; N.C.H. Lo, C. Macky, and B. Wahlen for 

 statistical advice; H. Orr and R. Allen for assistance 



in preparing the figures; and G. Antonelis, J. Barlow, 

 D. DeMaster, J. Jehl, and two anonymous reviewers 

 for reviewing the manuscript. The research of BSS and 

 PKY was supported by USAF contract FO4701-88- 

 C-0026. 



