ANTONELIS ET AL.: SPRING AND SUMMER PREY OF CALIFORNIA SEA LIONS 



between the two areas may be an artifact of the dif- 

 ferent methods used for estimating fish length. 3) 

 What appears most probable to us, is the differential 

 geographical distribution of Pacific whiting accord- 

 ing to age. Generally, the younger fish occur in the 

 southern portion of their range, and, although there is 

 some overlap in age groups, the age and size of the 

 fish increase in a northward direction (Bailey et al. 

 1982). 



In cases where sufficient life history information is 

 available, seasonal or annual changes in the 

 occurrence of the four major prey (Fig. 2) can be 

 related to known changes in the prey's relative abun- 

 dance and availability to California sea lions. During 

 both years of this study, the decrease in the 

 occurrence of Pacific whiting in the scats from spring 

 to summer appears to reflect known changes in the 

 migration pattern of the species when adults and a 

 portion of the juvenile population migrate toward 

 shore and north of Point Conception (T. Dark'). For 

 market squid and juvenile rockfish, however, the 

 movement patterns off the coast of California are 

 conspicuously different than Pacific whiting. Gener- 

 ally, market squid increase in abundance in shallow 

 waters (5-50 m depth) near the northern California 

 Channel Islands in late spring, and peak numbers 

 occur in the early summer during spawning (S.Kato 4 ). 

 Inspection of the unpublished data from the 1970-75 

 commercial catches of market squid within 30 nmi of 

 Point Bennett, San Miguel Island, also indicated that 

 peak abundance occurs during the summer months. 5 

 Similarly, in spring through summer, juvenile rock- 

 fish (S. paucispinis and 5. jordani) from the three- 

 species complex identified in this study begin to move 

 into more shallow waters (5-50 m depth) as they com- 

 plete the pelagic stage of their life cycles (E. Hob- 

 son 6 ). In these three instances, seasonal changes in 

 the relative availability of Pacific whiting, market 

 squid, and juvenile rockfish are reflected in the fre- 

 quency of their occurrence in sea lion scats. A similar 

 relationship was also suggested by Bailey and Ainley 

 (1982), when they observed a seasonal change in the 

 prey consumed by California sea lions near the 

 Farallon Islands. 



Although the percentage of occurrence of northern 



'T. Dark, Northwest and Alaska Fisheries Center, National Marine 

 Fisheries Service, NOAA, Seattle, WA 98112, pers. commun. 

 1982. 



4 S. Kato, Southwest Fisheries Center Tiburon Laboratory, Na- 

 tional Marine Fisheries Service, NOAA, Tiburon, CA 94920, pers. 

 commun. 1981. 



'Data provided bv E. Knaggs, Calif. Dep. Fish and Game, Long 

 Beach, Calif., 1982". 



fi E. Hobson, Southwest Fisheries Center Tiburon Laboratory, Na- 

 tional Marine Fisheries Service, NOAA, Tiburon, CA 94920, pers. 

 commun. 1981. 



anchovy in the scats showed no significant seasonal 

 changes from spring to summer, the annual occurrence 

 of otoliths from northern anchovies in the sea lion 

 scats was significantly greater in 1979 than in 1978. 

 Their low numbers in the 1978 scats could be related 

 to a decline in the northern anchovy population 

 resulting from poor recruitment of the 1974-77 year 

 classes (Mais 1981). In 1978, however, the year-class 

 recruitment was strong (Mais 1981), and the 

 increased abundance appears to be reflected in an 

 increased percentage of occurrence in the 1979 

 collection. This explanation is corroborated by our 

 comparison of the northern anchovy otoliths collect- 

 ed during the 2 years, where we found that the 1979 

 scats contained significantly smaller fish which were 

 mostly (81%) yearlings from the 1978 year class. 



Differences in the annual occurrence of Pacific 

 whiting and market squid were also noted in this 

 study. For market squid, there was no fishery infor- 

 mation available during the time of this study which 

 would provide us with a possible explanation for 

 these differences. With Pacific whiting, however, the 

 decrease in occurrence in the scats from 1978 to 

 1979 appears to be related to exceptionally high re- 

 cruitment of the 1977 year class which was followed 

 by an average, or possibly somewhat less-than- 

 average, recruitment in 1978 (T. Dark footnote 3). 

 This information is corroborated by a comparison of 

 the Pacific whiting otoliths collected during the 2 

 years of our study. In 1978, sea lions preyed upon 

 significantly smaller fish which were mostly (98.5%) 

 yearlings from the 1977 year class. 



Our analysis of the frequency occurrence of prey 

 species per individual scat (Fig. 3) suggests that 

 California sea lions commonly feed on single prey 

 species during the spring and feed more frequently 

 on multiple prey species in the summer. This shift 

 from single to multiple occurrence of prey species in 

 scats could reflect a decrease in the overall 

 availability of the potential prey species in the sum- 

 mer which may necessitate foraging on a greater 

 variety of food items for survival (Morse 1980). Alter- 

 natively, numerous potential prey species may 

 become more available (Morse 1980) during the 

 summer; thus, California sea lions could forage 

 opportunistically on a greater variety of schooling 

 fishes or squids which concentrate in a comparatively 

 small area of high productivity. 



There are, however, a variety of factors which could 

 affect prey-species availability. Seasonal migration, 

 diel vertical migration, variability in schooling 

 behavior, or physiological changes associated with 

 spawning (Moyle and Cech 1982) are probably some 

 of the more important factors related to prey selec- 



73 



